Assume L 5 1.20 m and ... toward the spring with speed v as shown in Figure .... simple situation is the first example of a nonisolated system, for wh...

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Challenge Problems 6 6. A particle of mass m 5 1.18 kg is attached between two identical springs on a frictionless, horizontal tabletop. Both springs have spring constant k and are initially unstressed, and the particle is at x 5 0. (a) The particle is pulled a distance x along a direction perpendicular to the initial configuration of the springs as shown in Figure P7.66. Show that the force exerted by the springs on the particle is "x 2 1 L2

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F 5 22kx a1 2

Overhead view k

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Figure P7.66 67. Review. A light spring Q/C has unstressed length 15.5 cm. It is described by Hooke’s law with spring constant 4.30 N/m. One end of the horizontal spring is held on a fixed vertical axle, and the other end is attached to a puck of mass m that can move without friction over a horizontal surface. The puck is set into motion in a circle with a period of 1.30 s. (a) Find the extension of the spring x as it depends on m. Evaluate x for (b) m 5 0.070 0 kg, (c) m 5 0.140 kg, (d) m 5 0.180 kg, and (e) m 5 0.190 kg. (f) Describe the pattern of variation of x as it depends on m.

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(c) Make a plot of U(x) versus x and identify all equilibrium points. Assume L 5 1.20 m and k 5 40.0 N/m. (d) If the particle is pulled 0.500 m to the right and then released, what is its speed when it reaches x 5 0?

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a function of the particle position x. The force on the particle is given by (8e22x) ^i. (b) Explain whether the force is conservative or nonconservative and how you can tell.

U 1 x 2 5 kx 2 1 2kL 1 L 2 "x 2 1 L2 2

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65. (a) Take U 5 5 for a system with a particle at position x 5

Q/C 0 and calculate the potential energy of the system as

(b) Show that the potential energy of the system is

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spring is parallel to the surface. A block of mass m is placed on the plane at a distance d from the spring. From this position, the block is projected downward toward the spring with speed v as shown in Figure P7.63. By what distance is the spring compressed when the block momentarily comes to rest?

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210 Chapter 7

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8

Conservation of Energy

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8.1 Analysis Model: Nonisolated System (Energy) 8.2 Analysis Model: Isolated System (Energy)

8.4 Changes in Mechanical Energy for Nonconservative Forces 8.5 Power

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8.3 Situations Involving Kinetic Friction

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In Chapter 7, we introduced three methods for storing energy in a system: kinetic energy, associated with movement of members of the system; potential energy, determined by the configuration of the system; and internal energy, which is related to the temperature of the system. We now consider analyzing physical situations using the energy approach for two types of systems: nonisolated and isolated systems. For nonisolated systems, we shall investigate ways that energy can cross the boundary of the system, resulting in a change in the system’s total energy. This analysis leads to a critically important principle called conservation of energy. The conservation of energy principle extends well beyond physics and can be applied to biological organisms, technological systems, and engineering situations. In isolated systems, energy does not cross the boundary of the system. For these systems, the total energy of the system is constant. If no nonconservative forces act within the system, we can use conservation of mechanical energy to solve a variety of problems.

Three youngsters enjoy the transformation of potential energy to kinetic energy on a waterslide. We can analyze processes such as these with the techniques developed in this chapter. (Jade Lee/Asia Images/Getty Images)

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212 Chapter 8 Conservation of Energy Situations involving the transformation of mechanical energy to internal energy due to nonconservative forces require special handling. We investigate the procedures for these types of problems. Finally, we recognize that energy can cross the boundary of a system at different rates. We describe the rate of energy transfer with the quantity power.

8.1 Analysis Model: Nonisolated System (Energy)

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As we have seen, an object, modeled as a particle, can be acted on by various forces, resulting in a change in its kinetic energy according to the work–kinetic energy theorem from Chapter 7. If we choose the object as the system, this very simple situation is the first example of a nonisolated system, for which energy crosses the boundary of the system during some time interval due to an interaction with the environment. This scenario is common in physics problems. If a system does not interact with its environment, it is an isolated system, which we will study in Section 8.2. The work–kinetic energy theorem is our first example of an energy equation appropriate for a nonisolated system. In the case of that theorem, the interaction of the system with its environment is the work done by the external force, and the quantity in the system that changes is the kinetic energy. So far, we have seen only one way to transfer energy into a system: work. We mention below a few other ways to transfer energy into or out of a system. The details of these processes will be studied in other sections of the book. We illustrate mechanisms to transfer energy in Figure 8.1 and summarize them as follows. Work, as we have learned in Chapter 7, is a method of transferring energy to a system by applying a force to the system such that the point of application of the force undergoes a displacement (Fig. 8.1a).

Energy leaves the radio from the speaker by mechanical waves.

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a Energy enters the automobile gas tank by matter transfer.

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c Energy enters the hair dryer by electrical transmission.

Energy leaves the lightbulb by electromagnetic radiation. © Cengage Learning/George Semple

© Cengage Learning/George Semple

fer mechanisms. In each case, the system into which or from which energy is transferred is indicated.

Cocoon/Photodisc/Getty Images

Figure 8.1 Energy trans-

Energy transfers to the handle of the spoon by heat. © Cengage Learning/George Semple

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© Cengage Learning/George Semple

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© Cengage Learning/George Semple

Energy is transferred to the block by work.

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8.1 Analysis Model: Nonisolated System (Energy) 213

Pitfall Prevention 8.1 Heat Is Not a Form of Energy The word heat is one of the most misused words in our popular language. Heat is a method of transferring energy, not a form of storing energy. Therefore, phrases such as “heat content,” “the heat of the summer,” and “the heat escaped” all represent uses of this word that are inconsistent with our physics definition. See Chapter 20.

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Mechanical waves (Chapters 16–18) are a means of transferring energy by allowing a disturbance to propagate through air or another medium. It is the method by which energy (which you detect as sound) leaves the system of your clock radio through the loudspeaker and enters your ears to stimulate the hearing process (Fig. 8.1b). Other examples of mechanical waves are seismic waves and ocean waves. Heat (Chapter 20) is a mechanism of energy transfer that is driven by a temperature difference between a system and its environment. For example, imagine dividing a metal spoon into two parts: the handle, which we identify as the system, and the portion submerged in a cup of coffee, which is part of the environment (Fig. 8.1c). The handle of the spoon becomes hot because fast-moving electrons and atoms in the submerged portion bump into slower ones in the nearby part of the handle. These particles move faster because of the collisions and bump into the next group of slow particles. Therefore, the internal energy of the spoon handle rises from energy transfer due to this collision process. Matter transfer (Chapter 20) involves situations in which matter physically crosses the boundary of a system, carrying energy with it. Examples include filling your automobile tank with gasoline (Fig. 8.1d) and carrying energy to the rooms of your home by circulating warm air from the furnace, a process called convection. Electrical transmission (Chapters 27 and 28) involves energy transfer into or out of a system by means of electric currents. It is how energy transfers into your hair dryer (Fig. 8.1e), home theater system, or any other electrical device. Electromagnetic radiation (Chapter 34) refers to electromagnetic waves such as light (Fig. 8.1f), microwaves, and radio waves crossing the boundary of a system. Examples of this method of transfer include cooking a baked potato in your microwave oven and energy traveling from the Sun to the Earth by light through space.1 A central feature of the energy approach is the notion that we can neither create nor destroy energy, that energy is always conserved. This feature has been tested in countless experiments, and no experiment has ever shown this statement to be incorrect. Therefore, if the total amount of energy in a system changes, it can only be because energy has crossed the boundary of the system by a transfer mechanism such as one of the methods listed above. Energy is one of several quantities in physics that are conserved. We will see other conserved quantities in subsequent chapters. There are many physical quantities that do not obey a conservation principle. For example, there is no conservation of force principle or conservation of velocity principle. Similarly, in areas other than physical quantities, such as in everyday life, some quantities are conserved and some are not. For example, the money in the system of your bank account is a conserved quantity. The only way the account balance changes is if money crosses the boundary of the system by deposits or withdrawals. On the other hand, the number of people in the system of a country is not conserved. Although people indeed cross the boundary of the system, which changes the total population, the population can also change by people dying and by giving birth to new babies. Even if no people cross the system boundary, the births and deaths will change the number of people in the system. There is no equivalent in the concept of energy to dying or giving birth. The general statement of the principle of conservation of energy can be described mathematically with the conservation of energy equation as follows:

DE system 5

o T

(8.1)

where E system is the total energy of the system, including all methods of energy storage (kinetic, potential, and internal), and T (for transfer) is the amount of energy transferred across the system boundary by some mechanism. Two of our transfer mechanisms have well-established symbolic notations. For work, Twork 5 W as discussed in Chapter 7, and for heat, Theat 5 Q as defined in Chapter 20. (Now that we 1Electromagnetic

radiation and work done by field forces are the only energy transfer mechanisms that do not require molecules of the environment to be available at the system boundary. Therefore, systems surrounded by a vacuum (such as planets) can only exchange energy with the environment by means of these two possibilities.

WW Conservation of energy

214 Chapter 8 Conservation of Energy are familiar with work, we can simplify the appearance of equations by letting the simple symbol W represent the external work Wext on a system. For internal work, we will always use W int to differentiate it from W.) The other four members of our list do not have established symbols, so we will call them TMW (mechanical waves), TMT (matter transfer), TET (electrical transmission), and TER (electromagnetic radiation). The full expansion of Equation 8.1 is

(8.2)

DK 1 DU 1 DE int 5 W 1 Q 1 TMW 1 TMT 1 TET 1 TER

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which is the primary mathematical representation of the energy version of the analysis model of the nonisolated system. (We will see other versions of the nonisolated system model, involving linear momentum and angular momentum, in later chapters.) In most cases, Equation 8.2 reduces to a much simpler one because some of the terms are zero for the specific situation. If, for a given system, all terms on the right side of the conservation of energy equation are zero, the system is an isolated system, which we study in the next section. The conservation of energy equation is no more complicated in theory than the process of balancing your checking account statement. If your account is the system, the change in the account balance for a given month is the sum of all the transfers: deposits, withdrawals, fees, interest, and checks written. You may find it useful to think of energy as the currency of nature! Suppose a force is applied to a nonisolated system and the point of application of the force moves through a displacement. Then suppose the only effect on the system is to change its speed. In this case, the only transfer mechanism is work (so that the right side of Eq. 8.2 reduces to just W ) and the only kind of energy in the system that changes is the kinetic energy (so that the left side of Eq. 8.2 reduces to just DK). Equation 8.2 then becomes DK 5 W

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which is the work–kinetic energy theorem. This theorem is a special case of the more general principle of conservation of energy. We shall see several more special cases in future chapters.

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Q uick Quiz 8.1 By what transfer mechanisms does energy enter and leave (a) your television set? (b) Your gasoline-powered lawn mower? (c) Your hand-cranked pencil sharpener?

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Q uick Quiz 8.2 Consider a block sliding over a horizontal surface with friction. Ignore any sound the sliding might make. (i) If the system is the block, this system is (a) isolated (b) nonisolated (c) impossible to determine (ii) If the system is the surface, describe the system from the same set of choices. (iii) If the system is the block and the surface, describe the system from the same set of choices.

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Analysis Model Nonisolated System (Energy) Imagine you have identified a system to be analyzed and have defined a system boundary. Energy can exist in the system in three forms: kinetic, potential, and internal. The total of that energy can be changed when energy crosses the system boundary by any of six transfer methods shown in the diagram here. The total change in the energy in the system is equal to the total amount of energy that has crossed the system boundary. The mathematical statement of that concept is expressed in the conservation of energy equation:

DE system 5 o T

(8.1)

System boundary The change in the total amount of energy in the system is equal to the total amount of energy that crosses the boundary of the system.

Work

Heat

Mechanical waves

Kinetic energy Potential energy Internal energy Matter transfer

Electrical Electromagnetic transmission radiation

8.2 Analysis Model: Isolated System (Energy) 215

Analysis Model Nonisolated System (Energy) (continued) The full expansion of Equation 8.1 shows the specific types of energy storage and transfer:

(8.2)

DK 1 DU 1 DE int 5 W 1 Q 1 TMW 1 TMT 1 TET 1 TER

For a specific problem, this equation is generally reduced to a smaller number of terms by eliminating the terms that are equal to zero because they are not appropriate to the situation. Examples:

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• a force does work on a system of a single object, changing its speed: the work–kinetic energy theorem, W 5 DK • a gas contained in a vessel has work done on it and experiences a transfer of energy by heat, resulting in a change in its temperature: the first law of thermodynamics, DE int 5 W 1 Q (Chapter 20) • an incandescent light bulb is turned on, with energy entering the filament by electricity, causing its temperature to increase, and leaving by light: DE int 5 TET 1 TER (Chapter 27) • a photon enters a metal, causing an electron to be ejected from the metal: the photoelectric effect, DK 1 DU 5 TER (Chapter 40)

8.2 Analysis Model: Isolated System (Energy)

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In this section, we study another very common scenario in physics problems: a system is chosen such that no energy crosses the system boundary by any method. We begin by considering a gravitational situation. Think about the book–Earth system in Figure 7.15 in the preceding chapter. After we have lifted the book, there is gravitational potential energy stored in the system, which can be calculated from the work done by the external agent on the system, using W 5 DUg . (Check to see that this equation, which we’ve seen before, is contained within Eq. 8.2 above.) Let us now shift our focus to the work done on the book alone by the gravitational force (Fig. 8.2) as the book falls back to its original height. As the book falls from yi to yf , the work done by the gravitational force on the book is

The book is held at rest here and then released.

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Physics

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Won book 5 1 m g 2 ? D r 5 1 2mg j^ 2 ? 3 1 yf 2 yi 2 j^ 4 5 mg yi 2 mg yf

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Physics

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(8.3)

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From the work–kinetic energy theorem of Chapter 7, the work done on the book is equal to the change in the kinetic energy of the book:

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At a lower position, the book is moving and has kinetic energy K.

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We can equate these two expressions for the work done on the book: DKbook 5 mgyi 2 mgyf

(8.4)

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Let us now relate each side of this equation to the system of the book and the Earth. For the right-hand side, mgyi 2 mgyf 5 2(mgyf 2 mgyi ) 5 2DUg

where Ug 5 mgy is the gravitational potential energy of the system. For the left-hand side of Equation 8.4, because the book is the only part of the system that is moving, we see that DKbook 5 DK, where K is the kinetic energy of the system. Therefore, with each side of Equation 8.4 replaced with its system equivalent, the equation becomes

DK 5 2DUg

(8.5)

This equation can be manipulated to provide a very important general result for solving problems. First, we move the change in potential energy to the left side of the equation: DK 1 DUg 5 0

Figure 8.2 A book is released from rest and falls due to work done by the gravitational force on the book.

216 Chapter 8 Conservation of Energy The left side represents a sum of changes of the energy stored in the system. The right-hand side is zero because there are no transfers of energy across the boundary of the system; the book–Earth system is isolated from the environment. We developed this equation for a gravitational system, but it can be shown to be valid for a system with any type of potential energy. Therefore, for an isolated system,

Conditions on Equation 8.6 Equation 8.6 is only true for a system in which conservative forces act. We will see how to handle nonconservative forces in Sections 8.3 and 8.4.

Mechanical energy of a system

(8.6)

DK 1 DU 5 0

(Check to see that this equation is contained within Eq. 8.2.) We defined in Chapter 7 the sum of the kinetic and potential energies of a system as its mechanical energy: E mech ; K 1 U

(8.7)

DE mech 5 0

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The mechanical energy of an isolated system with no nonconservative forces acting is conserved.

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where U represents the total of all types of potential energy. Because the system under consideration is isolated, Equations 8.6 and 8.7 tell us that the mechanical energy of the system is conserved: (8.8)

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Equation 8.8 is a statement of conservation of mechanical energy for an isolated system with no nonconservative forces acting. The mechanical energy in such a system is conserved: the sum of the kinetic and potential energies remains constant: Let us now write the changes in energy in Equation 8.6 explicitly:

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Pitfall Prevention 8.2

(Kf 2 Ki ) 1 (Uf 2 Ui ) 5 0 Kf 1 Uf 5 Ki 1 Ui

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(8.9)

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For the gravitational situation of the falling book, Equation 8.9 can be written as 1 2 2 mv f

1 mg yf 5 12mv i 2 1 mg yi

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As the book falls to the Earth, the book–Earth system loses potential energy and gains kinetic energy such that the total of the two types of energy always remains constant: E total,i 5 E total, f . If there are nonconservative forces acting within the system, mechanical energy is transformed to internal energy as discussed in Section 7.7. If nonconservative forces act in an isolated system, the total energy of the system is conserved although the mechanical energy is not. In that case, we can express the conservation of energy of the system as

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The total energy of an isolated system is conserved. 1

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DE system 5 0

(8.10)

where E system includes all kinetic, potential, and internal energies. This equation is the most general statement of the energy version of the isolated system model. It is equivalent to Equation 8.2 with all terms on the right-hand side equal to zero. Q uick Quiz 8.3 A rock of mass m is dropped to the ground from a height h. A second rock, with mass 2m, is dropped from the same height. When the second rock strikes the ground, what is its kinetic energy? (a) twice that of the first rock (b) four times that of the first rock (c) the same as that of the first rock (d) half as much as that of the first rock (e) impossible to determine

Figure 8.3 (Quick Quiz 8.4) Three identical balls are thrown with the same initial speed from the top of a building.

Q uick Quiz 8.4 Three identical balls are thrown from the top of a building, all with the same initial speed. As shown in Figure 8.3, the first is thrown horizontally, the second at some angle above the horizontal, and the third at some angle below the horizontal. Neglecting air resistance, rank the speeds of the balls at the instant each hits the ground.

8.2 Analysis Model: Isolated System (Energy) 217

Analysis Model Isolated System (Energy)

(8.10)

DE system 5 0

If no nonconservative forces act within the isolated system, the mechanical energy of the system is conserved, so

(8.8)

DE mech 5 0

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• an object is in free-fall; gravitational potential energy transforms to kinetic energy: DK 1 DU 5 0 • a basketball rolling across a gym floor comes to rest; kinetic energy transforms to internal energy: DK 1 DE int 5 0 • a pendulum is raised and released with an initial speed; its motion eventually stops due to air resistance; gravitational potential energy and kinetic energy transform to internal energy, DK 1 DU 1 DE int 5 0 (Chapter 15) • a battery is connected to a resistor; chemical potential energy in the battery transforms to internal energy in the resistor: DU 1 DE int 5 0 (Chapter 27)

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Examples:

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Imagine you have System identified a system boundary Kinetic energy to be analyzed and Potential energy have defined a system Internal energy boundary. Energy can exist in the system in three forms: kinetic, The total amount of energy potential, and interin the system is constant. nal. Imagine also a Energy transforms among the three possible types. situation in which no energy crosses the boundary of the system by any method. Then, the system is isolated; energy transforms from one form to another and Equation 8.2 becomes

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Problem-Solving Strategy Isolated and Nonisolated Systems with No Nonconservative Forces: Conservation of Energy

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Many problems in physics can be solved using the principle of conservation of energy. The following procedure should be used when you apply this principle:

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1. Conceptualize. Study the physical situation carefully and form a mental representation of what is happening. As you become more proficient working energy problems, you will begin to be comfortable imagining the types of energy that are changing in the system and the types of energy transfers occurring across the system boundary.

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2. Categorize. Define your system, which may consist of more than one object and may or may not include springs or other possibilities for storing potential energy. Identify the time interval over which you will analyze the energy changes in the problem. Determine if any energy transfers occur across the boundary of your system during this time interval. If so, use the nonisolated system model, DE system 5 o T, from Section 8.1. If not, use the isolated system model, DE system 5 0. Determine whether any nonconservative forces are present within the system. If so, use the techniques of Sections 8.3 and 8.4. If not, use the principle of conservation of energy as outlined below. 3. Analyze. Choose configurations to represent the initial and final conditions of the system based on your choice of time interval. For each object that changes elevation, select a reference position for the object that defines the zero configuration of gravitational potential energy for the system. For an object on a spring, the zero configuration for elastic potential energy is when the object is at its equilibrium position. If there is more than one conservative force, write an expression for the potential energy associated with each force. Begin with Equation 8.2 and retain only those terms in the equation that are appropriate for the situation in the problem. Express each change of energy stored in the system as the final value minus the initial value. Substitute appropriate expressions for each initial and final value of energy storage on the left side of the equation and for the energy transfers on the right side of the equation. Solve for the unknown quantity.

continued

218 Chapter 8

Conservation of Energy

▸ Problem-Solving Strategy c o n t i n u e d 4. Finalize. Make sure your results are consistent with your mental representation. Also make sure the values of your results are reasonable and consistent with connections to everyday experience.

Example 8.1

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A ball of mass m is dropped from a height h above the ground as shown in Figure 8.4.

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(A) Neglecting air resistance, determine the speed of the ball when it is at a height y above the ground. Choose the system as the ball and the Earth.

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Solution

Conceptualize Figure 8.4 and our everyday experience with falling objects

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allow us to conceptualize the situation. Although we can readily solve this problem with the techniques of Chapter 2, let us practice an energy approach.

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Categorize As suggested in the problem, we identify the system as the ball and the Earth. Because there is neither air resistance nor any other interaction between the system and the environment, the system is isolated and we use the isolated system model. The only force between members of the system is the gravitational force, which is conservative.

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acting within the system, we apply the principle of conservation of mechanical energy to the ball–Earth system. At the instant the ball is released, its kinetic energy is Ki 5 0 and the gravitational potential energy of the system is Ugi 5 mgh. When the ball is at a position y above the ground, its kinetic energy is K f 5 12mv f 2 and the potential energy relative to the ground is Ugf 5 mgy.

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Solve for vf :

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Substitute for the energies:

vf

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Figure 8.4 (Example 8.1) A ball is dropped from a height h above the ground. Initially, the total energy of the ball–Earth system is gravitational potential energy, equal to mgh relative to the ground. At the position y, the total energy is the sum of the kinetic and potential energies.

Analyze Because the system is isolated and there are no nonconservative forces

Write the appropriate reduction of Equation 8.2, noting that the only types of energy in the system that change are kinetic energy and gravitational potential energy:

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yf y Ugf mgy 1 Kf mvf2 2

DK 1 DUg 5 0

1 12mv f 2 2 0 2 1 1 mg y 2 mgh 2 5 0

v f 2 5 2g 1 h 2 y 2

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(B) F ind the speed of the ball again at height y by choosing the ball as the system.

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Categorize In this case, the only type of energy in the system that changes is kinetic energy. A single object that can be modeled as a particle cannot possess potential energy. The effect of gravity is to do work on the ball across the boundary of the system. We use the nonisolated system model. Analyze Write the appropriate reduction of Equation 8.2:

DK 5 W

Substitute for the initial and final kinetic energies and the work:

1 12mv f2 2 0 2 5 Fg ? DS r 5 2mg ^j ? Dy ^j 5 2mgDy 5 2mg(y 2 h) 5 mg(h 2 y)

Solve for vf :

v f 2 5 2g 1 h 2 y 2

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v f 5 "2g 1 h 2 y 2

8.2 Analysis Model: Isolated System (Energy) 219

▸ 8.1 c o n t i n u e d Finalize The final result is the same, regardless of the choice of system. In your future problem solving, keep in mind that the choice of system is yours to make. Sometimes the problem is much easier to solve if a judicious choice is made as to the system to analyze. W h at I f ?

at height y?

What if the ball were thrown downward from its highest position with a speed vi ? What would its speed be

Answer If the ball is thrown downward initially, we would expect its speed at height y to be larger than if simply dropped. Make your choice of system, either the ball alone or the ball and the Earth. You should find that either choice gives you the following result:

A Grand Entrance AM

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You are designing an apparatus to support an actor of mass 65.0 kg who is to “fly” down to the stage during the performance of a play. You attach the actor’s harness to a 130-kg sandbag by means of a lightweight steel cable running smoothly over two frictionless pulleys as in Figure 8.5a. You need 3.00 m of cable between the harness and the nearest pulley so that the pulley can be hidden behind a curtain. For the apparatus to work successfully, the sandbag must never lift above the floor as the actor swings from above the stage to the floor. Let us call the initial angle that the actor’s cable makes with the vertical u. What is the maximum value u can have before the sandbag lifts off the floor?

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Example 8.2

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v f 5 "v i2 1 2g 1 h 2 y 2

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S o l u ti o n

Conceptualize We must use several concepts to solve this problem.

Actor

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Imagine what happens as the actor approaches the bottom of the swing. At the bottom, the cable is vertical and must support his weight as well as provide centripetal acceleration of his body in the upward direction. At this point in his swing, the tension in the cable is the highest and the sandbag is most likely to lift off the floor.

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Categorize Looking first at the swinging of the actor from the initial point to the lowest point, we model the actor and the Earth as an isolated system. We ignore air resistance, so there are no nonconservative forces acting. You might initially be tempted to model the system as nonisolated because of the interaction of the system with the cable, which is in the environment. The force applied to the actor by the cable, however, is always perpendicular to each element of the displacement of the actor and hence does no work. Therefore, in terms of energy transfers across the boundary, the system is isolated.

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Figure 8.5 (Example 8.2) (a) An actor uses some clever staging to make his entrance. (b) The free-body diagram for the actor at the bottom of the circular path. (c) The free-body diagram for the sandbag if the normal force from the floor goes to zero.

Analyze We first find the actor’s speed as he arrives at the floor as a function of the initial angle u and the radius R of the circular path through which he swings. From the isolated system model, make the appropriate reduction of Equation 8.2 for the actor–Earth system:

DK 1 DUg 5 0

continued

220 Chapter 8 Conservation of Energy ▸ 8.2 c o n t i n u e d (1) 1 12m actor v f2 2 0 2 1 1 0 2 m actor g yi 2 5 0

From the geometry in Figure 8.5a, notice that yf 5 0, so yi 5 R 2 R cos u 5 R(1 2 cos u). Use this relationship in Equation (1) and solve for vf 2:

(2) v f 2 5 2gR 1 1 2 cos u 2

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Let yi be the initial height of the actor above the floor and vf be his speed at the instant before he lands. (Notice that Ki 5 0 because the actor starts from rest and that Uf 5 0 because we define the configuration of the actor at the floor as having a gravitational potential energy of zero.)

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Categorize Next, focus on the instant the actor is at the lowest point. Because the tension in the cable is transferred as a force applied to the sandbag, we model the actor at this instant as a particle under a net force. Because the actor moves along a circular arc, he experiences at the bottom of the swing a centripetal acceleration of vf 2/r directed upward. vf2 F 5 T 2 m g 5 m y actor actor a R

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Analyze Apply Newton’s second law from the particle under

ee b

a net force model to the actor at the bottom of his path, using the free-body diagram in Figure 8.5b as a guide, and recognizing the acceleration as centripetal:

(3) T 5 m actorg 1 m actor

vf2 R

s. w

Categorize Finally, notice that the sandbag lifts off the floor when the upward force exerted on it by the cable exceeds the gravitational force acting on it; the normal force from the floor is zero when that happens. We do not, however, want the sandbag to lift off the floor. The sandbag must remain at rest, so we model it as a particle in equilibrium.

ic

Analyze A force T of the magnitude given by Equation (3) is transmitted by the cable to the sandbag. If the sandbag

ys

remains at rest but is just ready to be lifted off the floor if any more force were applied by the cable, the normal force on it becomes zero and the particle in equilibrium model tells us that T 5 m bagg as in Figure 8.5c. m bag g 5 m actor g 1 m actor

Solve for cos u and substitute the given parameters:

cos u 5

sw

ar ph

Substitute this condition and Equation (2) into Equation (3):

3m actor 2 m bag 2m actor

u 5 60.08

2gR 1 1 2 cos u 2

5

R

3 1 65.0 kg 2 2 130 kg 5 0.500 2 1 65.0 kg 2

w

.a

Finalize Here we had to combine several analysis models from different areas of our study. Notice that the length R of the cable from the actor’s harness to the leftmost pulley did not appear in the final algebraic equation for cos u. Therefore, the final answer is independent of R.

w

w

Example 8.3

The Spring-Loaded Popgun AM

The launching mechanism of a popgun consists of a trigger-released spring (Fig. 8.6a). The spring is compressed to a position y A, and the trigger is fired. The projectile of mass m rises to a position y C above the position at which it leaves the spring, indicated in Figure 8.6b as position y B 5 0. Consider a firing of the gun for which m 5 35.0 g, y A 5 20.120 m, and y C 5 20.0 m. (A) Neglecting all resistive forces, determine the spring constant. S o l u ti o n

Conceptualize Imagine the process illustrated in parts (a) and (b) of Figure 8.6. The projectile starts from rest at A, speeds up as the spring pushes upward on it, leaves the spring at B, and then slows down as the gravitational force pulls downward on it, eventually coming to rest at point C.

8.2 Analysis Model: Isolated System (Energy) 221

▸ 8.3 c o n t i n u e d % 100 50 0

yC S

v

Kinetic Elastic Grav. Total energy pot. pot. energy energy energy

c % 100 50 0 yB 0

B

Kinetic Elastic Grav. Total energy pot. pot. energy energy energy

f

ic

b

Kinetic Elastic Grav. Total energy pot. pot. energy energy energy

ys

a

s. w

e % 100 50 0

ly

Kinetic Elastic Grav. Total energy pot. pot. energy energy energy

Isolated system: total energy constant

ee b

% 100 50 0

Figure 8.6 (Example 8.3) A spring-loaded popgun (a) before firing and (b) when the spring extends to its relaxed length. (c) An energy bar chart for the popgun–projectile–Earth system before the popgun is loaded. The energy in the system is zero. (d) The popgun is loaded by means of an external agent doing work on the system to push the spring downward. Therefore the system is nonisolated during this process. After the popgun is loaded, elastic potential energy is stored in the spring and the gravitational potential energy of the system is lower because the projectile is below point B. (e) as the projectile passes through point B, all of the energy of the isolated system is kinetic. (f) When the projectile reaches point C, all of the energy of the isolated system is gravitational potential.

.c

d

yA

A

Nonisolated system: total energy changes

om

C

ar ph

Categorize We identify the system as the projectile, the spring, and the Earth. We ignore both air resistance on the projectile and friction in the gun, so we model the system as isolated with no nonconservative forces acting. Analyze Because the projectile starts from rest, its initial kinetic energy is zero. We choose the zero configuration for

sw

the gravitational potential energy of the system to be when the projectile leaves the spring at B. For this configuration, the elastic potential energy is also zero. After the gun is fired, the projectile rises to a maximum height y C. The final kinetic energy of the projectile is zero. (1) DK 1 DUg 1 DUs 5 0

w

w

.a

From the isolated system model, write a conservation of mechanical energy equation for the system between configurations when the projectile is at points A and C:

w

Substitute for the initial and final energies: Solve for k:

Substitute numerical values:

1 0 2 0 2 1 1 mg y C 2 mg y A 2 1 1 0 2 12kx 2 2 5 0

k5 k5

2mg 1 y C 2 y A 2 x2

2 1 0.035 0 kg 2 1 9.80 m/s2 2 3 20.0 m 2 1 20.120 m 2 4 5 958 N/m 1 0.120 m 2 2

(B) Find the speed of the projectile as it moves through the equilibrium position B of the spring as shown in Figure 8.6b. S o l u ti o n

Analyze The energy of the system as the projectile moves through the equilibrium position of the spring includes only the kinetic energy of the projectile 12mv B2. Both types of potential energy are equal to zero for this configuration of the system.

continued

222 Chapter 8 Conservation of Energy ▸ 8.3 c o n t i n u e d Write Equation (1) again for the system between points A and B:

DK 1 DUg 1 DUs 5 0

Substitute for the initial and final energies:

1 21mv B2 2 0 2 1 1 0 2 mg y A 2 1 1 0 2 21kx 2 2 5 0

Solve for v B:

vB 5

Substitute numerical values:

1 958 N/m 2 1 0.120 m 2 2 1 2 1 9.80 m/s2 2 1 20.120 m 2 5 19.8 m/s 1 0.035 0 kg 2 Å

om

vB 5

kx 2 1 2g y A Å m

ly

.c

Finalize This example is the first one we have seen in which we must include two different types of potential energy. Notice in part (A) that we never needed to consider anything about the speed of the ball between points A and C, which is part of the power of the energy approach: changes in kinetic and potential energy depend only on the initial and final values, not on what happens between the configurations corresponding to these values.

ee b

8.3 Situations Involving Kinetic Friction

ys

ic

s. w

Consider again the book in Figure 7.18a sliding to the right on the surface of a heavy table and slowing down due to the friction force. Work is done by the friction force on the book because there is a force and a displacement. Keep in mind, however, that our equations for work involve the displacement of the point of application of the force. A simple model of the friction force between the book and the surface is shown in Figure 8.7a. We have represented the entire friction force between the book and surface as being due to two identical teeth that have been spot-welded together.2 One tooth projects upward from the surface, the other downward from the book, and they are welded at the points where they touch. The friction force acts at the junction of the two teeth. Imagine that the book slides a small distance d to the right as in Figure 8.7b. Because the teeth are modeled as identical, the junction of the teeth moves to the right by a distance d/2. Therefore, the displacement of the point of application of the friction force is d/2, but the displacement of the book is d! In reality, the friction force is spread out over the entire contact area of an object sliding on a surface, so the force is not localized at a point. In addition, because the magnitudes of the friction forces at various points are constantly changing as individual spot welds occur, the surface and the book deform locally, and so on, the displacement of the point of application of the friction force is not at all the same as the displacement of the book. In fact, the displacement of the point of application of the friction force is not calculable and so neither is the work done by the friction force. The work–kinetic energy theorem is valid for a particle or an object that can be modeled as a particle. When a friction force acts, however, we cannot calculate the work done by friction. For such situations, Newton’s second law is still valid for the system even though the work–kinetic energy theorem is not. The case of a nondeformable object like our book sliding on the surface3 can be handled in a relatively straightforward way. Starting from a situation in which forces, including friction, are applied to the book, we can follow a similar procedure to that done in developing Equation 7.17. Let us start by writing Equation 7.8 for all forces on an object other than friction:

sw

ar ph

The entire friction force is modeled to be applied at the interface between two identical teeth projecting from the book and the surface.

Book

w

d

w

a

w

.a

Surface

d 2

The point of application of the friction force moves through a displacement of magnitude d/2. b

Figure 8.7 (a) A simplified model of friction between a book and a surface. (b) The book is moved to the right by a distance d.

2Figure

S a Wother forces 5 3 1 a F other forces 2 ? d r S

(8.11)

8.7 and its discussion are inspired by a classic article on friction: B. A. Sherwood and W. H. Bernard, “Work and heat transfer in the presence of sliding friction,” American Journal of Physics, 52:1001, 1984.

3The overall shape of the book remains the same, which is why we say it is nondeformable. On a microscopic level, however, there is deformation of the book’s face as it slides over the surface.

8.3 Situations Involving Kinetic Friction 223

The d S r in this equation is the displacement of the object because for forces other than friction, under the assumption that these forces do not deform the object, this displacement is the same as the displacement of the point of application of the forces. To each side of Equation 8.11 let us add the integral of the scalar product of r . In doing so, we are not defining this quantity the force of kinetic friction and d S as work! We are simply saying that it is a quantity that can be calculated mathematically and will turn out to be useful to us in what follows. S S S a Wother forces 1 3 fk ? d r 5 3 1 a F other forces 2 ? d r 1 3 fk ? d r S

S

S

5 3 1 a F other forces 1 fk 2 ? d S r S The integrand on the right side of this equation is the net force g F on the object, so S

om

S

S S a Wother forces 1 3 fk ? d r 5 3 a F ? d r S

Incorporating Newton’s second law g F 5 mS a gives S

f S dS v S dS v a Wother forces 1 3 fk ? d S ? v dt r 5 3 mS a ?d S r 53m ?d S r 53 m dt dt ti

ly

t

.c

S

ee b

(8.12)

s. w

r as S v dt. The scalar product obeys where we have used Equation 4.3 to rewrite d S the product rule for differentiation (See Eq. B.30 in Appendix B.6), so the derivav with itself can be written tive of the scalar product of S dS v S S dS v dS v S d S S 1v?v2 5 ?v 1 v? 52 ?v dt dt dt dt

ys

ic

We used the commutative property of the scalar product to justify the final expression in this equation. Consequently,

ar ph

dS v S 1 d S S dv 2 ? v 5 2 1 v ? v 2 5 12 dt dt dt Substituting this result into Equation 8.12 gives

f f 2 S S 1 dv 1 1 1 2 2 2 W 1 f ? d r 5 m dt 5 m a b 3 3 d 1 v 2 5 2mv f 2 2mv i 5 DK 2 2 a other forces 3 k dt ti vi

t

v

w

.a

sw

Looking S at the left side of this equation, notice that in the inertial frame of the r will be in opposite directions for every increment d S r of the surface, f k and d S S S path followed by the object. Therefore, fk ? d r 5 2fk dr. The previous expression now becomes a Wother forces 2 3 fk dr 5 DK

w

w

In our model for friction, the magnitude of the kinetic friction force is constant, so f k can be brought out of the integral. The remaining integral e dr is simply the sum of increments of length along the path, which is the total path length d. Therefore,

o Wother forces 2 f kd 5 DK

(8.13)

Equation 8.13 can be used when a friction force acts on an object. The change in kinetic energy is equal to the work done by all forces other than friction minus a term f kd associated with the friction force. Considering the sliding book situation again, let’s identify the larger system of the book and the surface as the book slows down under the influence of a friction force alone. There is no work done across the boundary of this system by other forces because the system does not interact with the environment. There are no other types of energy transfer occurring across the boundary of the system, assuming we ignore the inevitable sound the sliding book makes! In this case, Equation 8.2 becomes

DE system 5 DK 1 DE int 5 0

224 Chapter 8 Conservation of Energy The change in kinetic energy of this book–surface system is the same as the change in kinetic energy of the book alone because the book is the only part of the system that is moving. Therefore, incorporating Equation 8.13 with no work done by other forces gives

Change in internal energy due to a constant friction force within the system

2f kd 1 DE int 5 0

DE int 5 f kd

(8.14)

o Wother forces 5 W 5 DK 1 DEint

(8.15)

.c

om

Equation 8.14 tells us that the increase in internal energy of the system is equal to the product of the friction force and the path length through which the block moves. In summary, a friction force transforms kinetic energy in a system to internal energy. If work is done on the system by forces other than friction, Equation 8.13, with the help of Equation 8.14, can be written as

ee b

ly

which is a reduced form of Equation 8.2 and represents the nonisolated system model for a system within which a nonconservative force acts.

S

n

A Block Pulled on a Rough Surface AM

ys

Example 8.4

ic

s. w

Q uick Quiz 8.5 You are traveling along a freeway at 65 mi/h. Your car has kinetic energy. You suddenly skid to a stop because of congestion in traffic. Where is the kinetic energy your car once had? (a) It is all in internal energy in the road. (b) It is all in internal energy in the tires. (c) Some of it has transformed to internal energy and some of it transferred away by mechanical waves. (d) It is all transferred away from your car by various mechanisms.

ar ph

A 6.0-kg block initially at rest is pulled to the right along a horizontal surface by a constant horizontal force of 12 N.

S

fk

F

a

sw

.a

w

w

w

Categorize T he block is pulled by a force and the

x

S

mg

Conceptualize This example is similar to Example 7.6 (page 190), but modified so that the surface is no longer frictionless. The rough surface applies a friction force on the block opposite to the applied force. As a result, we expect the speed to be lower than that found in Example 7.6.

vf

S

(A) Find the speed of the block after it has moved 3.0 m if the surfaces in contact have a coefficient of kinetic friction of 0.15. S o l u ti o n

S

S

F

S

n

S

fk

Figure 8.8 (Example 8.4) (a) A block pulled to the right on a rough surface by a constant horizontal force. (b) The applied force is at an angle u to the horizontal.

S

vf

u

S

x

mg b

surface is rough, so the block and the surface are modeled as a nonisolated system with a nonconservative force acting.

Analyze Figure 8.8a illustrates this situation. Neither the normal force nor the gravitational force does work on the system because their points of application are displaced horizontally. Find the work done on the system by the applied force just as in Example 7.6:

o W

other forces

Apply the particle in equilibrium model to the block in the vertical direction:

o F

5 0 S n 2 mg 5 0 S n 5 mg

Find the magnitude of the friction force:

f k 5 mkn 5 mkmg 5 (0.15)(6.0 kg)(9.80 m/s2) 5 8.82 N

y

5 W F 5 F Dx

8.3 Situations Involving Kinetic Friction 225

▸ 8.4 c o n t i n u e d FDx 5 DK 1 DE int 5 1 12mvf2 2 0 2 1 fkd

Substitute the energies into Equation 8.15 and solve for the final speed of the block:

vf 5 Substitute numerical values:

vf 5

2 1 2fkd 1 F Dx 2 Åm

2 3 2 1 8.82 N 2 1 3.0 m 2 1 1 12 N 2 1 3.0 m 2 4 5 1.8 m/s Å 6.0 kg

om

Finalize A s expected, this value is less than the 3.5 m/s found in the case of the block sliding on a frictionless surface (see Example 7.6). The difference in kinetic energies between the block in Example 7.6 and the block in this example is equal to the increase in internal energy of the block–surface system in this example. S

.c

(B) S uppose the force F is applied at an angle u as shown in Figure 8.8b. At what angle should the force be applied to achieve the largest possible speed after the block has moved 3.0 m to the right?

ly

S o l u ti o n

Conceptualize You might guess that u 5 0 would give the largest speed because the force would have the largest comS

s. w

ee b

ponent possible in the direction parallel to the surface. Think about F applied at an arbitrary nonzero angle, however. Although the horizontal component of the force would be reduced, the vertical component of the force would reduce the normal force, in turn reducing the force of friction, which suggests that the speed could be maximized by pulling at an angle other than u 5 0.

Categorize A s in part (A), we model the block and the surface as a nonisolated system with a nonconservative force acting. (1)

ic

Analyze Find the work done by the applied force, noting

ys

that Dx 5 d because the path followed by the block is a straight line:

ar ph

Apply the particle in equilibrium model to the block in the vertical direction: Solve for n:

sw

Use Equation 8.15 to find the final kinetic energy for this situation: Substitute the results in Equations (1) and (2):

o F

y

other forces

w

w

Evaluate u for mk 5 0.15:

5 W F 5 F Dx cos u 5 Fd cos u

5 n 1 F sin u 2 mg 5 0

(2) n 5 mg 2 F sin u W F 5 DK 1 D E int 5 (Kf 2 0) 1 f kd S Kf 5 W F 2 f kd Kf 5 Fd cos u 2 mknd 5 Fd cos u 2 mk(mg 2 F sin u)d dK f du

5 2Fd sin u 2 mk(0 2 F cos u)d 5 0 2 sin u 1 mk cos u 5 0

w

.a

Maximizing the speed is equivalent to maximizing the final kinetic energy. Consequently, differentiate Kf with respect to u and set the result equal to zero:

o W

tan u 5 mk u 5 tan21(mk) 5 tan21(0.15) 5 8.58

Finalize Notice that the angle at which the speed of the block is a maximum is indeed not u 5 0. When the angle exceeds 8.58, the horizontal component of the applied force is too small to be compensated by the reduced friction force and the speed of the block begins to decrease from its maximum value.

Conceptual Example 8.5

Useful Physics for Safer Driving

A car traveling at an initial speed v slides a distance d to a halt after its brakes lock. If the car’s initial speed is instead 2v at the moment the brakes lock, estimate the distance it slides.

continued

226 Chapter 8 Conservation of Energy ▸ 8.5 c o n t i n u e d S o l u ti o n

Let us assume the force of kinetic friction between the car and the road surface is constant and the same for both speeds. According to Equation 8.13, the friction force multiplied by the distance d is equal to the initial kinetic energy of the car (because Kf 5 0 and there is no work done by other forces). If the speed is doubled, as it is in this example, the kinetic energy is quadrupled. For a given friction force, the distance traveled is four times as great when the initial speed is doubled, and so the estimated distance the car slides is 4d.

A block of mass 1.6 kg is attached to a horizontal spring that has a force constant of 1 000 N/m as shown in Figure 8.9a. The spring is compressed 2.0 cm and is then released from rest as in Figure 8.9b.

ee b

(A) C alculate the speed of the block as it passes through the equilibrium position x 5 0 if the surface is frictionless.

.c

A Block–Spring System AM

ly

Example 8.6

om

x x0

a

before, and it is easy to visualize the block being pushed to the right by the spring and moving with some speed at x 5 0.

S

Fs

x x x0 b

ar ph

model the block as a nonisolated system.

(a) A block attached to a spring is pushed inward from an initial position x 5 0 by an external agent. (b) At position x, the block is released from rest and the spring pushes it to the right.

ys

Categorize We identify the system as the block and

Figure 8.9 (Example 8.6)

ic

Conceptualize This situation has been discussed

s. w

S o l u ti o n

Analyze In this situation, the block starts with vi 5 0 at xi 5 22.0 cm, and we want to find vf at xf 5 0.

Ws 5 12kx 2max

Work is done on the block, and its speed changes. The conservation of energy equation, Equation 8.2, reduces to the work–kinetic energy theorem. Use that theorem to find the speed at x 5 0:

Ws 5 12mv f 2 2 12mv i 2

w

.a

sw

Use Equation 7.11 to find the work done by the spring on the system with x max 5 xi :

w

w

Substitute numerical values:

vf 5 vf 5

Å Å

vi2 1

01

2 2 2 2 W 5 v 2 1 1 12kx max m s Å i m

2 3 1 1 1 000 N/m 2 1 0.020 m 2 2 4 5 0.50 m/s 1.6 kg 2

Finalize A lthough this problem could have been solved in Chapter 7, it is presented here to provide contrast with the following part (B), which requires the techniques of this chapter. (B) C alculate the speed of the block as it passes through the equilibrium position if a constant friction force of 4.0 N retards its motion from the moment it is released. S o l u ti o n

Conceptualize The correct answer must be less than that found in part (A) because the friction force retards the motion.

Categorize We identify the system as the block and the surface, a nonisolated system because of the work done by the spring. There is a nonconservative force acting within the system: the friction between the block and the surface.

8.4 Changes in Mechanical Energy for Nonconservative Forces

227

▸ 8.6 c o n t i n u e d Analyze Write Equation 8.15:

Ws 5 DK 1 DE int 5 1 12mv f 2 2 0 2 1 fk d

Substitute for the work done by the spring:

vf 5

Substitute numerical values:

vf 5

2 1 Ws 2 fkd 2 Åm

2 1 2 1 kx 2 fkd 2 Å m 2 max

2 3 1 1 1 000 N/m 2 1 0.020 m 2 2 2 1 4.0 N 2 1 0.020 m 2 4 5 0.39 m/s Å 1.6 kg 2

om

vf 5

Solve for vf :

Finalize A s expected, this value is less than the 0.50 m/s found in part (A).

which is equal in magnitude to the kinetic energy at x 5 0 for the frictionless case. (Verify it!). Therefore, all the

s. w

ly

f kd 5 (10.0 N)(0.020 m) 5 0.20 J

kinetic energy has been transformed to internal energy by friction when the block arrives at x 5 0, and its speed at this point is v 5 0. In this situation as well as that in part (B), the speed of the block reaches a maximum at some position other than x 5 0. Problem 53 asks you to locate these positions.

ee b

Answer In this case, the value of f kd as the block moves to x 5 0 is

.c

What if the friction force were increased to 10.0 N? What is the block’s speed at x 5 0?

W h at I f ?

ys

ic

8.4 Changes in Mechanical Energy for Nonconservative Forces

w

.a

sw

ar ph

Consider the book sliding across the surface in the preceding section. As the book moves through a distance d, the only force in the horizontal direction is the force of kinetic friction. This force causes a change 2f kd in the kinetic energy of the book as described by Equation 8.13. Now, however, suppose the book is part of a system that also exhibits a change in potential energy. In this case, 2f k d is the amount by which the mechanical energy of the system changes because of the force of kinetic friction. For example, if the book moves on an incline that is not frictionless, there is a change in both the kinetic energy and the gravitational potential energy of the book–Earth system. Consequently, DE mech 5 DK 1 DUg 5 2f kd 5 2DE int

w

w

In general, if a nonconservative force acts within an isolated system, DK 1 DU 1 DE int 5 0

(8.16)

where DU is the change in all forms of potential energy. We recognize Equation 8.16 as Equation 8.2 with no transfers of energy across the boundary of the system. If the system in which nonconservative forces act is nonisolated and the external influence on the system is by means of work, the generalization of Equation 8.13 is

o Wother forces 2 f kd 5 DEmech

This equation, with the help of Equations 8.7 and 8.14, can be written as

o Wother forces 5 W 5 DK 1 DU 1 DEint

(8.17)

This reduced form of Equation 8.2 represents the nonisolated system model for a system that possesses potential energy and within which a nonconservative force acts.

228 Chapter 8 Conservation of Energy Example 8.7

Crate Sliding Down a Ramp AM S

A 3.00-kg crate slides down a ramp. The ramp is 1.00 m in length and inclined at an angle of 30.08 as shown in Figure 8.10. The crate starts from rest at the top, experiences a constant friction force of magnitude 5.00 N, and continues to move a short distance on the horizontal floor after it leaves the ramp.

vi 0

d 1.00 m 0.500 m

(A) Use energy methods to determine the speed of the crate at the bottom of the ramp.

S

vf

The larger the friction force, the more slowly the crate will slide.

Categorize We identify the crate, the surface, and the Earth as an isolated system with a nonconservative force acting.

Figure 8.10 (Example 8.7) A crate slides down a ramp under the influence of gravity. The potential energy of the system decreases, whereas the kinetic energy increases.

.c

Conceptualize Imagine the crate sliding down the ramp in Figure 8.10.

ly

S o l u ti o n

om

30.0

ee b

Analyze Because vi 5 0, the initial kinetic energy of the system when the crate is at the top of the ramp is zero. If the y

coordinate is measured from the bottom of the ramp (the final position of the crate, for which we choose the gravitational potential energy of the system to be zero) with the upward direction being positive, then yi 5 0.500 m.

Write the conservation of energy equation (Eq. 8.2) for this system:

s. w

DK 1 DU 1 DE int 5 0

1 12mv f2 2 0 2 1 1 0 2 mg yi 2 1 fkd 5 0

Substitute for the energies:

ic

Solve for vf : Substitute numerical values:

2 1 mgyi 2 fkd 2 Åm

2 3 1 3.00 kg 2 1 9.80 m/s2 2 1 0.500 m 2 2 1 5.00 N 2 1 1.00 m 2 4 5 2.54 m/s Å 3.00 kg

ar ph

vf 5

ys

(1) v f 5

(B) H ow far does the crate slide on the horizontal floor if it continues to experience a friction force of magnitude 5.00 N?

sw

S o l u ti o n

Analyze This part of the problem is handled in exactly the same way as part (A), but in this case we can consider the

w

.a

mechanical energy of the system to consist only of kinetic energy because the potential energy of the system remains fixed. DK 1 DE int 5 0

Substitute for the energies:

1 0 2 12mv i2 2 1 fkd 5 0

w

w

Write the conservation of energy equation for this situation:

Solve for the distance d and substitute numerical values:

d5

1 3.00 kg 2 1 2.54 m/s 2 2 mv i2 5 5 1.94 m 2fk 2 1 5.00 N 2

Finalize For comparison, you may want to calculate the speed of the crate at the bottom of the ramp in the case in which the ramp is frictionless. Also notice that the increase in internal energy of the system as the crate slides down the ramp is f kd 5 (5.00 N)(1.00 m) 5 5.00 J. This energy is shared between the crate and the surface, each of which is a bit warmer than before. Also notice that the distance d the object slides on the horizontal surface is infinite if the surface is frictionless. Is that consistent with your conceptualization of the situation? W h at I f ? A cautious worker decides that the speed of the crate when it arrives at the bottom of the ramp may be so large that its contents may be damaged. Therefore, he replaces the ramp with a longer one such that the new

229

8.4 Changes in Mechanical Energy for Nonconservative Forces

▸ 8.7 c o n t i n u e d ramp makes an angle of 25.08 with the ground. Does this new ramp reduce the speed of the crate as it reaches the ground?

Answer Because the ramp is longer, the friction force acts over a longer distance and transforms more of the mechanical energy into internal energy. The result is a reduction in the kinetic energy of the crate, and we expect a lower speed as it reaches the ground. sin 25.08 5

Find the length d of the new ramp:

vf 5

d5

0.500 m 5 1.18 m sin 25.08

2 3 1 3.00 kg 2 1 9.80 m/s2 2 1 0.500 m 2 2 1 5.00 N 2 1 1.18 m 2 4 5 2.42 m/s Å 3.00 kg

.c

The final speed is indeed lower than in the higher-angle case.

ee b

Block–Spring Collision AM

ly

Example 8.8

S

om

Find vf from Equation (1) in part (A):

0.500 m d

s. w

A block having a mass of 0.80 kg is given an initial velocity v A 5 1.2 m/s to the right and collides with a spring whose mass is negligible and whose force constant is k 5 50 N/m as shown in Figure 8.11.

8.8) A block sliding on a frictionless, horizontal surface collides with a light spring. (a) Initially, the mechanical energy is all kinetic energy. (b) The mechanical energy is the sum of the kinetic energy of the block and the elastic potential energy in the spring. (c) The energy is entirely potential energy. (d) The energy is transformed back to the kinetic energy of the block. The total energy of the system remains constant throughout the motion.

ar ph

Conceptualize The various parts of Figure 8.11 help us imagine what the block will do in this situation. All motion takes place in a horizontal plane, so we do not need to consider changes in gravitational potential energy.

.a

sw

Categorize We identify the system to be the block and the spring and model it as an isolated system with no nonconservative forces acting.

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Analyze Before the collision, when

x0 S

Figure 8.11 (Example

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S o l u ti o n

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(A) A ssuming the surface to be frictionless, calculate the maximum compression of the spring after the collision.

vA

a

1

2 E 2 mvA

A S

vB

b

1

1

2 2 E 2 mv B 2 kx B

B xB S

vC 0

c

C

1

2 E 2 kx max

x max S

S

vD vA

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1 1 2 2 the block is at A, it has kinetic E D d 2 mv D 2 mvA energy and the spring is uncompressed, so the elastic potential energy stored in the system is zero. Therefore, the total mechanical energy of the system before the collision is just 12mv A2. After the collision, when the block is at C, the spring is fully compressed; now the block is at rest and so has zero kinetic energy. The elastic potential energy stored in the system, however, has its maximum value 12kx 2 5 12kx 2max, where the origin of coordinates x 5 0 is chosen to be the equilibrium position of the spring and xmax is the maximum compression of the spring, which in this case happens to be x C. The total mechanical energy of the system is conserved because no nonconservative forces act on objects within the isolated system.

Write the conservation of energy equation for this situation:

DK 1 DU 5 0

Substitute for the energies:

2 1 0 2 12mv A2 2 1 1 12kx max 2 02 5 0

Solve for x max and evaluate:

x max 5

0.80 kg m 1 1.2 m/s 2 5 0.15 m vA 5 Åk Å 50 N/m

continued

230 Chapter 8 Conservation of Energy ▸ 8.8 c o n t i n u e d (B) Suppose a constant force of kinetic friction acts between the block and the surface, with mk 5 0.50. If the speed of the block at the moment it collides with the spring is v A 5 1.2 m/s, what is the maximum compression x C in the spring? S o l u ti o n

Conceptualize Because of the friction force, we expect the compression of the spring to be smaller than in part (A) because some of the block’s kinetic energy is transformed to internal energy in the block and the surface. Categorize We identify the system as the block, the surface, and the spring. This is an isolated system but now involves a nonconservative force.

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Analyze In this case, the mechanical energy E mech 5 K 1 Us of the system is not conserved because a friction force acts on the block. From the particle in equilibrium model in the vertical direction, we see that n 5 mg. f k 5 mkn 5 mkmg

Write the conservation of energy equation for this situation:

DK 1 DU 1 DE int 5 0

Substitute the initial and final energies:

1 0 2 12mv A2 2 1 1 12kx C2 2 0 2 1 mkmg x C 5 0

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Evaluate the magnitude of the friction force:

Rearrange the terms into a qaudratic equation:

kx C2 1 2mkmg x C 2 mv A2 5 0

Substitute numerical values:

50x C2 1 2 1 0.50 2 1 0.80 2 1 9.80 2 x C 2 1 0.80 2 1 1.2 2 2 5 0

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50x C2 1 7.84x C 2 1.15 5 0

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Solving the quadratic equation for x C gives x C 5 0.092 m and x C 5 20.25 m. The physically meaningful root is x C 5 0.092 m.

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Finalize The negative root does not apply to this situation because the block must be to the right of the origin (positive value of x) when it comes to rest. Notice that the value of 0.092 m is less than the distance obtained in the frictionless case of part (A) as we expected.

Connected Blocks in Motion AM

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Example 8.9

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Two blocks are connected by a light string that passes over a frictionless pulley as shown in Figure 8.12. The block of mass m1 lies on a horizontal surface and is connected to a spring of force constant k. The system is released from rest when the spring is unstretched. If the hanging block of mass m 2 falls a distance h before coming to rest, calculate the coefficient of kinetic friction between the block of mass m1 and the surface.

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S o l u ti o n

Conceptualize The key word rest appears twice in the problem statement. This word suggests that the configurations of the system associated with rest are good candidates for the initial and final configurations because the kinetic energy of the system is zero for these configurations.

Categorize In this situation, the system consists of the two blocks, the spring, the surface, and the Earth. This is an isolated system with a nonconservative force acting. We also model the sliding block as a particle in equilibrium in the vertical direction, leading to n 5 m1g.

k m1

m2

h

Figure 8.12 (Example 8.9) As the hanging block moves from its highest elevation to its lowest, the system loses gravitational potential energy but gains elastic potential energy in the spring. Some mechanical energy is transformed to internal energy because of friction between the sliding block and the surface.

Analyze We need to consider two forms of potential energy for the system, gravitational and elastic: DUg 5 Ugf 2 Ugi is

the change in the system’s gravitational potential energy, and DUs 5 Usf 2 Usi is the change in the system’s elastic potential energy. The change in the gravitational potential energy of the system is associated with only the falling block

8.4 Changes in Mechanical Energy for Nonconservative Forces

231

▸ 8.9 c o n t i n u e d because the vertical coordinate of the horizontally sliding block does not change. The initial and final kinetic energies of the system are zero, so DK 5 0. Write the appropriate reduction of Equation 8.2:

(1) DUg 1 DUs 1 DE int 5 0

Substitute for the energies, noting that as the hanging block falls a distance h, the horizontally moving block moves the same distance h to the right, and the spring stretches by a distance h:

1 0 2 m 2gh 2 1 1 12kh2 2 0 2 1 fkh 5 0

Substitute for the friction force:

2m 2gh 1 12kh2 1 mkm 1gh 5 0

Solve for mk:

mk 5

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m 2g 2 12kh m 1g

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Finalize This setup represents a method of measuring the coefficient of kinetic friction between an object and some surface. Notice how we have solved the examples in this chapter using the energy approach. We begin with Equation 8.2 and then tailor it to the physical situation. This process may include deleting terms, such as the kinetic energy term and all terms on the right-hand side of Equation 8.2 in this example. It can also include expanding terms, such as rewriting DU due to two types of potential energy in this example.

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Conceptual Example 8.10 Interpreting the Energy Bars

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The energy bar charts in Figure 8.13 show three instants in the motion of the system in Figure 8.12 and described in Example 8.9. For each bar chart, identify the configuration of the system that corresponds to the chart. S o l u ti o n

% 100 50 0

Kinetic Elastic Grav. Internal Total energy pot. pot. energy energy energy energy

a

w

w

w

.a

sw

In Figure 8.13a, there is no kinetic energy in the system. % Therefore, nothing in the system is moving. The bar chart 100 Isolated 50 shows that the system contains only gravitational potential system: 0 total energy and no internal energy yet, which corresponds to the Kinetic Elastic Grav. Internal Total energy configuration with the darker blocks in Figure 8.12 and repenergy pot. pot. energy energy constant energy energy resents the instant just after the system is released. b In Figure 8.13b, the system contains four types of energy. The height of the gravitational potential energy bar is at % 100 50%, which tells us that the hanging block has moved half50 way between its position corresponding to Figure 8.13a and 0 the position defined as y 5 0. Therefore, in this configuraKinetic Elastic Grav. Internal Total energy pot. pot. energy energy tion, the hanging block is between the dark and light images energy energy of the hanging block in Figure 8.12. The system has gained c kinetic energy because the blocks are moving, elastic potenFigure 8.13 (Conceptual Example 8.10) Three energy bar tial energy because the spring is stretching, and internal charts are shown for the system in Figure 8.12. energy because of friction between the block of mass m1 and the surface. In Figure 8.13c, the height of the gravitational potential energy bar is zero, telling us that the hanging block is at y 5 0. In addition, the height of the kinetic energy bar is zero, indicating that the blocks have stopped moving momentarily. Therefore, the configuration of the system is that shown by the light images of the blocks in Figure 8.12. The height of the elastic potential energy bar is high because the spring is stretched its maximum amount. The height of the internal energy bar is higher than in Figure 8.13b because the block of mass m1 has continued to slide over the surface after the configuration shown in Figure 8.13b.

232 Chapter 8 Conservation of Energy

8.5 Power

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Consider Conceptual Example 7.7 again, which involved rolling a refrigerator up a ramp into a truck. Suppose the man is not convinced the work is the same regardless of the ramp’s length and sets up a long ramp with a gentle rise. Although he does the same amount of work as someone using a shorter ramp, he takes longer to do the work because he has to move the refrigerator over a greater distance. Although the work done on both ramps is the same, there is something different about the tasks: the time interval during which the work is done. The time rate of energy transfer is called the instantaneous power P and is defined as dE P; (8.18) dt

Definition of power

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We will focus on work as the energy transfer method in this discussion, but keep in mind that the notion of power is valid for any means of energy transfer discussed in Section 8.1. If an external force is applied to an object (which we model as a particle) and if the work done by this force on the object in the time interval Dt is W, the average power during this interval is

Pavg 5

W Dt

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Therefore, in Conceptual Example 7.7, although the same work is done in rolling the refrigerator up both ramps, less power is required for the longer ramp. In a manner similar to the way we approached the definition of velocity and acceleration, the instantaneous power is the limiting value of the average power as Dt approaches zero: W dW 5 P 5 lim dt Dt S 0 Dt where we have represented theSinfinitesimal value of the work done by dW. We find from Equation 7.3 that dW 5 F ? d S r . Therefore, the instantaneous power can be written S S dr S dW 5 F? 5 F ?S v (8.19) P5 dt dt S S where v 5 d r /dt. The SI unit of power is joules per second (J/s), also called the watt (W) after James Watt: 1 W 5 1 J/s 5 1 kg ? m2/s3

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w

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The watt

Pitfall Prevention 8.3

W, W, and watts Do not confuse the symbol W for the watt with the italic symbol W for work. Also, remember that the watt already represents a rate of energy transfer, so “watts per second” does not make sense. The watt is the same as a joule per second.

A unit of power in the U.S. customary system is the horsepower (hp):

1 hp 5 746 W

A unit of energy (or work) can now be defined in terms of the unit of power. One kilowatt-hour (kWh) is the energy transferred in 1 h at the constant rate of 1 kW 5 1 000 J/s. The amount of energy represented by 1 kWh is

1 kWh 5 (103 W)(3 600 s) 5 3.60 3 106 J

A kilowatt-hour is a unit of energy, not power. When you pay your electric bill, you are buying energy, and the amount of energy transferred by electrical transmission into a home during the period represented by the electric bill is usually expressed in kilowatt-hours. For example, your bill may state that you used 900 kWh of energy during a month and that you are being charged at the rate of 10¢ per kilowatt-hour. Your obligation is then $90 for this amount of energy. As another example, suppose an electric bulb is rated at 100 W. In 1.00 h of operation, it would have energy transferred to it by electrical transmission in the amount of (0.100 kW)(1.00 h) 5 0.100 kWh 5 3.60 3 105 J.

8.5 Power 233

Example 8.11 Power Delivered by an Elevator Motor AM An elevator car (Fig. 8.14a) has a mass of 1 600 kg and is carrying passengers having a combined mass of 200 kg. A constant friction force of 4 000 N retards its motion.

Motor

(A) H ow much power must a motor deliver to lift the elevator car and its passengers at a constant speed of 3.00 m/s?

Analyze The free-body diagram in Figure 8.14b specifies the upward direction as positive. The total mass M of the system (car plus passengers) is equal to 1 800 kg. Using the particle in equilibrium model, apply Newton’s second law to the car:

o F

Solve for T :

T 5 Mg 1 f

a

S

f

S

Mg

b

s. w

5 T 2 f 2 Mg 5 0

S

S

v 5 Tv 5 1 Mg 1 f 2 v P 5 T ?S

ic

Use Equation 8.19 and that T is in the same direction as S v to find the power:

P 5 [(1 800 kg)(9.80 m/s2) 1 (4 000 N)](3.00 m/s) 5 6.49 3 104 W

ys

Substitute numerical values:

y

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Categorize The friction force increases the power necessary to lift the elevator. The problem states that the speed of the elevator is constant, which tells us that a 5 0. We model the elevator as a particle in equilibrium.

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nitude T that pulls the elevator car upward.

8.11) (a) The motor exerts S an upward force T on the elevator car. The magnitude of this force is the total tension T in the cables connecting the car and motor. The downward forces acting on S the car are a friction force f and the gravitational force S Fg 5 M S g . (b) The free-body diagram for the elevator car.

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Conceptualize The motor must supply the force of mag-

Figure 8.14 (Example

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S o l u ti o n

S

T

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(B) W hat power must the motor deliver at the instant the speed of the elevator is v if the motor is designed to provide the elevator car with an upward acceleration of 1.00 m/s2? S o l u ti o n

sw

Conceptualize In this case, the motor must supply the force of magnitude T that pulls the elevator car upward with an increasing speed. We expect that more power will be required to do that than in part (A) because the motor must now perform the additional task of accelerating the car.

.a

Categorize In this case, we model the elevator car as a particle under a net force because it is accelerating.

w

Analyze Using the particle under a net force model,

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apply Newton’s second law to the car:

o F

y

5 T 2 f 2 Mg 5 Ma

T 5 M(a 1 g) 1 f

Use Equation 8.19 to obtain the required power:

P 5 Tv 5 [M(a 1 g) 1 f ]v

Substitute numerical values:

P 5 [(1 800 kg)(1.00 m/s2 1 9.80 m/s2) 1 4 000 N]v

w

Solve for T :

5 (2.34 3 104)v where v is the instantaneous speed of the car in meters per second and P is in watts.

Finalize To compare with part (A), let v 5 3.00 m/s, giving a power of

P 5 (2.34 3 104 N)(3.00 m/s) 5 7.02 3 104 W

which is larger than the power found in part (A), as expected.

234 Chapter 8 Conservation of Energy

Summary Definitions The instantaneous power P is defined as the time rate of energy transfer: P;

dE dt

(8.18)

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A nonisolated system is one for which energy crosses the boundary of the system. An isolated system is one for which no energy crosses the boundary of the system.

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Concepts and Principles

If a friction force of magnitude f k acts over a distance d within a system, the change in internal energy of the system is

ee b

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For a nonisolated system, we can equate the change in the total energy stored in the system to the sum of all the transfers of energy across the system boundary, which is a statement of conservation of energy. For an isolated system, the total energy is constant.

(8.14)

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DE int 5 f kd

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Analysis Models for Problem Solving

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Kinetic energy Potential energy Internal energy

Electrical Electromagnetic transmission radiation

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Nonisolated System (Energy). The most general statement describing the behavior of a nonisolated system is the conservation of energy equation: DE system 5 o T

(8.1)

Including the types of energy storage and energy transfer that we have discussed gives DK 1 DU 1 DE int 5 W 1 Q 1 TMW 1 TMT 1 TET 1 TER (8.2) For a specific problem, this equation is generally reduced to a smaller number of terms by eliminating the terms that are not appropriate to the situation.

Kinetic energy Potential energy Internal energy

The total amount of energy in the system is constant. Energy transforms among the three possible types.

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Matter transfer

.a

The change in the total amount of energy in the system is equal to the total amount of energy that crosses the boundary of the system.

Heat

ys

Work

System boundary

System boundary

Mechanical waves

Isolated System (Energy). The total energy of an isolated system is conserved, so

DE system 5 0

(8.10)

which can be written as

DK 1 DU 1 DE int 5 0

(8.16)

If no nonconservative forces act within the isolated system, the mechanical energy of the system is conserved, so

DE mech 5 0

(8.8)

which can be written as

DK 1 DU 5 0

(8.6)

Conceptual Questions

Objective Questions

235

1. denotes answer available in Student Solutions Manual/Study Guide

5. Answer yes or no to each of the following questions. (a) Can an object–Earth system have kinetic energy and not gravitational potential energy? (b) Can it have gravitational potential energy and not kinetic energy? (c) Can it have both types of energy at the same moment? (d) Can it have neither?

2. Two children stand on a platform at the top of a curving slide next to a backyard swimming pool. At the same moment the smaller child hops off to jump straight down into the pool, the bigger child releases herself at the top of the frictionless slide. (i) Upon reaching the water, the kinetic energy of the smaller child compared with that of the larger child is (a) greater (b) less (c) equal. (ii) Upon reaching the water, the speed of the smaller child compared with that of the larger child is (a) greater (b) less (c) equal. (iii) During their motions from the platform to the water, the average acceleration of the smaller child compared with that of the larger child is (a) greater (b) less (c) equal.

6. In a laboratory model of cars skidding to a stop, data are measured for four trials using two blocks. The blocks have identical masses but different coefficients of kinetic friction with a table: mk 5 0.2 and 0.8. Each block is launched with speed vi 5 1 m/s and slides across the level table as the block comes to rest. This process represents the first two trials. For the next two trials, the procedure is repeated but the blocks are launched with speed vi 5 2 m/s. Rank the four trials (a) through (d) according to the stopping distance from largest to smallest. If the stopping distance is the same in two cases, give them equal rank. (a) vi 5 1 m/s, mk 5 0.2 (b) vi 5 1 m/s, mk 5 0.8 (c) vi 5 2 m/s, mk 5 0.2 (d) vi 5 2 m/s, mk 5 0.8

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7. What average power is generated by a 70.0-kg mountain climber who climbs a summit of height 325 m in 95.0 min? (a) 39.1 W (b) 54.6 W (c) 25.5 W (d) 67.0 W (e) 88.4 W

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3. At the bottom of an air track tilted at angle u, a glider of mass m is given a push to make it coast a distance d up the slope as it slows down and stops. Then the glider comes back down the track to its starting point. Now the experiment is repeated with the same original speed but with a second identical glider set on top of the first. The airflow from the track is strong enough to support the stacked pair of gliders so that the combination moves over the track with negligible friction. Static friction holds the second glider stationary relative to the first glider throughout the motion. The coefficient of static friction between the two gliders is ms. What is the change in mechanical energy of the two-glider–Earth system in the up- and down-slope motion after the pair of gliders is released? Choose one. (a) 22msmg (b) 22mgd cos u (c) 22msmgd cos u (d) 0 (e) 12msmgd cos u

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1. You hold a slingshot at arm’s length, pull the light elastic band back to your chin, and release it to launch a pebble horizontally with speed 200 cm/s. With the same procedure, you fire a bean with speed 600 cm/s. What is the ratio of the mass of the bean to the mass of the pebble? (a) 19 (b) 13 (c) 1 (d) 3 (e) 9

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8. A ball of clay falls freely to the hard floor. It does not bounce noticeably, and it very quickly comes to rest. What, then, has happened to the energy the ball had while it was falling? (a) It has been used up in producing the downward motion. (b) It has been transformed back into potential energy. (c) It has been transferred into the ball by heat. (d) It is in the ball and floor (and walls) as energy of invisible molecular motion. (e) Most of it went into sound.

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4. An athlete jumping vertically on a trampoline leaves the surface with a velocity of 8.5 m/s upward. What maximum height does she reach? (a) 13 m (b) 2.3 m (c) 3.7 m (d) 0.27 m (e) The answer can’t be determined because the mass of the athlete isn’t given.

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Conceptual Questions

9. A pile driver drives posts into the ground by repeatedly dropping a heavy object on them. Assume the object is dropped from the same height each time. By what factor does the energy of the pile driver–Earth system change when the mass of the object being dropped is doubled? (a) 12 (b) 1; the energy is the same (c) 2 (d) 4

1. denotes answer available in Student Solutions Manual/Study Guide

1. One person drops a ball from the top of a building while another person at the bottom observes its motion. Will these two people agree (a) on the value of the gravitational potential energy of the ball– Earth system? (b) On the change in potential energy? (c) On the kinetic energy of the ball at some point in its motion? 2. A car salesperson claims that a 300-hp engine is a necessary option in a compact car, in place of the conventional 130-hp engine. Suppose you intend to drive the

car within speed limits (# 65 mi/h) on flat terrain. How would you counter this sales pitch? 3. Does everything have energy? Give the reasoning for your answer. 4. You ride a bicycle. In what sense is your bicycle solar-powered? 5. A bowling ball is suspended from the ceiling of a lecture hall by a strong cord. The ball is drawn away from its equilibrium position and released from rest at the

236 Chapter 8 Conservation of Energy

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9. A block is connected to a spring that is suspended from the ceiling. Assuming air resistance is ignored, describe the energy transformations that occur within the system consisting of the block, the Earth, and the spring when the block is set into vertical motion.

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10. In Chapter 7, the work–kinetic energy theorem, W 5 DK, was introduced. This equation states that work done on a system appears as a change in kinetic energy. It is a special-case equation, valid if there are no changes in any other type of energy such as potential or internal. Give two or three examples in which work is done on a system but the change in energy of the system is not a change in kinetic energy.

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8. Consider the energy transfers and transformations listed below in parts (a) through (e). For each part, (i) describe human-made devices designed to produce each of the energy transfers or transformations

and, (ii) whenever possible, describe a natural process in which the energy transfer or transformation occurs. Give details to defend your choices, such as identifying the system and identifying other output energy if the device or natural process has limited efficiency. (a) Chemical potential energy transforms into internal energy. (b) Energy transferred by electrical transmission becomes gravitational potential energy. (c) Elastic potential energy transfers out of a system by heat. (d) Energy transferred by mechanical waves does work on a system. (e) Energy carried by electromagnetic waves becomes kinetic energy in a system.

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tip of the demonstrator’s nose as shown in Figure CQ8.5. The demonstrator remains stationary. (a) Explain why the ball does not strike her on its return swing. (b) Would this demonstrator be safe if the ball were given a push from its starting position at her nose? 6. Can a force of static friction do work? If not, why not? If so, give an example. Figure CQ8.5 7. In the general conservation of energy equation, state which terms predominate in describing each of the following devices and processes. For a process going on continuously, you may consider what happens in a 10-s time interval. State which terms in the equation represent original and final forms of energy, which would be inputs, and which outputs. (a) a slingshot firing a pebble (b) a fire burning (c) a portable radio operating (d) a car braking to a stop (e) the surface of the Sun shining visibly (f) a person jumping up onto a chair

1. straightforward; 2. intermediate; 3. challenging

GP Guided Problem M Master It tutorial available in Enhanced WebAssign W Watch It video solution available in Enhanced WebAssign

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Solutions Manual/Study Guide

Enhanced WebAssign

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1. full solution available in the Student

AMT Analysis Model tutorial available in

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Problems

BIO Q/C S

Section 8.2 Analysis Model: Isolated System (Energy)

1. For each of the following systems and time intervals, S write the appropriate version of Equation 8.2, the conservation of energy equation. (a) the heating coils in your toaster during the first five seconds after you turn the toaster on (b) your automobile from just before you fill it with gasoline until you pull away from the gas station at speed v (c) your body while you sit quietly and eat a peanut butter and jelly sandwich for lunch (d) your home during five minutes of a sunny afternoon while the temperature in the home remains fixed

3. A block of mass 0.250 kg is placed on top of a light, verW tical spring of force constant 5 000 N/m and pushed downward so that the spring is compressed by 0.100 m. After the block is released from rest, it travels upward and then leaves the spring. To what maximum height above the point of release does it rise? 4. A 20.0-kg cannonball is fired from a cannon with muzW zle speed of 1 000 m/s at an angle of 37.08 with the horizontal. A second ball is fired at an angle of 90.08. Use the isolated system model to find (a) the maximum height reached by each ball and (b) the total mechanical energy of the ball–Earth system at the maximum height for each ball. Let y 5 0 at the cannon. A 5. Review. A bead slides without fric- h R AMT tion around a loop-the-loop (Fig. M P8.5). The bead is released from Figure P8.5 rest at a height h 5 3.50R. (a) What

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Section 8.1 Analysis Model: Nonisolated System (Energy)

2. A ball of mass m falls from a height h to the floor. S (a) Write the appropriate version of Equation 8.2 for the system of the ball and the Earth and use it to calculate the speed of the ball just before it strikes the Earth. (b) Write the appropriate version of Equation 8.2 for the system of the ball and use it to calculate the speed of the ball just before it strikes the Earth.

237

Problems

duced high-frequency “microtremor” vibrations that were rapidly damped and did not travel far. Assume 0.01% of the total energy was carried away by longrange seismic waves. The magnitude of an earthquake on the Richter scale is given by

is its speed at point A? (b) How large is the normal force on the bead at point A if its mass is 5.00 g? 6. A block of mass m 5 5.00 kg is released from point A W and slides on the frictionless track shown in Figure P8.6. Determine (a) the block’s speed at points B and C and (b) the net work done by the gravitational force on the block as it moves from point A to point C. A

B

1.5

11. Review. The system shown in Figure S P8.11 consists of a light, inextensible cord, light, frictionless pulleys, and blocks of equal mass. Notice that block B is attached to one of the pulleys. The system is initially held at rest so that the blocks are at the same height above the ground. The blocks are then released. Find the speed of block A at the moment the vertical separation of the blocks is h.

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C 2.00 m

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3.20 m

log E 2 4.8

where E is the seismic wave energy in joules. According to this model, what was the magnitude of the demonstration quake?

m

5.00 m

M5

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A

B

Figure P8.11

Section 8.3 Situations Involving Kinetic Friction

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m2

12. A sled of mass m is given a kick on a frozen pond. The kick imparts to the sled an initial speed of 2.00 m/s. The coefficient of kinetic friction between sled and ice is 0.100. Use energy considerations to find the distance the sled moves before it stops.

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m1

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7. Two objects are connected M by a light string passing over a light, frictionless pulley as shown in Figure P8.7. The object of mass m1 5 5.00 kg is released from rest at a height h 5 4.00 m above the table. Using the isolated system model, (a) determine the speed of the object of mass m 2 5 3.00 kg just as the 5.00-kg object hits the table and (b) find the maximum height above the table to which the 3.00-kg object rises.

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Figure P8.6

Figure P8.7

Problems 7 and 8.

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.a

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8. Two objects are connected by a light string passing S over a light, frictionless pulley as shown in Figure P8.7. The object of mass m1 is released from rest at height h above the table. Using the isolated system model, (a) determine the speed of m 2 just as m1 hits the table and (b) find the maximum height above the table to which m 2 rises.

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9. A light, rigid rod is 77.0 cm long. Its top end is pivoted on a frictionless, horizontal axle. The rod hangs straight down at rest with a small, massive ball attached to its bottom end. You strike the ball, suddenly giving it a horizontal velocity so that it swings around in a full circle. What minimum speed at the bottom is required to make the ball go over the top of the circle? 10. At 11:00 a.m. on September 7, 2001, more than one million British schoolchildren jumped up and down for one minute to simulate an earthquake. (a) Find the energy stored in the children’s bodies that was converted into internal energy in the ground and their bodies and propagated into the ground by seismic waves during the experiment. Assume 1 050 000 children of average mass 36.0 kg jumped 12 times each, raising their centers of mass by 25.0 cm each time and briefly resting between one jump and the next. (b) Of the energy that propagated into the ground, most pro-

13. A sled of mass m is given a kick on a frozen pond. The S kick imparts to the sled an initial speed of v. The coefficient of kinetic friction between sled and ice is mk . Use energy considerations to find the distance the sled moves before it stops.

14. A crate of mass 10.0 kg is pulled up a rough incline with M an initial speed of 1.50 m/s. The pulling force is 100 N parallel to the incline, which makes an angle of 20.08 with the horizontal. The coefficient of kinetic friction is 0.400, and the crate is pulled 5.00 m. (a) How much work is done by the gravitational force on the crate? (b) Determine the increase in internal energy of the crate–incline system owing to friction. (c) How much work is done by the 100-N force on the crate? (d) What is the change in kinetic energy of the crate? (e) What is the speed of the crate after being pulled 5.00 m? 15. A block of mass m 5 2.00 kg k W is attached to a spring of m force constant k 5 500 N/m as shown in Figure P8.15. The block is pulled to a posix 0 x xi tion xi 5 5.00 cm to the right of equilibrium and released Figure P8.15 from rest. Find the speed the block has as it passes through equilibrium if (a) the horizontal surface is frictionless and (b) the coefficient of friction between block and surface is mk 5 0.350. 16. A 40.0-kg box initially at rest is pushed 5.00 m along a rough, horizontal floor with a constant applied horizontal force of 130 N. The coefficient of friction

238 Chapter 8 Conservation of Energy

22. The coefficient of friction

m1

AMT between the block of mass W m1 5 3.00 kg and the surface

in Figure P8.22 is mk 5 0.400. The system starts from rest. What is the speed of the ball of mass m 2 5 5.00 kg when it has fallen a distance h 5 1.50 m?

m2

Figure P8.22

om

23. A 5.00-kg block is set into vi d M motion up an inclined plane with an initial speed of vi 5 8.00 m/s (Fig. P8.23). The block comes to rest after travu eling d 5 3.00 m along the plane, which is inclined at an angle of u 5 30.08 to the Figure P8.23 horizontal. For this motion, determine (a) the change in the block’s kinetic energy, (b) the change in the potential energy of the block– Earth system, and (c) the friction force exerted on the block (assumed to be constant). (d) What is the coefficient of kinetic friction? 24. A 1.50-kg object is held 1.20 m above a relaxed massless, vertical spring with a force constant of 320 N/m. The object is dropped onto the spring. (a) How far does the object compress the spring? (b) What If? Repeat part (a), but this time assume a constant air-resistance force of 0.700 N acts on the object during its motion. (c) What If? How far does the object compress the spring if the same experiment is performed on the Moon, where g 5 1.63 m/s2 and air resistance is neglected?

ic

is 10.0 J. At some later time tf , the kinetic energy of the particle is 18.0 J. (a) If only conservative forces act on the particle, what are the potential energy and the total energy of the system at time tf ? (b) If the potential energy of the system at time tf is 5.00 J, are any nonconservative forces acting on the particle? (c) Explain your answer to part (b).

s. w

18. At time ti , the kinetic energy of a particle is 30.0 J and

Q/C the potential energy of the system to which it belongs

ee b

ly

Section 8.4 Changes in Mechanical Energy for Nonconservative Forces

of the cannon? (b) At what point does the ball have maximum speed? (c) What is this maximum speed?

.c

between box and floor is 0.300. Find (a) the work done by the applied force, (b) the increase in internal energy in the box–floor system as a result of friction, (c) the work done by the normal force, (d) the work done by the gravitational force, (e) the change in kinetic energy of the box, and (f) the final speed of the box. 17. A smooth circular hoop with a radius of 0.500 m is placed flat on the floor. A 0.400-kg particle slides around the inside edge of the hoop. The particle is given an initial speed of 8.00 m/s. After one revolution, its speed has dropped to 6.00 m/s because of friction with the floor. (a) Find the energy transformed from mechanical to internal in the particle–hoop– floor system as a result of friction in one revolution. (b) What is the total number of revolutions the particle makes before stopping? Assume the friction force remains constant during the entire motion.

shown

in

Figure

A

sw

20. As

Q/C P8.20, a green bead of

ar ph

ys

19. A boy in a wheelchair (total mass 47.0 kg) has speed 1.40 m/s at the crest of a slope 2.60 m high and 12.4 m long. At the bottom of the slope his speed is 6.20 m/s. Assume air resistance and rolling resistance can be modeled as a constant friction force of 41.0 N. Find the work he did in pushing forward on his wheels during the downhill ride.

w

w

w

.a

mass 25 g slides along a straight wire. The length of the wire from point A to point B is 0.600 m, B and point A is 0.200 m Figure P8.20 higher than point B. A constant friction force of magnitude 0.025 0 N acts on the bead. (a) If the bead is released from rest at point A, what is its speed at point B? (b) A red bead of mass 25 g slides along a curved wire, subject to a friction force with the same constant magnitude as that on the green bead. If the green and red beads are released simultaneously from rest at point A, which bead reaches point B with a higher speed? Explain.

21. A toy cannon uses a spring to project a 5.30-g soft rubW ber ball. The spring is originally compressed by 5.00 cm and has a force constant of 8.00 N/m. When the cannon is fired, the ball moves 15.0 cm through the horizontal barrel of the cannon, and the barrel exerts a constant friction force of 0.032 0 N on the ball. (a) With what speed does the projectile leave the barrel

25. A 200-g block is pressed against a spring of force M constant 1.40 kN/m until the block compresses the spring 10.0 cm. The spring rests at the bottom of a ramp inclined at 60.08 to the horizontal. Using energy considerations, determine how far up the incline the block moves from its initial position before it stops (a) if the ramp exerts no friction force on the block and (b) if the coefficient of kinetic friction is 0.400. 26. An 80.0-kg skydiver jumps out of a balloon at an alti-

Q/C tude of 1 000 m and opens his parachute at an altitude

of 200 m. (a) Assuming the total retarding force on the skydiver is constant at 50.0 N with the parachute closed and constant at 3 600 N with the parachute open, find the speed of the skydiver when he lands on the ground. (b) Do you think the skydiver will be injured? Explain. (c) At what height should the parachute be opened so that the final speed of the skydiver when he hits the ground is 5.00 m/s? (d) How realistic is the assumption that the total retarding force is constant? Explain.

27. A child of mass m starts from rest and slides without GP friction from a height h along a slide next to a pool Q/C (Fig. P8.27). She is launched from a height h/5 into S the air over the pool. We wish to find the maximum height she reaches above the water in her projectile motion. (a) Is the child–Earth system isolated or

Problems

lifetime of the energy-efficient bulb is 10 000 h and its purchase price is $4.50, whereas the conventional bulb has a lifetime of 750 h and costs $0.42. Determine the total savings obtained by using one energy-efficient bulb over its lifetime as opposed to using conventional bulbs over the same time interval. Assume an energy cost of $0.200 per kilowatt-hour.

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3 4. An electric scooter has a battery capable of supplying 120 Wh of energy. If friction forces and other losses account for 60.0% of the energy usage, what altitude change can a rider achieve when driving in hilly terrain if the rider and scooter have a combined weight of 890 N? 35. Make an order-of-magnitude estimate of the power a car engine contributes to speeding the car up to highway speed. In your solution, state the physical quantities you take as data and the values you measure or estimate for them. The mass of a vehicle is often given in the owner’s manual.

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.c

nonisolated? Why? (b) Is there a nonconservative force acting within the system? (c) Define the configuration of the system when the child is at the water level as having zero gravitational potential energy. Express the total energy of the system when the child is at the top of the waterslide. (d) Express the total energy of the system when the child is at the launching point. (e) Express the total energy of the system when the child is at the highest point in her projectile motion. (f) From parts (c) and (d), determine her initial speed vi at the launch point in terms of g and h. (g) From parts (d), (e), and (f), determine her maximum airborne height y max in terms of h and the launch angle u. (h) Would your answers be the same if the waterslide were not frictionless? Explain.

239

h h/5

36. An older-model car accelerates from 0 to speed v in S a time interval of Dt. A newer, more powerful sports car accelerates from 0 to 2v in the same time period. Assuming the energy coming from the engine appears only as kinetic energy of the cars, compare the power of the two cars.

ee b

u

y max

s. w

Figure P8.27 Section 8.5 Power

37. For saving energy, bicycling and walking are far more

ar ph

ys

ic

28. Sewage at a certain pumping station is raised vertically by 5.49 m at the rate of 1 890 000 liters each day. The sewage, of density 1 050 kg/m3, enters and leaves the pump at atmospheric pressure and through pipes of equal diameter. (a) Find the output mechanical power of the lift station. (b) Assume an electric motor continuously operating with average power 5.90 kW runs the pump. Find its efficiency.

BIO efficient means of transportation than is travel by

sw

29. An 820-N Marine in basic training climbs a 12.0-m W vertical rope at a constant speed in 8.00 s. What is his power output? 30. The electric motor of a model train accelerates the

.a

Q/C train from rest to 0.620 m/s in 21.0 ms. The total mass

w

w

of the train is 875 g. (a) Find the minimum power delivered to the train by electrical transmission from the metal rails during the acceleration. (b) Why is it the minimum power?

w

31. When an automobile moves with constant speed down a highway, most of the power developed by the engine is used to compensate for the energy transformations due to friction forces exerted on the car by the air and the road. If the power developed by an engine is 175 hp, estimate the total friction force acting on the car when it is moving at a speed of 29 m/s. One horsepower equals 746 W. 32. A certain rain cloud at an altitude of 1.75 km contains 3.20 3 107 kg of water vapor. How long would it take a 2.70-kW pump to raise the same amount of water from the Earth’s surface to the cloud’s position? 33. An energy-efficient lightbulb, taking in 28.0 W of power, can produce the same level of brightness as a conventional lightbulb operating at power 100 W. The

automobile. For example, when riding at 10.0 mi/h, a cyclist uses food energy at a rate of about 400 kcal/h above what he would use if merely sitting still. (In exercise physiology, power is often measured in kcal/h rather than in watts. Here 1 kcal 5 1 nutritionist’s Calorie = 4 186 J.) Walking at 3.00 mi/h requires about 220 kcal/h. It is interesting to compare these values with the energy consumption required for travel by car. Gasoline yields about 1.30 3 108 J/gal. Find the fuel economy in equivalent miles per gallon for a person (a) walking and (b) bicycling.

38. A 650-kg elevator starts from rest. It moves upward for 3.00 s with constant acceleration until it reaches its cruising speed of 1.75 m/s. (a) What is the average power of the elevator motor during this time interval? (b) How does this power compare with the motor power when the elevator moves at its cruising speed? 39. A 3.50-kN piano is lifted by three workers at constant speed to an apartment 25.0 m above the street using a pulley system fastened to the roof of the building. Each worker is able to deliver 165 W of power, and the pulley system is 75.0% efficient (so that 25.0% of the mechanical energy is transformed to other forms due to friction in the pulley). Neglecting the mass of the pulley, find the time required to lift the piano from the street to the apartment. 40. Energy is conventionally measured in Calories as well

BIO as in joules. One Calorie in nutrition is one kilocalo-

rie, defined as 1 kcal 5 4 186 J. Metabolizing 1 g of fat can release 9.00 kcal. A student decides to try to lose weight by exercising. He plans to run up and down the stairs in a football stadium as fast as he can and

240 Chapter 8 Conservation of Energy

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h

d

.c

0

ly

Figure P8.45 46. Review. As shown in Fig-

Q/C ure P8.46, a light string

m1

that does not stretch changes from horizonm2 h tal to vertical as it passes over the edge of a table. d The string connects m1, a 3.50-kg block originally at rest on the horizontal table at a height h 5 1.20 m Figure P8.46 above the floor, to m 2, a hanging 1.90-kg block originally a distance d 5 0.900 m above the floor. Neither the surface of the table nor its edge exerts a force of kinetic friction. The blocks start to move from rest. The sliding block m1 is projected horizontally after reaching the edge of the table. The hanging block m 2 stops without bouncing when it strikes the floor. Consider the two blocks plus the Earth as the system. (a) Find the speed at which m1 leaves the edge of the table. (b) Find the impact speed of m1 on the floor. (c) What is the shortest length of the string so that it does not go taut while m1 is in flight? (d) Is the energy of the system when it is released from rest equal to the energy of the system just before m1 strikes the ground? (e) Why or why not?

ic

Additional Problems

H

ee b

41. A loaded ore car has a mass of 950 kg and rolls on rails AMT with negligible friction. It starts from rest and is pulled M up a mine shaft by a cable connected to a winch. The shaft is inclined at 30.08 above the horizontal. The car accelerates uniformly to a speed of 2.20 m/s in 12.0 s and then continues at constant speed. (a) What power must the winch motor provide when the car is moving at constant speed? (b) What maximum power must the winch motor provide? (c) What total energy has transferred out of the motor by work by the time the car moves off the end of the track, which is of length 1 250 m?

45. Review. A boy starts at rest and slides down a frictionS less slide as in Figure P8.45. The bottom of the track is a height h above the ground. The boy then leaves the track horizontally, striking the ground at a distance d as shown. Using energy methods, determine the initial height H of the boy above the ground in terms of h and d.

s. w

as many times as necessary. To evaluate the program, suppose he runs up a flight of 80 steps, each 0.150 m high, in 65.0 s. For simplicity, ignore the energy he uses in coming down (which is small). Assume a typical efficiency for human muscles is 20.0%. This statement means that when your body converts 100 J from metabolizing fat, 20 J goes into doing mechanical work (here, climbing stairs). The remainder goes into extra internal energy. Assume the student’s mass is 75.0 kg. (a) How many times must the student run the flight of stairs to lose 1.00 kg of fat? (b) What is his average power output, in watts and in horsepower, as he runs up the stairs? (c) Is this activity in itself a practical way to lose weight?

42. Make an order-of-magnitude estimate of your power

ys

BIO output as you climb stairs. In your solution, state the

ar ph

physical quantities you take as data and the values you measure or estimate for them. Do you consider your peak power or your sustainable power?

w

.a

sw

43. A small block of mass m 5 200 g is released from rest at point A along the horizontal diameter on the inside of a frictionless, hemispherical bowl of radius R 5 30.0 cm (Fig. P8.43). Calculate (a) the gravitational potential energy of the block–Earth system when the block is at point A relative to point B, (b) the kinetic energy of the block at point B, (c) its speed at point B, and (d) its kinetic energy and the potential energy when the block is at point C.

Figure P8.43 Problems 43 and 44.

47. A 4.00-kg particle moves along the x axis. Its position 3 M varies with time according to x 5 t 1 2.0t , where x is in meters and t is in seconds. Find (a) the kinetic energy of the particle at any time t, (b) the acceleration of the particle and the force acting on it at time t, (c) the power being delivered to the particle at time t, and (d) the work done on the particle in the interval t 5 0 to t 5 2.00 s.

4 4. What If? The block of mass m 5 200 g described in Q/C Problem 43 (Fig. P8.43) is released from rest at point A, and the surface of the bowl is rough. The block’s speed at point B is 1.50 m/s. (a) What is its kinetic energy at point B? (b) How much mechanical energy is transformed into internal energy as the block moves from point A to point B? (c) Is it possible to determine the coefficient of friction from these results in any simple manner? (d) Explain your answer to part (c).

4 8. Why is the following situation impossible? A softball pitcher has a strange technique: she begins with her hand at rest at the highest point she can reach and then quickly rotates her arm backward so that the ball moves through a half-circle path. She releases the ball when her hand reaches the bottom of the path. The pitcher maintains a component of force on the 0.180-kg ball of constant magnitude 12.0 N in the direction of motion around the complete path. As the ball arrives

w

A

w

R

B

C 2R/3

241

Problems

ys ar ph B C

sw

Figure P8.49

50. Heedless of danger, a child leaps onto a pile of old

Q/C mattresses to use them as a trampoline. His motion

.a

between two particular points is described by the energy conservation equation kg 2 1 2.40 m/s 2 2 1 1 46.0 kg 2 1 9.80 m/s2 2 1 2.80 m 1 x 2 5 1 4 2 2 1 1.94 3 10 N/m 2 x

w

1 2 1 46.0

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52. Jonathan is riding a bicycle and encounters a hill S of height h. At the base of the hill, he is traveling at a speed vi . When he reaches the top of the hill, he is traveling at a speed vf . Jonathan and his bicycle together have a mass m. Ignore friction in the bicycle mechanism and between the bicycle tires and the road. (a) What is the total external work done on the system of Jonathan and the bicycle between the time he starts up the hill and the time he reaches the top? (b) What is the change in potential energy stored in Jonathan’s body during this process? (c) How much work does Jonathan do on the bicycle pedals within the Jonathan– bicycle–Earth system during this process?

ee b

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.c

53. Consider the block–spring–surface system in part (B) of Example 8.6. (a) Using an energy approach, find the position x of the block at which its speed is a maximum. (b) In the What If? section of this example, we explored the effects of an increased friction force of 10.0 N. At what position of the block does its maximum speed occur in this situation? 54. As it plows a parking lot, a

Q/C snowplow pushes an everS growing pile of snow in

ic

D

A

Jonathan do on the bicycle pedals within the Jonathan–bicycle–Earth system during this process?

s. w

at the bottom of the path, it leaves her hand with a speed of 25.0 m/s. 9. A skateboarder with his board can be modeled as a 4 particle of mass 76.0 kg, located at his center of mass (which we will study in Chapter 9). As shown in Figure P8.49, the skateboarder starts from rest in a crouching position at one lip of a half-pipe (point A). The half-pipe is one half of a cylinder of radius 6.80 m with its axis horizontal. On his descent, the skateboarder moves without friction so that his center of mass moves through one quarter of a circle of radius 6.30 m. (a) Find his speed at the bottom of the half-pipe (point B). (b) Immediately after passing point B, he stands up and raises his arms, lifting his center of mass from 0.500 m to 0.950 m above the concrete (point C). Next, the skateboarder glides upward with his center of mass moving in a quarter circle of radius 5.85 m. His body is horizontal when he passes point D, the far lip of the half-pipe. As he passes through point D, the speed of the skateboarder is 5.14 m/s. How much chemical potential energy in the body of the skateboarder was converted to mechanical energy in the skateboarder–Earth system when he stood up at point B? (c) How high above point D does he rise? Caution: Do not try this stunt yourself without the required skill and protective equipment.

w

w

(a) Solve the equation for x. (b) Compose the statement of a problem, including data, for which this equation gives the solution. (c) Add the two values of x obtained in part (a) and divide by 2. (d) What is the significance of the resulting value in part (c)? 51. Jonathan is riding a bicycle and encounters a hill of

AMT height 7.30 m. At the base of the hill, he is traveling

at 6.00 m/s. When he reaches the top of the hill, he is traveling at 1.00 m/s. Jonathan and his bicycle together have a mass of 85.0 kg. Ignore friction in the bicycle mechanism and between the bicycle tires and the road. (a) What is the total external work done on the system of Jonathan and the bicycle between the time he starts up the hill and the time he reaches the top? (b) What is the change in potential energy stored in Jonathan’s body during this process? (c) How much work does

v t

S front of it. Suppose a car v moving through the air is similarly modeled as a A cylinder of area A pushFigure P8.54 ing a growing disk of air in front of it. The originally stationary air is set into motion at the constant speed v of the cylinder as shown in Figure P8.54. In a time interval Dt, a new disk of air of mass Dm must be moved a distance v Dt and hence must be given a kinetic energy 12 1 Dm 2 v 2. Using this model, show that the car’s power loss owing to air resistance is 12 rAv 3 and that the resistive force acting on the car is 12 rAv 2, where r is the density of air. Compare this result with the empirical expression 12 DrAv 2 for the resistive force.

55. A wind turbine on a wind farm turns in response to a force of high-speed air resistance, R 5 12 DrAv 2. The power available is P 5 Rv 5 12 Drpr 2v 3, where v is the wind speed and we have assumed a circular face for the wind turbine of radius r. Take the drag coefficient as D 5 1.00 and the density of air from the front endpaper. For a wind turbine having r 5 1.50 m, calculate the power available with (a) v 5 8.00 m/s and (b) v 5 24.0 m/s. The power delivered to the generator is limited by the efficiency of the system, about 25%. For comparison, a large American home uses about 2 kW of electric power. 56. Consider the popgun in Example 8.3. Suppose the projectile mass, compression distance, and spring constant remain the same as given or calculated in the example. Suppose, however, there is a friction force of magnitude 2.00 N acting on the projectile as it rubs against the interior of the barrel. The vertical length from point A to the end of the barrel is 0.600 m.

242 Chapter 8 Conservation of Energy

58. Review. Why is the following situation impossible? A new high-speed roller coaster is claimed to be so safe that the passengers do not need to wear seat belts or any other restraining device. The coaster is designed with a vertical circular section over which the coaster travels on the inside of the circle so that the passengers are upside down for a short time interval. The radius of the circular section is 12.0 m, and the coaster enters the bottom of the circular section at a speed of 22.0 m/s. Assume the coaster moves without friction on the track and model the coaster as a particle.

om

ic

59. A horizontal spring attached to a wall has a force con-

61. A child’s pogo stick (Fig. P8.61) C stores energy in a spring with a force constant of 2.50 3 B A 104 N/m. At position A (x A 5 20.100 m), the spring compression is a maximum and the child is momentarily at rest. At position B (x B 5 0), the spring xC is relaxed and the child is movxA ing upward. At position C, the child is again momentarily at rest at the top of the jump. The combined mass of child and Figure P8.61 pogo stick is 25.0 kg. Although the boy must lean forward to remain balanced, the angle is small, so let’s assume the pogo stick is vertical. Also assume the boy does not bend his legs during the motion. (a) Calculate the total energy of the child–stick–Earth system, taking both gravitational and elastic potential energies as zero for x 5 0. (b) Determine x C. (c) Calculate the speed of the child at x 5 0. (d) Determine the value of x for which the kinetic energy of the system is a maximum. (e) Calculate the child’s maximum upward speed.

.c

where t is in seconds and a is in m/s2. (a) What is the change in kinetic energy of the car during the interval from t 5 0 to t 5 2.50 s? (b) What is the minimum average power output of the engine over this time interval? (c) Why is the value in part (b) described as the minimum value?

ly

a 5 1.16t 2 0.210t 2 1 0.240t 3

(a) Show that Aristotle’s proportions are included in the equation P Dt 5 bwd, where b is a proportionality constant. (b) Show that our theory of motion includes this part of Aristotle’s theory as one special case. In particular, describe a situation in which it is true, derive the equation representing Aristotle’s proportions, and determine the proportionality constant.

ee b

57. As the driver steps on the gas pedal, a car of mass Q/C 1 160 kg accelerates from rest. During the first few seconds of motion, the car’s acceleration increases with time according to the expression

load w a distance d/2 in time interval Dt/2, then (4) P/2 will move w/2 the given distance d in the given time interval Dt.

s. w

(a) After the spring is compressed and the popgun fired, to what height does the projectile rise above point B? (b) Draw four energy bar charts for this situation, analogous to those in Figures 8.6c–d.

Q/C stant of k 5 850 N/m. A block of mass m 5 1.00 kg

w

w

w

k

.a

sw

ar ph

ys

is attached to the spring and rests on a frictionless, horizontal surface as in Figure P8.59. (a) The block is pulled to a position xi 5 6.00 cm from equilibrium and released. Find the elastic potential energy stored in the spring when the block is 6.00 cm from equilibrium and when the block passes through equilibrium. (b) Find the speed of the block as it passes through the equilibrium point. (c) What is the speed of the block when it is at a position xi /2 5 3.00 cm? (d) Why isn’t the answer to part (c) half the answer to part (b)?

x0

m

x xi /2

x xi

Figure P8.59

60. More than 2 300 years ago, the Greek teacher Aristo-

Q/C tle wrote the first book called Physics. Put into more S precise terminology, this passage is from the end of its

Section Eta:

Let P be the power of an agent causing motion; w, the load moved; d, the distance covered; and Dt, the time interval required. Then (1) a power equal to P will in an interval of time equal to Dt move w/2 a distance 2d; or (2) it will move w/2 the given distance d in the time interval Dt/2. Also, if (3) the given power P moves the given

62. A 1.00-kg object slides k a m W to the right on a surface having a coeffiS vi cient of kinetic friction 0.250 (Fig. P8.62a). b The object has a speed of vi 5 3.00 m/s when d it makes contact with S a light spring (Fig. c vf 0 P8.62b) that has a force S constant of 50.0 N/m. v The object comes to d rest after the spring has been compressed S a distance d (Fig. v0 P8.62c). The object is e D then forced toward the left by the spring (Fig. P8.62d) and continues Figure P8.62 to move in that direction beyond the spring’s unstretched position. Finally, the object comes to rest a distance D to the left of the unstretched spring (Fig. P8.62e). Find (a) the distance of compression d, (b) the speed v at the unstretched position when the object is moving to the left (Fig. P8.62d), and (c) the distance D where the object comes to rest.

Problems

63. A 10.0-kg block is released from rest at point A in FigM ure P8.63. The track is frictionless except for the portion between points B and C, which has a length of 6.00 m. The block travels down the track, hits a spring of force constant 2 250 N/m, and compresses the spring 0.300 m from its equilibrium position before coming to rest momentarily. Determine the coefficient of kinetic friction between the block and the rough surface between points B and C.

A 6.00 m

67. Review. The mass of a car is 1 500 kg. The shape of the car’s body is such that its aerodynamic drag coefficient is D 5 0.330 and its frontal area is 2.50 m2. Assuming the drag force is proportional to v 2 and ignoring other sources of friction, calculate the power required to maintain a speed of 100 km/h as the car climbs a long hill sloping at 3.208.

C

68. A pendulum, comprising a light S string of length L and a small u sphere, swings in the vertical d L plane. The string hits a peg located Peg a distance d below the point of suspension (Fig. P8.68). (a) Show that if the sphere is released from a height below that of the peg, it Figure P8.68 will return to this height after the string strikes the peg. (b) Show that if the pendulum is released from rest at the horizontal position (u 5 908) and is to swing in a complete circle centered on the peg, the minimum value of d must be 3L/5.

.c

Figure P8.63

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6 4. A block of mass m1 5 20.0 kg is

AMT connected to a block of mass M m 2 5 30.0 kg by a massless

m1

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string that passes over a light, m2 frictionless pulley. The 30.0-kg k u block is connected to a spring that has negligible mass and a force constant of k 5 250 N/m Figure P8.64 as shown in Figure P8.64. The spring is unstretched when the system is as shown in the figure, and the incline is frictionless. The 20.0-kg block is pulled a distance h 5 20.0 cm down the incline of angle u 5 40.08 and released from rest. Find the speed of each block when the spring is again unstretched.

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The line from the center of curvature of the cap to the pumpkin makes an angle ui 5 08 with the vertical. While you happen to be standing nearby in the middle of a rainy night, a breath of wind makes the pumpkin start sliding downward from rest. It loses contact with the cap when the line from the center of the hemisphere to the pumpkin makes a certain angle with the vertical. What is this angle?

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3.00 m

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65. A block of mass 0.500 kg is pushed against a horizonQ/C tal spring of negligible mass until the spring is compressed a distance x (Fig. P8.65). The force constant of the spring is 450 N/m. When it is released, the block travels along a frictionless, horizontal surface to point A, the bottom of a vertical circular track of radius R 5 1.00 m, and continues to move up the track. The block’s speed at the bottom of the track is v A 5 12.0 m/s, and the block experiences an average friction force of 7.00 N while sliding up the track. (a) What is x? (b) If the block were to reach the top of the track, what would be its speed at that point? (c) Does the block actually reach the top of the track, or does it fall off before reaching the top?

R

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Figure P8.65 66. Review. As a prank, someone has balanced a pumpkin at the highest point of a grain silo. The silo is topped with a hemispherical cap that is frictionless when wet.

69. A block of mass M rests on a table. It is fastened to the S lower end of a light, vertical spring. The upper end of the spring is fastened to a block of mass m. The upper block is pushed down by an additional force 3mg, so the spring compression is 4mg/k. In this configuration, the upper block is released from rest. The spring lifts the lower block off the table. In terms of m, what is the greatest possible value for M?

70. Review. Why is the following situation impossible? An athlete tests her hand ui strength by having an assistant hang weights from her belt as she hangs onto a horizontal bar with her hands. When the weights hanging on her belt have increased to 80% of her body weight, her hands can Figure P8.70 no longer support her and she drops to the floor. Frustrated at not meeting her hand-strength goal, she decides to swing on a trapeze. The trapeze consists of a bar suspended by two parallel ropes, each of length ,, allowing performers to swing in a vertical circular arc (Fig. P8.70). The athlete holds the bar and steps off an elevated platform, starting from rest with the ropes at an angle ui 5 60.08 with respect to the vertical. As she swings several times back and forth in a circular arc, she forgets her frustration related to the hand-strength test. Assume the size of the

244 Chapter 8 Conservation of Energy the maximum value of the coefficient of friction that would allow the block to return to x 5 0?

performer’s body is small compared to the length , and air resistance is negligible. 71. While running, a person transforms about 0.600 J of

BIO chemical energy to mechanical energy per step per

76. In bicycling for aerobic exercise, a woman wants her

BIO heart rate to be between 136 and 166 beats per min-

ute. Assume that her heart rate is directly proportional to her mechanical power output within the range relevant here. Ignore all forces on the woman–bicycle system except for static friction forward on the drive wheel of the bicycle and an air resistance force proportional to the square of her speed. When her speed is 22.0 km/h, her heart rate is 90.0 beats per minute. In what range should her speed be so that her heart rate will be in the range she wants?

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77. Review. In 1887 in Bridgeport, Connecticut, C. J. Belknap built the water slide shown in Figure P8.77. A rider on a small sled, of total mass 80.0 kg, pushed off to start at the top of the slide (point A) with a speed of 2.50 m/s. The chute was 9.76 m high at the top and 54.3 m long. Along its length, 725 small wheels made friction negligible. Upon leaving the chute horizontally at its bottom end (point C), the rider skimmed across the water of Long Island Sound for as much as 50 m, “skipping along like a flat pebble,” before at last coming to rest and swimming ashore, pulling his sled after him. (a) Find the speed of the sled and rider at point C. (b) Model the force of water friction as a constant retarding force acting on a particle. Find the magnitude of the friction force the water exerts on the sled. (c) Find the magnitude of the force the chute exerts on the sled at point B. (d) At point C, the chute is horizontal but curving in the vertical plane. Assume its radius of curvature is 20.0 m. Find the force the chute exerts on the sled at point C.

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Figure P8.72

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Engraving from Scientific American

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72. A roller-coaster car shown in Figure P8.72 is released S from rest from a height h and then moves freely with negligible friction. The roller-coaster track includes a circular loop of radius R in a vertical plane. (a) First suppose the car barely makes it around the loop; at the top of the loop, the riders are upside down and feel weightless. Find the required height h of the release point above the bottom of the loop in terms of R. (b) Now assume the release point is at or above the minimum required height. Show that the normal force on the car at the bottom of the loop exceeds the normal force at the top of the loop by six times the car’s weight. The normal force on each rider follows the same rule. Such a large normal force is dangerous and very uncomfortable for the riders. Roller coasters are therefore not built with circular loops in vertical planes. Figure P6.17 (page 170) shows an actual design.

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kilogram of body mass. If a 60.0-kg runner transforms energy at a rate of 70.0 W during a race, how fast is the person running? Assume that a running step is 1.50 m long.

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73. A ball whirls around in a vertical circle at the end of a S string. The other end of the string is fixed at the center of the circle. Assuming the total energy of the ball– Earth system remains constant, show that the tension in the string at the bottom is greater than the tension at the top by six times the ball’s weight.

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74. An airplane of mass 1.50 3 104 kg is in level flight, ini-

a

Q/C tially moving at 60.0 m/s. The resistive force exerted

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by air on the airplane has a magnitude of 4.0 3 104 N. By Newton’s third law, if the engines exert a force on the exhaust gases to expel them out of the back of the engine, the exhaust gases exert a force on the engines in the direction of the airplane’s travel. This force is called thrust, and the value of the thrust in this situation is 7.50 3 104 N. (a) Is the work done by the exhaust gases on the airplane during some time interval equal to the change in the airplane’s kinetic energy? Explain. (b) Find the speed of the airplane after it has traveled 5.0 3 102 m.

75. Consider the block–spring collision discussed in Example 8.8. (a) For the situation in part (B), in which the surface exerts a friction force on the block, show that the block never arrives back at x 5 0. (b) What is

20.0 m

A 9.76 m

B 54.3 m

C 50.0 m

b

Figure P8.77 78. In a needle biopsy, a narrow strip of tissue is extracted

BIO from a patient using a hollow needle. Rather than

being pushed by hand, to ensure a clean cut the needle can be fired into the patient’s body by a spring. Assume that the needle has mass 5.60 g, the light spring has

245

Problems S

force constant 375 N/m, and the spring is originally compressed 8.10 cm to project the needle horizontally without friction. After the needle leaves the spring, the tip of the needle moves through 2.40 cm of skin and soft tissue, which exerts on it a resistive force of 7.60 N. Next, the needle cuts 3.50 cm into an organ, which exerts on it a backward force of 9.20 N. Find (a) the maximum speed of the needle and (b) the speed at which the flange on the back end of the needle runs into a stop that is set to limit the penetration to 5.90 cm.

a wind exerting constant horizontal force F , on a vine having length L and initially making an angle u with the vertical (Fig. P8.81). Take D 5 50.0 m, F 5 110 N, L 5 40.0 m, and u 5 50.08. (a) With what minimum speed must Jane begin her swing to just make it to the other side? (b) Once the rescue is complete, Tarzan and Jane must swing back across the river. With what minimum speed must they begin their swing? Assume Tarzan has a mass of 80.0 kg.

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Challenge Problems

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Figure P8.81

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L

Tarzan

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Boundary

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Wind S f F

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79. Review. A uniform board of length L is sliding along a S smooth, frictionless, horizontal plane as shown in Figure P8.79a. The board then slides across the boundary with a rough horizontal surface. The coefficient of kinetic friction between the board and the second surface is mk . (a) Find the acceleration of the board at the moment its front end has traveled a distance x beyond the boundary. (b) The board stops at the moment its back end reaches the boundary as shown in Figure P8.79b. Find the initial speed v of the board.

Figure P8.79

8 0. Starting from rest, a 64.0-kg person bungee jumps

Q/C from a tethered hot-air balloon 65.0 m above the

Pivot

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ground. The bungee cord has negligible mass and unstretched length 25.8 m. One end is tied to the basket of the balloon and the other end to a harness around the person’s body. The cord is modeled as a spring that obeys Hooke’s law with a spring constant of 81.0 N/m, and the person’s body is modeled as a particle. The hot-air balloon does not move. (a) Express the gravitational potential energy of the person–Earth system as a function of the person’s variable height y above the ground. (b) Express the elastic potential energy of the cord as a function of y. (c) Express the total potential energy of the person–cord–Earth system as a function of y. (d) Plot a graph of the gravitational, elastic, and total potential energies as functions of y. (e) Assume air resistance is negligible. Determine the minimum height of the person above the ground during his plunge. (f) Does the potential energy graph show any equilibrium position or positions? If so, at what elevations? Are they stable or unstable? (g) Determine the jumper’s maximum speed.

82. A ball of mass m 5 300 g is connected by a strong string of length L 5 80.0 cm to a pivot and held in place with the string vertical. A wind exerts constant force F to the right on the ball as shown in Figure P8.82. The ball is released from rest. The wind makes it swing up to attain maximum height H above its starting point before it swings down again. (a) Find H as a function of F. Evaluate H for (b) F 5 1.00 N and (c) F 5 10.0 N. How does H behave (d) as F approaches zero and (e) as F approaches infinity? (f) Now consider the equilibrium height of the ball with the wind blowing. Determine it as a function of F. Evaluate the equilibrium height for (g) F 5 10 N and (h) F going to infinity.

81. Jane, whose mass is 50.0 kg, needs to swing across a river (having width D) filled with person-eating crocodiles to save Tarzan from danger. She must swing into

L

S

Pivot L S

F

F

m H

m a

b

Figure P8.82 83. What If? Consider the roller coaster described in Prob-

Q/C lem 58. Because of some friction between the coaster

and the track, the coaster enters the circular section at a speed of 15.0 m/s rather than the 22.0 m/s in Problem 58. Is this situation more or less dangerous for the passengers than that in Problem 58? Assume the circular section is still frictionless.

246 Chapter 8 Conservation of Energy cord, tied to a harness around his body, to stop his fall at a point 10.0 m above the ground. Model his body as a particle and the cord as having negligible mass and obeying Hooke’s law. In a preliminary test he finds that when hanging at rest from a 5.00-m length of the cord, his body weight stretches it by 1.50 m. He will drop from rest at the point where the top end of a longer section of the cord is attached to the stationary balloon. (a) What length of cord should he use? (b) What maximum acceleration will he experience?

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8 4. A uniform chain of length 8.00 m initially lies stretched out on a horizontal table. (a) Assuming the coefficient of static friction between chain and table is 0.600, show that the chain will begin to slide off the table if at least 3.00 m of it hangs over the edge of the table. (b) Determine the speed of the chain as its last link leaves the table, given that the coefficient of kinetic friction between the chain and the table is 0.400. 85. A daredevil plans to bungee jump from a balloon 65.0 m above the ground. He will use a uniform elastic

c h a p t e r

9

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Linear Momentum and Collisions

9.1 Linear Momentum

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9.2 Analysis Model: Isolated System (Momentum)

9.4 Collisions in One Dimension 9.5 Collisions in Two Dimensions 9.6 The Center of Mass 9.7 Systems of Many Particles 9.8 Deformable Systems 9.9 Rocket Propulsion

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9.3 Analysis Model: Nonisolated System (Momentum)

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Consider what happens when two cars collide as in the opening photograph for this chapter. Both cars change their motion from having a very large velocity to being at rest because of the collision. Because each car experiences a large change in velocity over a very short time interval, the average force on it is very large. By Newton’s third law, each of the cars experiences a force of the same magnitude. By Newton’s second law, the results of those forces on the motion of the car depends on the mass of the car. One of the main objectives of this chapter is to enable you to understand and analyze such events in a simple way. First, we introduce the concept of momentum, which is useful for describing objects in motion. The momentum of an object is related to both its mass and its velocity. The concept of momentum leads us to a second conservation law, that of conservation of momentum. In turn, we identify new momentum versions of analysis models for isolated and nonisolated system. These models are especially useful for treating problems that involve collisions between objects and for analyzing rocket propulsion. This chapter also introduces the concept of the center of mass of a system of particles. We find that the motion of a system of particles can be described by the motion of one particle located at the center of mass that represents the entire system.

9.1 Linear Momentum

The concept of momentum allows the analysis of car collisions even without detailed knowledge of the forces involved. Such analysis can determine the relative velocity of the cars before the collision, and in addition aid engineers in designing safer vehicles. (The English translation of the German text on the side of the trailer in the background is: “Pit stop for your vehicle.”) (AP Photos/Keystone/ Regina Kuehne)

In Chapter 8, we studied situations that are difficult to analyze with Newton’s laws. We were able to solve problems involving these situations by identifying a system and

247

248 Chapter 9

Linear Momentum and Collisions

applying a conservation principle, conservation of energy. Let us consider another situation and see if we can solve it with the models we have developed so far: A 60-kg archer stands at rest on frictionless ice and fires a 0.030-kg arrow horizontally at 85 m/s. With what velocity does the archer move across the ice after firing the arrow? From Newton’s third law, we know that the force that the bow exerts on the arrow is paired with a force in the opposite direction on the bow (and the archer). This force causes the archer to slide backward on the ice with the speed requested in the problem. We cannot determine this speed using motion models such as the particle under constant acceleration because we don’t have any information about the acceleration of the archer. We cannot use force models such as the particle under a net force because we don’t know anything about forces in this situation. Energy models are of no help because we know nothing about the work done in pulling the bowstring back or the elastic potential energy in the system related to the taut bowstring. Despite our inability to solve the archer problem using models learned so far, this problem is very simple to solve if we introduce a new quantity that describes motion, linear momentum. To generate this new quantity, consider an isolated system of two particles (Fig. 9.1) with masses m1 and m 2 moving with velocities S v 1 and S v 2 at an instant of time. Because the system is isolated, the only force on one particle is that from the other particle. If a force from particle 1 (for example, a gravitational force) acts on particle 2, there must be a second force—equal in magnitude but opposite in direction—that particle 2 exerts on particle 1. ThatSis, the forces on the S particles form a Newton’s third law action–reaction pair, and F 12 5 2 F 21. We can express this condition as

S

m1

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v1

S

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F21 S

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F12

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m2 S

v2

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Figure 9.1 Two particles interact with each other. According to Newton’s third law, we must have S S F 12 5 2 F 21.

S

S

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F 21 1 F 12 5 0

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From a system point of view, this equation says that if we add up the forces on the particles in an isolated system, the sum is zero. Let us further analyze this situation by incorporating Newton’s second law. At the instant shown in Figure 9.1, the interacting particles in the system have accelerations corresponding to the forces on them. Therefore, replacing the force on each particle with mS a for the particle gives a 1 1 m 2S a2 5 0 m 1S

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Now we replace each acceleration with its definition from Equation 4.5: m1

dS v2 dS v1 1 m2 50 dt dt

If the masses m1 and m 2 are constant, we can bring them inside the derivative operation, which gives v22 d 1 m 2S d 1 m 1S v12 1 50 dt dt d 1m S v 1 m 2S v 2 2 5 0 (9.1) dt 1 1

Notice that the derivative of the sum m 1S v 1 1 m 2S v 2 with respect to time is zero. Consequently, this sum must be constant. We learn from this discussion that the quantity mS v for a particle is important in that the sum of these quantities for an isolated system of particles is conserved. We call this quantity linear momentum: The linear momentum of a particle or an object that can be modeled as a v is defined to be the product of the particle of mass m moving with a velocity S mass and velocity of the particle:

Definition of linear momentum of a particle

S

p ; mS v

(9.2)

9.1 Linear Momentum 249

Linear momentum is a vector quantity because it equals the product of a scalar quantity m and a vector quantity S v . Its direction is along S v , it has dimensions ML/T, and its SI unit is kg ? m/s. If a particle is moving in an arbitrary direction, S p has three components, and Equation 9.2 is equivalent to the component equations px 5 mv x

py 5 mv y

pz 5 mv z

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As you can see from its definition, the concept of momentum1 provides a quantitative distinction between heavy and light particles moving at the same velocity. For example, the momentum of a bowling ball is much greater than that of a tennis ball moving at the same speed. Newton called the product mS v quantity of motion; this term is perhaps a more graphic description than our present-day word momentum, which comes from the Latin word for movement. We have seen another quantity, kinetic energy, that is a combination of mass and speed. It would be a legitimate question to ask why we need another quantity, momentum, based on mass and velocity. There are clear differences between kinetic energy and momentum. First, kinetic energy is a scalar, whereas momentum is a vector. Consider a system of two equal-mass particles heading toward each other along a line with equal speeds. There is kinetic energy associated with this system because members of the system are moving. Because of the vector nature of momentum, however, the momentum of this system is zero. A second major difference is that kinetic energy can transform to other types of energy, such as potential energy or internal energy. There is only one type of linear momentum, so we see no such transformations when using a momentum approach to a problem. These differences are sufficient to make models based on momentum separate from those based on energy, providing an independent tool to use in solving problems. Using Newton’s second law of motion, we can relate the linear momentum of a particle to the resultant force acting on the particle. We start with Newton’s second law and substitute the definition of acceleration: dS v S F 5 m a 5 m a dt S

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In Newton’s second law, the mass m is assumed to be constant. Therefore, we can bring m inside the derivative operation to give us aF 5 S

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p dS v2 d 1 mS 5 dt dt

(9.3)

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This equation shows that the time rate of change of the linear momentum of a particle is equal to the net force acting on the particle. In Chapter 5, we identified force as that which causes a change in the motion of an object (Section 5.2). In Newton’s second law (Eq. 5.2), we used acceleration S a to represent the change in motion. We see now in Equation 9.3 that we can use the derivative of momentum S p with respect to time to represent the change in motion. This alternative form of Newton’s second law is the form in which Newton presented the law, and it is actually more general than the form introduced in Chapter 5. In addition to situations in which the velocity vector varies with time, we can use Equation 9.3 to study phenomena in which the mass changes. For example, the mass ofSa rocket changes as fuel is burned and ejected from the rocket. We cannot a to analyze rocket propulsion; we must use a momentum approach, use g F 5 mS as we will show in Section 9.9.

1In

this chapter, the terms momentum and linear momentum have the same meaning. Later, in Chapter 11, we shall use the term angular momentum for a different quantity when dealing with rotational motion.

WW Newton’s second law for a particle

250 Chapter 9

Linear Momentum and Collisions

Q uick Quiz 9.1 Two objects have equal kinetic energies. How do the magnitudes of their momenta compare? (a) p1 , p 2 (b) p1 5 p 2 (c) p1 . p 2 (d) not enough information to tell

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Q uick Quiz 9.2 Your physical education teacher throws a baseball to you at a certain speed and you catch it. The teacher is next going to throw you a medicine ball whose mass is ten times the mass of the baseball. You are given the following choices: You can have the medicine ball thrown with (a) the same speed as the baseball, (b) the same momentum, or (c) the same kinetic energy. Rank these choices from easiest to hardest to catch.

9.2 Analysis Model: Isolated System (Momentum) d S 1p 1 S p2 2 5 0 dt 1

momentum of an isolated system is conserved, the momentum of one particle within an isolated system is not necessarily conserved because other particles in the system may be interacting with it. Avoid applying conservation of momentum to a single particle.

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Momentum of an Isolated System Is Conserved Although the

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Using the definition of momentum, Equation 9.1 can be written

Pitfall Prevention 9.1

Because the time derivative of the total momentum S p tot 5 S p1 1 S p 2 is zero, we conclude that the total momentum of the isolated system of the two particles in Figure 9.1 must remain constant: S

p tot 5 constant

s. w

(9.4)

or, equivalently, over some time interval,

S Dp tot 5 0

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(9.5)

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Equation 9.5 can be written as

S

p 1i 1 S p 2i 5 S p 1f 1 S p 2f

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sw

where S p 1f and S p 2f are the final values of the p 1i and S p 2i are the initial values and S momenta for the two particles for the time interval during which the particles interact. This equation in component form demonstrates that the total momenta in the x, y, and z directions are all independently conserved: p1ix 1 p 2ix 5 p1fx 1 p 2fx p1iy 1 p 2iy 5 p1fy 1 p 2fy p1iz 1 p 2iz 5 p1fz 1 p 2fz (9.6)

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Equation 9.5 is the mathematical statement of a new analysis model, the isolated system (momentum). It can be extended to any number of particles in an isolated system as we show in Section 9.7. We studied the energy version of the isolated system model in Chapter 8 (DE system 5 0) and now we have a momentum version. In general, Equation 9.5 can be stated in words as follows:

The momentum version of the isolated system model

Whenever two or more particles in an isolated system interact, the total momentum of the system does not change. This statement tells us that the total momentum of an isolated system at all times equals its initial momentum. Notice that we have made no statement concerning the type of forces acting on the particles of the system. Furthermore, we have not specified whether the forces are conservative or nonconservative. We have also not indicated whether or not the forces are constant. The only requirement is that the forces must be internal to the system. This single requirement should give you a hint about the power of this new model.

9.2 Analysis Model: Isolated System (Momentum) 251

Analysis Model Isolated System (Momentum) Imagine you have identified a system to be analyzed and have defined a system boundary. If there are no external forces on the system, the system is isolated. In that case, the total momentum of the system, which is the vector sum of the momenta of all members of the system, is conserved: S Dp tot 5 0

￼

System boundary Momentum

(9.5)

Examples:

If no external forces act on the system, the total momentum of the system is constant.

The Archer AM

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Example 9.1

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• a cue ball strikes another ball on a pool table • a spacecraft fires its rockets and moves faster through space • molecules in a gas at a specific temperature move about and strike each other (Chapter 21) • an incoming particle strikes a nucleus, creating a new nucleus and a different outgoing particle (Chapter 44) • an electron and a positron annihilate to form two outgoing photons (Chapter 46)

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Let us consider the situation proposed at the beginning of Section 9.1. A 60-kg archer stands at rest on frictionless ice and fires a 0.030-kg arrow horizontally at 85 m/s (Fig. 9.2). With what velocity does the archer move across the ice after firing the arrow?

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S o l u ti o n

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Conceptualize You may have conceptualized this problem already when it was introduced at the beginning of Section 9.1. Imagine the arrow being fired one way and the archer recoiling in the opposite direction. Categorize A s discussed in Section 9.1, we cannot solve this problem with models based on motion, force, or energy. Nonetheless, we can solve this problem very easily with an approach involving momentum. Let us take the system to consist of the archer (including the bow) and the arrow. The system is not isolated because the gravitational force and the normal force from the ice act on the system. These forces, however, are vertical and perpendicular to the motion of the system. There are no external forces in the horizontal direction, and we can apply the isolated system (momentum) model in terms of momentum components in this direction.

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Figure 9.2 (Example 9.1) An archer fires an arrow horizontally to the right. Because he is standing on frictionless ice, he will begin to slide to the left across the ice.

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Analyze The total horizontal momentum of the system before the arrow is fired is zero because nothing in the sys-

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tem is moving. Therefore, the total horizontal momentum of the system after the arrow is fired must also be zero. We choose the direction of firing of the arrow as the positive x direction. Identifying the archer as particle 1 and the arrow as particle 2, we have m1 5 60 kg, m 2 5 0.030 kg, and S v 2f 5 85 ^i m/s.

Using the isolated system (momentum) model, begin with Equation 9.5:

S Dp 50

Solve this equation for S v 1f and substitute numerical values:

S

v 1f 5 2

S

S

pf 2 S pi 5 0

S

S

pf 5 S pi

v 1f 1 m 2S v 2f 5 0 S m 1S

0.030 kg m2 S v 2f 5 2a b 1 85 ^i m/s 2 5 20.042 ^i m/s m1 60 kg

Finalize The negative sign for S v 1f indicates that the archer is moving to the left in Figure 9.2 after the arrow is fired, in

the direction opposite the direction of motion of the arrow, in accordance with Newton’s third law. Because the archer

continued

252 Chapter 9

Linear Momentum and Collisions

▸ 9.1 c o n t i n u e d is much more massive than the arrow, his acceleration and consequent velocity are much smaller than the acceleration and velocity of the arrow. Notice that this problem sounds very simple, but we could not solve it with models based on motion, force, or energy. Our new momentum model, however, shows us that it not only sounds simple, it is simple! What if the arrow were fired in a direction that makes an angle u with the horizontal? How will that change the recoil velocity of the archer?

leading to

Answer The recoil velocity should decrease in magni-

For u 5 0, cos u 5 1 and the final velocity of the archer reduces to the value when the arrow is fired horizontally. For nonzero values of u, the cosine function is less than 1 and the recoil velocity is less than the value calculated for u 5 0. If u 5 908, then cos u 5 0 and v1f 5 0, so there is no recoil velocity. In this case, the archer is simply pushed downward harder against the ice as the arrow is fired.

tude because only a component of the velocity of the arrow is in the x direction. Conservation of momentum in the x direction gives

m2 v cos u m 1 2f

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m1v1f 1 m 2v 2f cos u 5 0

v 1f 5 2

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W h at I f ?

Can We Really Ignore the Kinetic Energy of the Earth? AM

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Example 9.2

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In Section 7.6, we claimed that we can ignore the kinetic energy of the Earth when considering the energy of a system consisting of the Earth and a dropped ball. Verify this claim. S o l u ti o n

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Conceptualize Imagine dropping a ball at the surface of the Earth. From your point of view, the ball falls while the Earth remains stationary. By Newton’s third law, however, the Earth experiences an upward force and therefore an upward acceleration while the ball falls. In the calculation below, we will show that this motion is extremely small and can be ignored.

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Categorize We identify the system as the ball and the Earth. We assume there are no forces on the system from outer space, so the system is isolated. Let’s use the momentum version of the isolated system model. Analyze We begin by setting up a ratio of the kinetic energy of the Earth to that of the ball. We identify vE and vb as the speeds of the Earth and the ball, respectively, after the ball has fallen through some distance.

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Use the definition of kinetic energy to set up this ratio:

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Apply the isolated system (momentum) model, recognizing that the initial momentum of the system is zero:

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Solve the equation for the ratio of speeds:

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Substitute this expression for vE /vb in Equation (1): Substitute order-of-magnitude numbers for the masses:

1 2 KE m E vE 2 2 m Ev E 5 1 5 a ba b (1) 2 m b vb Kb 2 m bv b

S 50 Dp

S pi 5 pf

S

0 5 m b vb 1 m E v E

mb vE 52 vb mE mb 2 mb mE KE 5 a b a2 b 5 mb mE mE Kb

1 kg mb KE , 10225 5 , 25 mE Kb 10 kg

Finalize The kinetic energy of the Earth is a very small fraction of the kinetic energy of the ball, so we are justified in ignoring it in the kinetic energy of the system.

9.3 Analysis Model: Nonisolated System (Momentum) According to Equation 9.3, the momentum of a particle changes if a net force acts on the particle. The same can be said about a net force applied to a system as we

9.3 Analysis Model: Nonisolated System (Momentum)

253

om

will show explicitly in Section 9.7: the momentum of a system changes if a net force from the environment acts on the system. This may sound similar to our discussion of energy in Chapter 8: the energy of a system changes if energy crosses the boundary of the system to or from the environment. In this section, we consider a nonisolated system. For energy considerations, a system is nonisolated if energy transfers across the boundary of the system by any of the means listed in Section 8.1. For momentum considerations, a system is nonisolated if a net force acts on the system for a time interval. In this case, we can imagine momentum being transferred to the system from the environment by means of the net force. Knowing the change in momentum caused by a force is useful in solving some types of problems. To build S a better understanding of this important concept, let us assume a net force g F acts on a particle and this force may vary withStime. According to Newton’s second law, in the form expressed in Equation 9.3, g F 5 d S p /dt, we can write

(9.7) dS p 5 a F dt We can integrate2 this expression to find the change in the momentum of a particle when the force acts over some time interval. If the momentum of the particle changes from S p i at time ti to S p f at time tf , integrating Equation 9.7 gives S

tf

S

ti

ee b

S Dp 5S pf 2 S p i 5 3 a F dt

ly

.c

(9.8)

s. w

To evaluate the integral, we need to know how the net force varies with time. The quantitySon the right side of this equation is a vector called the impulse of the net force g F acting on a particle over the time interval Dt 5 tf 2 ti : I ; 3 a F dt The impulse imparted to the tf

S

(9.9)

ic

S

ti

WW Impulse of a force

particle by the force is the

S

sw

ar ph

ys

under the curve.having a magniFrom its definition, we see that impulse I is a area vector quantity tude equal to the area under the force–time curve as described in Figure 9.3a. It is F assumed the force varies in time in the general manner shown in the figure and is nonzero in the time interval Dt 5 tf 2 ti . The direction of the impulse vector is the same as the direction of the change in momentum. Impulse has the dimensions of momentum, that is, ML/T. Impulse is not a property of a particle; rather, it is a measure of the degree to which an external force changes the particle’s momentum. Because the net force imparting an impulse to a particle can generally vary in time, it is convenient to define a time-averaged net force:

1 a F 2 avg

w

.a

S

t

tf

t

(9.10)

The time-averaged net force gives the same impulse to a particle as does the timevarying force in (a).

The impulse imparted to the particle by the force is the area under the curve.

w w

S ti 1 f ; 3 a aF dt Dt ti

F

F

( F )avg

ti

tf

a 2Here

t

ti

tf

t

b

we are integrating force with respect to time. Compare this strategy with our efforts in Chapter 7, where we The time-averaged net force integrated force with respect to position to find the work done by the force.

gives the same impulse to a particle as does the timevarying force in (a).

F

Figure 9.3 (a) A net force acting on a particle may vary in time. (b) The value of the constant force (o F )avg (horizontal dashed line) is chosen so that the area (o F )avg Dt of the rectangle is the same as the area under the curve in (a).

254 Chapter 9

Linear Momentum and Collisions

where Dt 5 tf 2 ti . (This equation is an application of the mean value theorem of calculus.) Therefore, we can express Equation 9.9 as I 5 1 a F 2 avg Dt

S

S

(9.11)

This time-averaged force, shown in Figure 9.3b, can be interpreted as the constant force that would give to the particle in the time interval Dt the same impulse that the time-varying force gives over this same interval. S In principle, if g F is known as a function of time, the impulse can be calculated from Equation 9.9. The calculation becomesSespeciallySsimple if the force S acting on the particle is constant. In this case, 1 g F 2 avg 5 g F , where g F is the constant net force, and Equation 9.11 becomes

om

I 5 a F Dt

S

S

(9.12)

.c

Combining Equations 9.8 and 9.9 gives us an important statement known as the impulse–momentum theorem: The change in the momentum of a particle is equal to the impulse of the net force acting on the particle:

ee b

ly

Impulse–momentum theorem for a particle

S

S 5 I Dp

sw

ar ph

ys

ic

s. w

This statement is equivalent to Newton’s second law. When we say that an impulse is given to a particle, we mean that momentum is transferred from an external agent to that particle. Equation 9.13 is identical in form to the conservation of energy equation, Equation 8.1, and its full expansion, Equation 8.2. Equation 9.13 is the most general statement of the principle of conservation of momentum and is called the conservation of momentum equation. In the case of a momentum approach, isolated systems tend to appear in problems more often than nonisolated systems, so, in practice, the conservation of momentum equation is often identified as the special case of Equation 9.5. The left side of Equation 9.13 represents the change in the momentum of the system, which in this case is a single particle. The right side is a measure of how much momentum crosses the boundary of the system due to the net force being applied to the system. Equation 9.13 is the mathematical statement of a new analysis model, the nonisolated system (momentum) model. Although this equation is similar in form to Equation 8.1, there are several differences in its application to problems. First, Equation 9.13 is a vector equation, whereas Equation 8.1 is a scalar equation. Therefore, directions are important for Equation 9.13. Second, there is only one type of momentum and therefore only one way to store momentum in a system. In contrast, as we see from Equation 8.2, there are three ways to store energy in a system: kinetic, potential, and internal. Third, there is only one way to transfer momentum into a system: by the application of a force on the system over a time interval. Equation 8.2 shows six ways we have identified as transferring energy into a system. Therefore, there is no expansion of Equation 9.13 analogous to Equation 8.2. In many physical situations, we shall use what is called the impulse approximation, in which we assume one of the forces exerted on a particle acts for a short time but is much greater than any other force present. In this case, the net force S S g F in Equation 9.9 is replaced with a single force F to find the impulse on the particle. This approximation is especially useful in treating collisions in which the duration of the collision is very short. When this approximation is made, the single force is referred to as an impulsive force. For example, when a baseball is struck with a bat, the time of the collision is about 0.01 s and the average force that the bat exerts on the ball in this time is typically several thousand newtons. Because this contact force is much greater than the magnitude of the gravitational force, the impulse approximation justifies our ignoring the gravitational forces exerted on

.a w w © David Woods/CORBIS

w Air bags in automobiles have saved countless lives in accidents. The air bag increases the time interval during which the passenger is brought to rest, thereby decreasing the force on (and resultant injury to) the passenger.

(9.13)

9.3 Analysis Model: Nonisolated System (Momentum)

255

the ball and bat during the collision. When we use this approximation, it is imporp i and S p f represent the momenta immediately before and tant to remember that S after the collision, respectively. Therefore, in any situation in which it is proper to use the impulse approximation, the particle moves very little during the collision.

om

Q uick Quiz 9.3 Two objects are at rest on a frictionless surface. Object 1 has a greater mass than object 2. (i) When a constant force is applied to object 1, it accelerates through a distance d in a straight line. The force is removed from object 1 and is applied to object 2. At the moment when object 2 has accelerated through the same distance d, which statements are true? (a) p1 , p 2 (b) p1 5 p 2 (c) p1 . p 2 (d) K1 , K 2 (e) K1 5 K 2 (f) K1 . K 2 (ii) When a force is applied to object 1, it accelerates for a time interval Dt. The force is removed from object 1 and is applied to object 2. From the same list of choices, which statements are true after object 2 has accelerated for the same time interval Dt?

Analysis Model Nonisolated System (Momentum)

ee b

ly

.c

Q uick Quiz 9.4 Rank an automobile dashboard, seat belt, and air bag, each used alone in separate collisions from the same speed, in terms of (a) the impulse and (b) the average force each delivers to a front-seat passenger, from greatest to least.

Impulse System boundary

(9.13)

Momentum

ys

ic

S

S Dp 5 I

￼

s. w

Imagine you have identified a system to be analyzed and have defined a system boundary. If external forces are applied on the system, the system is nonisolated. In that case, the change in the total momentum of the system is equal to the impulse on the system, a statement known as the impulse–momentum theorem:

Examples:

ar ph

• a baseball is struck by a bat • a spool sitting on a table is pulled by a string (Example 10.14 in Chapter 10) • a gas molecule strikes the wall of the container holding the gas (Chapter 21) • photons strike an absorbing surface and exert pressure on the surface (Chapter 34)

sw

How Good Are the Bumpers? AM

.a

Example 9.3

The change in the total momentum of the system is equal to the total impulse on the system.

Before

w

In a particular crash test, a car of mass 1 500 kg collides with a wall as shown in Figure 9.4. The initial and final velocities of the car are S v i 5 215.0 i^ m/s S ^ and v f 5 2.60 i m/s, respectively. If the collision lasts 0.150 s, find the impulse caused by the collision and the average net force exerted on the car.

After + 2.60 m/s

S o l u ti o n

Conceptualize The collision time is short, so we can imagine the car being brought to rest very rapidly and then moving back in the opposite direction with a reduced speed. Categorize Let us assume the net force exerted on the car by the wall and friction from the ground is large compared with other forces on the car (such as

a

Hyundai Motors/HO/Landov

w

w

–15.0 m/s

b

Figure 9.4 (Example 9.3) (a) This car’s momentum changes as a result of its collision with the wall. (b) In a crash test, much of the car’s initial kinetic energy is transformed into energy associated with the damage to the car.

continued

256 Chapter 9

Linear Momentum and Collisions

▸ 9.3 c o n t i n u e d air resistance). Furthermore, the gravitational force and the normal force exerted by the road on the car are perpendicular to the motion and therefore do not affect the horizontal momentum. Therefore, we categorize the problem as one in which we can apply the impulse approximation in the horizontal direction. We also see that the car’s momentum changes due to an impulse from the environment. Therefore, we can apply the nonisolated system (momentum) model.

Analyze S

S 5S pf 2 S p i 5 mS v f 2 mS vi 5 m1S vf 2 S vi2 I 5 Dp

Use Equation 9.13 to find the impulse on the car:

5 1 1 500 kg 2 3 2.60 ^i m/s 2 1 215.0 ^i m/s 2 4 5 2.64 3 104 ^i kg # m/s

Use Equation 9.11 to evaluate the average net force exerted on the car:

S

2.64 3 104 ^i kg # m/s I 5 5 1.76 3 105 ^i N Dt 0.150 s

om

1 a F 2 avg 5 S

ee b

ly

.c

Finalize The net force found above is a combination of the normal force on the car from the wall and any friction force between the tires and the ground as the front of the car crumples. If the brakes are not operating while the crash occurs and the crumpling metal does not interfere with the free rotation of the tires, this friction force could be relatively small due to the freely rotating wheels. Notice that the signs of the velocities in this example indicate the reversal of directions. What would the mathematics be describing if both the initial and final velocities had the same sign? W h at I f ? What if the car did not rebound from the wall? Suppose the final velocity of the car is zero and the time interval of the collision remains at 0.150 s. Would that represent a larger or a smaller net force on the car?

s. w

Answer In the original situation in which the car rebounds, the net force on the car does two things during the time

The average net force exerted on the car is

1 a F 2 avg 5

S

ys

ic

interval: (1) it stops the car, and (2) it causes the car to move away from the wall at 2.60 m/s after the collision. If the car does not rebound, the net force is only doing the first of these steps—stopping the car—which requires a smaller force. Mathematically, in the case of the car that does not rebound, the impulse is S S I 5 Dp 5S pf 2 S p i 5 0 2 1 1 500 kg 2 1 215.0 ^i m/s 2 5 2.25 3 104 i^ kg # m/s

2.25 3 104 i^ kg # m/s I 5 5 1.50 3 105 i^ N Dt 0.150 s which is indeed smaller than the previously calculated value, as was argued conceptually.

ar ph

S

sw

9.4 Collisions in One Dimension

w

.a

In this section, we use the isolated system (momentum) model to describe what happens when two particles collide. The term collision represents an event during which two particles come close to each other and interact by means of forces. The interaction forces are assumed to be much greater than any external forces present, so we can use the impulse approximation. A collision may involve physical contact between two macroscopic objects as described in Figure 9.5a, but the notion of what is meant by a collision must be generalized because “physical contact” on a submicroscopic scale is ill-defined and hence meaningless. To understand this concept, consider a collision on an atomic scale (Fig. 9.5b) such as the collision of a proton with an alpha particle (the nucleus of a helium atom). Because the particles are both positively charged, they repel each other due to the strong electrostatic force between them at close separations and never come into “physical contact.” When two particles of masses m1 and m 2 collide as shown in Figure 9.5, the impulsive forces may vary in time in complicated ways, such as that shown in Figure 9.3. Regardless of the complexity of the time behavior of the impulsive force, however, this force is internal to the system of two particles. Therefore, the two particles form an isolated system and the momentum of the system must be conserved in any collision.

S

S

F12

m2

w

m1

w

F21

a

p ++ 4

He

b

Figure 9.5 (a) The collision between two objects as the result of direct contact. (b) The “collision” between two charged particles.

9.4 Collisions in One Dimension 257

om

In contrast, the total kinetic energy of the system of particles may or may not be conserved, depending on the type of collision. In fact, collisions are categorized as being either elastic or inelastic depending on whether or not kinetic energy is conserved. An elastic collision between two objects is one in which the total kinetic energy (as well as total momentum) of the system is the same before and after the collision. Collisions between certain objects in the macroscopic world, such as billiard balls, are only approximately elastic because some deformation and loss of kinetic energy take place. For example, you can hear a billiard ball collision, so you know that some of the energy is being transferred away from the system by sound. An elastic collision must be perfectly silent! Truly elastic collisions occur between atomic and subatomic particles. These collisions are described by the isolated system model for both energy and momentum. Furthermore, there must be no transformation of kinetic energy into other types of energy within the system. An inelastic collision is one in which the total kinetic energy of the system is not the same before and after the collision (even though the momentum of the system is conserved). Inelastic collisions are of two types. When the objects stick together after they collide, as happens when a meteorite collides with the Earth, the collision is called perfectly inelastic. When the colliding objects do not stick together but some kinetic energy is transformed or transferred away, as in the case of a rubber ball colliding with a hard surface, the collision is called inelastic (with no modifying adverb). When the rubber ball collides with the hard surface, some of the ball’s kinetic energy is transformed when the ball is deformed while it is in contact with the surface. Inelastic collisions are described by the momentum version of the isolated system model. The system could be isolated for energy, with kinetic energy transformed to potential or internal energy. If the system is nonisolated, there could be energy leaving the system by some means. In this latter case, there could also be some transformation of energy within the system. In either of these cases, the kinetic energy of the system changes. In the remainder of this section, we investigate the mathematical details for collisions in one dimension and consider the two extreme cases, perfectly inelastic and elastic collisions.

Inelastic Collisions Generally, inelastic collisions are hard to analyze without additional information. Lack of this information appears in the mathematical representation as having more unknowns than equations.

ar ph

ys

ic

s. w

ee b

ly

.c

Pitfall Prevention 9.2

Perfectly Inelastic Collisions

w

.a

sw

Consider two particles of masses m1 and m 2 moving with initial velocities S v 1i and S v 2i along the same straight line as shown in Figure 9.6. The two particles collide headon, stick together, and then move with some common velocity S v f after the collision. Because the momentum of an isolated system is conserved in any collision, we can say that the total momentum before the collision equals the total momentum of the composite system after the collision: S Dp 50

S

S

pi 5 S pf

w

w

Solving for the final velocity gives

S S S S m 1 v 1i 1 m 2 v 2i 5 1 m 1 1 m 2 2 v f

v 1i 1 m 2S v 2i m 1S S vf 5 m1 1 m2

(9.14) m1

(9.15)

v 1i and S v 2i Consider two particles of masses m1 and m 2 moving with initial velocities S along the same straight line as shown in Figure 9.7 on page 258. The two particles collide head-on and then leave the collision site with different velocities, S v 1f and S v 2f . In an elastic collision, both the momentum and kinetic energy of the system are conserved. Therefore, considering velocities along the horizontal direction in Figure 9.7, we have

pi 5 pf S m1v1i 1 m 2v 2i 5 m1v1f 1 m 2v 2f Ki 5 K f S

S

v1i

S

v2i

m2

a

After the collision, the particles move together.

Elastic Collisions

Before the collision, the particles move separately.

1 2 2 m 1v 1i

1 12m 2v 2i 2 5 12m 1v 1f 2 1 12m 2v 2f 2

(9.16) (9.17)

S

m1 m2

vf

b

Figure 9.6 Schematic representation of a perfectly inelastic head-on collision between two particles.

Linear Momentum and Collisions

Because all velocities in Figure 9.7 are either to the left or the right, they can be represented by the corresponding speeds along with algebraic signs indicating direction. We shall indicate v as positive if a particle moves to the right and negative if it moves to the left. In a typical problem involving elastic collisions, there are two unknown quantities, and Equations 9.16 and 9.17 can be solved simultaneously to find them. An alternative approach, however—one that involves a little mathematical manipulation of Equation 9.17—often simplifies this process. To see how, let us cancel the factor 12 in Equation 9.17 and rewrite it by gathering terms with subscript 1 on the left and 2 on the right:

Before the collision, the particles move separately. S

S

v1i

v2i

m1

m2

a

After the collision, the particles continue to move separately with new velocities. S

Factoring both sides of this equation gives

S

v1f

m1(v1i2 2 v1f 2) 5 m 2(v 2f 2 2 v 2i2)

v2f

m1(v1i 2 v1f ) (v1i 1 v1f ) 5 m 2(v 2f 2 v 2i )(v 2f 1 v 2i )

(9.18)

.c

om

258 Chapter 9

b tation of an elastic head-on collision between two particles.

ly

Figure 9.7 Schematic represen-

Next, let us separate the terms containing m1 and m 2 in Equation 9.16 in a similar way to obtain

ee b

m1(v1i 2 v1f ) 5 m 2(v 2f 2 v 2i )

(9.19)

To obtain our final result, we divide Equation 9.18 by Equation 9.19 and obtain v1i 1 v1f 5 v 2f 1 v 2i

s. w

Now rearrange terms once again so as to have initial quantities on the left and final quantities on the right: v1i 2 v 2i 5 2(v1f 2 v 2f ) (9.20)

ic

Pitfall Prevention 9.3 Not a General Equation Equation

sw

ar ph

ys

9.20 can only be used in a very specific situation, a one-d imensional, elastic collision between two objects. The general concept is conservation of momentum (and conservation of kinetic energy if the collision is elastic) for an isolated system.

This equation, in combination with Equation 9.16, can be used to solve problems dealing with elastic collisions. This pair of equations (Eqs. 9.16 and 9.20) is easier to handle than the pair of Equations 9.16 and 9.17 because there are no quadratic terms like there are in Equation 9.17. According to Equation 9.20, the relative velocity of the two particles before the collision, v1i 2 v 2i , equals the negative of their relative velocity after the collision, 2(v1f 2 v 2f ). Suppose the masses and initial velocities of both particles are known. Equations 9.16 and 9.20 can be solved for the final velocities in terms of the initial velocities because there are two equations and two unknowns:

w

w

w

.a

Elastic collision: particle 2 initially at rest

v 1f 5 a v 2f 5 a

m1 2 m2 2m 2 bv 1i 1 a bv m1 1 m2 m 1 1 m 2 2i m2 2 m1 2m 1 bv 1i 1 a bv m1 1 m2 m 1 1 m 2 2i

(9.21) (9.22)

It is important to use the appropriate signs for v1i and v 2i in Equations 9.21 and 9.22. Let us consider some special cases. If m1 5 m 2, Equations 9.21 and 9.22 show that v1f 5 v 2i and v 2f 5 v1i , which means that the particles exchange velocities if they have equal masses. That is approximately what one observes in head-on billiard ball collisions: the cue ball stops and the struck ball moves away from the collision with the same velocity the cue ball had. If particle 2 is initially at rest, then v 2i 5 0, and Equations 9.21 and 9.22 become

v 1f 5 a

v 2f 5 a

m1 2 m2 bv m 1 1 m 2 1i 2m 1 bv m 1 1 m 2 1i

(9.23) (9.24)

If m1 is much greater than m 2 and v 2i 5 0, we see from Equations 9.23 and 9.24 that v1f < v1i and v 2f < 2v1i . That is, when a very heavy particle collides head-on with a

9.4 Collisions in One Dimension 259

ly

.c

Q uick Quiz 9.5 In a perfectly inelastic one-dimensional collision between two moving objects, what condition alone is necessary so that the final kinetic energy of the system is zero after the collision? (a) The objects must have initial momenta with the same magnitude but opposite directions. (b) The objects must have the same mass. (c) The objects must have the same initial velocity. (d) The objects must have the same initial speed, with velocity vectors in opposite directions.

om

very light one that is initially at rest, the heavy particle continues its motion unaltered after the collision and the light particle rebounds with a speed equal to about twice the initial speed of the heavy particle. An example of such a collision is that of a moving heavy atom, such as uranium, striking a light atom, such as hydrogen. If m 2 is much greater than m1 and particle 2 is initially at rest, then v 1f < –v1i and v 2f < 0. That is, when a very light particle collides head-on with a very heavy particle that is initially at rest, the light particle has its velocity reversed and the heavy one remains approximately at rest. For example, imagine what happens when you throw a table tennis ball at a bowling ball as in Quick Quiz 9.6 below.

ic

s. w

ee b

Q uick Quiz 9.6 A table-tennis ball is thrown at a stationary bowling ball. The table-tennis ball makes a one-dimensional elastic collision and bounces back along the same line. Compared with the bowling ball after the collision, does the table-tennis ball have (a) a larger magnitude of momentum and more kinetic energy, (b) a smaller magnitude of momentum and more kinetic energy, (c) a larger magnitude of momentum and less kinetic energy, (d) a smaller magnitude of momentum and less kinetic energy, or (e) the same magnitude of momentum and the same kinetic energy?

ys

Problem-Solving Strategy One-Dimensional Collisions

ar ph

You should use the following approach when solving collision problems in one dimension:

1. Conceptualize. Imagine the collision occurring in your mind. Draw simple diagrams of the particles before and after the collision and include appropriate velocity vectors. At first, you may have to guess at the directions of the final velocity vectors.

sw

2. Categorize. Is the system of particles isolated? If so, use the isolated system

.a

(momentum) model. Further categorize the collision as elastic, inelastic, or perfectly inelastic.

3. Analyze. Set up the appropriate mathematical representation for the problem.

w

w

If the collision is perfectly inelastic, use Equation 9.15. If the collision is elastic, use Equations 9.16 and 9.20. If the collision is inelastic, use Equation 9.16. To find the final velocities in this case, you will need some additional information.

w

4. Finalize. Once you have determined your result, check to see if your answers are consistent with the mental and pictorial representations and that your results are realistic.

Example 9.4

The Executive Stress Reliever AM

An ingenious device that illustrates conservation of momentum and kinetic energy is shown in Figure 9.8 on page 260. It consists of five identical hard balls supported by strings of equal lengths. When ball 1 is pulled out and released, after the almost-elastic collision between it and ball 2, ball 1 stops and ball 5 moves out as shown in Figure 9.8b. If balls 1 and 2 are pulled out and released, they stop after the collision and balls 4 and 5 swing out, and so forth. Is it ever possible that when ball 1 is released, it stops after the collision and balls 4 and 5 will swing out on the opposite side and travel with half the speed of ball 1 as in Figure 9.8c?

continued

260 Chapter 9

Linear Momentum and Collisions

▸ 9.4 c o n t i n u e d S o l u ti o n

This can happen

v

v b

.c

om

This cannot happen

S

S

v/2

ly

v c

Figure 9.8 (Example 9.4) (a) An executive stress reliever. (b) If one ball swings

ee b

Analyze Let’s consider the situation

a

S

S

down, we see one ball swing out at the other end. (c) Is it possible for one ball to swing down and two balls to leave the other end with half the speed of the first ball? In (b) and (c), the velocity vectors shown represent those of the balls immediately before and immediately after the collision.

s. w

Categorize Because of the very short time interval between the arrival of the ball from the left and the departure of the ball(s) from the right, we can use the impulse approximation to ignore the gravitational forces on the balls and model the five balls as an isolated system in terms of both momentum and energy. Because the balls are hard, we can categorize the collisions between them as elastic for purposes of calculation.

© Cengage Learning/Charles D. Winters

Conceptualize With the help of Figure 9.8c, imagine one ball coming in from the left and two balls exiting the collision on the right. That is the phenomenon we want to test to see if it could ever happen.

ar ph

ys

ic

shown in Figure 9.8c. The momentum of the system before the collision is mv, where m is the mass of ball 1 and v is its speed immediately before the collision. After the collision, we imagine that ball 1 stops and balls 4 and 5 swing out, each moving with speed v/2. The total momentum of the system after the collision would be m(v/2) 1 m(v/2) 5 mv. Therefore, the momentum of the system is conserved in the situation shown in Figure 9.8c! The kinetic energy of the system immediately before the collision is K i 5 12mv 2 and that after the collision is K f 5 12m 1 v/2 2 2 1 12m 1 v/2 2 2 5 14mv 2. That shows that the kinetic energy of the system is not conserved, which is inconsistent with our assumption that the collisions are elastic.

Finalize Our analysis shows that it is not possible for balls 4 and 5 to swing out when only ball 1 is released. The only way to conserve both momentum and kinetic energy of the system is for one ball to move out when one ball is released, two balls to move out when two are released, and so on.

sw

Consider what would happen if balls 4 and 5 are glued together. Now what happens when ball 1 is pulled out and released?

W h at I f ?

w

w

w

.a

Answer In this situation, balls 4 and 5 must move together as a single object after the collision. We have argued that both momentum and energy of the system cannot be conserved in this case. We assumed, however, ball 1 stopped after striking ball 2. What if we do not make this assumption? Consider the conservation equations with the assumption that ball 1 moves after the collision. For conservation of momentum, pi 5 pf mv1i 5 mv1f 1 2mv4,5

where v4,5 refers to the final speed of the ball 4–ball 5 combination. Conservation of kinetic energy gives us Ki 5 K f 1 2 2 mv 1i

5

1 2 2 mv 1f

Combining these equations gives v 4,5 5 23v 1i

1 12 1 2m 2 v 4,52 v 1f 5 213v 1i

Therefore, balls 4 and 5 move together as one object after the collision while ball 1 bounces back from the collision with one third of its original speed.

9.4 Collisions in One Dimension 261

Example 9.5

Carry Collision Insurance! AM

An 1 800-kg car stopped at a traffic light is struck from the rear by a 900-kg car. The two cars become entangled, moving along the same path as that of the originally moving car. If the smaller car were moving at 20.0 m/s before the collision, what is the velocity of the entangled cars after the collision? S o l u ti o n

om

Conceptualize This kind of collision is easily visualized, and one can predict that after the collision both cars will be moving in the same direction as that of the initially moving car. Because the initially moving car has only half the mass of the stationary car, we expect the final velocity of the cars to be relatively small.

.c

Categorize We identify the two cars as an isolated system in terms of momentum in the horizontal direction and apply the impulse approximation during the short time interval of the collision. The phrase “become entangled” tells us to categorize the collision as perfectly inelastic. because the larger car is initially at rest. S 50 Dp

S pi 5 pf

ee b

Use the isolated system model for momentum:

ly

Analyze The magnitude of the total momentum of the system before the collision is equal to that of the smaller car

Solve for vf and substitute numerical values:

1 900 kg 2 1 20.0 m/s 2 m 1v i 5 5 6.67 m/s m1 1 m2 900 kg 1 1 800 kg

s. w

vf 5

S m1vi 5 (m1 1 m 2)vf

ic

Finalize Because the final velocity is positive, the direction of the final velocity of the combination is the same as the velocity of the initially moving car as predicted. The speed of the combination is also much lower than the initial speed of the moving car.

ys

W h at I f ? Suppose we reverse the masses of the cars. What if a stationary 900-kg car is struck by a moving 1 800-kg car? Is the final speed the same as before?

ar ph

Answer Intuitively, we can guess that the final speed of the combination is higher than 6.67 m/s if the initially moving car is the more massive car. Mathematically, that should be the case because the system has a larger momentum if the initially moving car is the more massive one. Solving for the new final velocity, we find

sw

vf 5

1 1 800 kg 2 1 20.0 m/s 2 m 1v i 5 5 13.3 m/s m1 1 m2 1 800 kg 1 900 kg

which is two times greater than the previous final velocity.

.a

The Ballistic Pendulum AM

w

w

Example 9.6

w

The ballistic pendulum (Fig. 9.9, page 262) is an apparatus used to measure the speed of a fast-moving projectile such as a bullet. A projectile of mass m1 is fired into a large block of wood of mass m 2 suspended from some light wires. The projectile embeds in the block, and the entire system swings through a height h. How can we determine the speed of the projectile from a measurement of h? S o l u ti o n

Conceptualize Figure 9.9a helps conceptualize the situation. Run the animation in your mind: the projectile enters the pendulum, which swings up to some height at which it momentarily comes to rest. Categorize The projectile and the block form an isolated system in terms of momentum if we identify configuration A as immediately before the collision and configuration B as immediately after the collision. Because the projectile imbeds in the block, we can categorize the collision between them as perfectly inelastic. Analyze To analyze the collision, we use Equation 9.15, which gives the speed of the system immediately after the colcontinued lision when we assume the impulse approximation.

262 Chapter 9

Linear Momentum and Collisions

S

v1A

m1

vB

m2

h

a

om

m1 m2 S

© Cengage Learning/Charles D. Winters

▸ 9.6 c o n t i n u e d

b

ly

.c

Figure 9.9 (Example 9.6) (a) Diagram of a ballistic pendulum. Notice that Sv 1A is the velocity of the projectile immevB is the velocity of the projectile–block system immediately after the perfectly inelasdiately before the collision and S tic collision. (b) Multiflash photograph of a ballistic pendulum used in the laboratory. m 1v 1A m1 1 m2

ee b

(1) v B 5

Noting that v 2A 5 0, solve Equation 9.15 for vB :

ys

the system immediately after the collision:

(2) K B 5 12 1 m 1 1 m 2 2 v B 2

ic

Analyze Write an expression for the total kinetic energy of

s. w

Categorize For the process during which the projectile–block combination swings upward to height h (ending at a configuration we’ll call C), we focus on a different system, that of the projectile, the block, and the Earth. We categorize this part of the problem as one involving an isolated system for energy with no nonconservative forces acting.

KB 5

ar ph

Substitute the value of vB from Equation (1) into Equation (2):

m 12v 1A 2 2 1 m1 1 m2 2

sw

This kinetic energy of the system immediately after the collision is less than the initial kinetic energy of the projectile as is expected in an inelastic collision. We define the gravitational potential energy of the system for configuration B to be zero. Therefore, UB 5 0, whereas UC 5 (m1 1 m 2)gh. Apply the isolated system model to the system:

w

.a

Substitute the energies:

w

Solve for v1A:

DK 1 DU 5 0 a0 2

S (K C 2 K B ) 1 (UC 2 UB ) 5 0

m 12v 1A 2 2 1 m1 1 m2 2

v 1A 5 a

b 1 3 1 m 1 1 m 2 2 gh 2 0 4 5 0

m1 1 m2 b"2gh m1

w

Finalize We had to solve this problem in two steps. Each step involved a different system and a different analysis model: isolated system (momentum) for the first step and isolated system (energy) for the second. Because the collision was assumed to be perfectly inelastic, some mechanical energy was transformed to internal energy during the collision. Therefore, it would have been incorrect to apply the isolated system (energy) model to the entire process by equating the initial kinetic energy of the incoming projectile with the final gravitational potential energy of the projectile– block–Earth combination.

Example 9.7

A Two-Body Collision with a Spring AM

A block of mass m1 5 1.60 kg initially moving to the right with a speed of 4.00 m/s on a frictionless, horizontal track collides with a light spring attached to a second block of mass m 2 5 2.10 kg initially moving to the left with a speed of 2.50 m/s as shown in Figure 9.10a. The spring constant is 600 N/m.

9.4 Collisions in One Dimension 263

▸ 9.7 c o n t i n u e d (A) Find the velocities of the two blocks after the collision. S o l u ti o n

v1i 4.00iˆ m/s

v2i –2.50iˆ m/s

S

Conceptualize With the help of Figure 9.10a, run an animation of the collision in your mind. Figure 9.10b shows an instant during the collision when the spring is compressed. Eventually, block 1 and the spring will again separate, so the system will look like Figure 9.10a again but with different velocity vectors for the two blocks.

S

k

m1

v1f 3.00iˆ m/s

S

m2

m1

S

v2f

k

m2

x

om

a b

the system is conserved, apply Equation 9.16:

(1) m1v1i 1 m 2v 2i 5 m1v1f 1 m 2v 2f

s. w

Analyze Because momentum of

ee b

ly

.c

Categorize Because the spring force is conservative, Figure 9.10 (Example 9.7) A moving block approaches a second kinetic energy in the system of two blocks and the moving block that is attached to a spring. spring is not transformed to internal energy during the compression of the spring. Ignoring any sound made when the block hits the spring, we can categorize the collision as being elastic and the two blocks and the spring as an isolated system for both energy and momentum.

(2) v1i 2 v 2i 5 2(v1f 2 v 2f )

Multiply Equation (2) by m1:

(3) m1v1i 2 m1v 2i 5 2m1v1f 1 m1v 2f

Add Equations (1) and (3):

2m1v1i 1 (m 2 2 m1)v 2i 5 (m1 1 m 2)v 2f

Solve for v 2f :

v 2f 5

ys

ic

Because the collision is elastic, apply Equation 9.20:

ar ph

2m 1v 1i 1 1 m 2 2 m 1 2 v 2i m1 1 m2

2 1 1.60 kg 2 1 4.00 m/s 2 1 1 2.10 kg 2 1.60 kg 2 1 22.50 m/s 2

5 3.12 m/s

v 2f 5

Solve Equation (2) for v1f and substitute numerical values:

v1f 5 v 2f 2 v1i 1 v 2i 5 3.12 m/s 2 4.00 m/s 1 (22.50 m/s) 5 2 3.38 m/s

sw

Substitute numerical values:

1.60 kg 1 2.10 kg

w

.a

(B) Determine the velocity of block 2 during the collision, at the instant block 1 is moving to the right with a velocity of 13.00 m/s as in Figure 9.10b. S o l u ti o n

w

w

Conceptualize Focus your attention now on Figure 9.10b, which represents the final configuration of the system for the time interval of interest. Categorize Because the momentum and mechanical energy of the isolated system of two blocks and the spring are conserved throughout the collision, the collision can be categorized as elastic for any final instant of time. Let us now choose the final instant to be when block 1 is moving with a velocity of 13.00 m/s. Analyze Apply Equation 9.16:

m1v1i 1 m 2v 2i 5 m1v1f 1 m 2v 2f

Solve for v 2f :

v 2f 5

Substitute numerical values:

v 2f 5

m 1v 1i 1 m 2v 2i 2 m 1v 1f m2

1 1.60 kg 2 1 4.00 m/s 2 1 1 2.10 kg 2 1 22.50 m/s 2 2 1 1.60 kg 2 1 3.00 m/s 2 2.10 kg

5 21.74 m/s

continued

264 Chapter 9

Linear Momentum and Collisions

▸ 9.7 c o n t i n u e d Finalize The negative value for v 2f means that block 2 is still moving to the left at the instant we are considering. (C) Determine the distance the spring is compressed at that instant. S o l u ti o n

Conceptualize Once again, focus on the configuration of the system shown in Figure 9.10b.

om

Categorize For the system of the spring and two blocks, no friction or other nonconservative forces act within the system. Therefore, we categorize the system as an isolated system in terms of energy with no nonconservative forces acting. The system also remains an isolated system in terms of momentum. Analyze We choose the initial configuration of the system to be that existing immediately before block 1 strikes the

.c

spring and the final configuration to be that when block 1 is moving to the right at 3.00 m/s. DK 1 DU 5 0

Evaluate the energies, recognizing that two objects in the system have kinetic energy and that the potential energy is elastic:

3 1 12m 1v 1f 2 1 12m 2v 2f2 2 2 1 12m 1v 1i2 1 12m 2v 2i2 2 4 1 1 12kx 2 2 0 2 5 0

Substitute numerical values:

ee b

x 2 5 1k 3 m 1 1 v 1i2 2 v 1f2 2 1 m 2 1 v 2i2 2 v 2f2 2 4

x2 5 a

s. w

Solve for x 2:

ly

Write the appropriate reduction of Equation 8.2:

1 b 5 1 1.60 kg 2 3 1 4.00 m/s 2 2 2 1 3.00 m/s 2 2 4 1 1 2.10 kg 2 3 1 2.50 m/s 2 2 2 1 1.74 m/s 2 2 4 6 600 N/m

ic

S x 5 0.173 m

ys

Finalize This answer is not the maximum compression of the spring because the two blocks are still moving toward each other at the instant shown in Figure 9.10b. Can you determine the maximum compression of the spring?

ar ph

9.5 Collisions in Two Dimensions In Section 9.2, we showed that the momentum of a system of two particles is conserved when the system is isolated. For any collision of two particles, this result implies that the momentum in each of the directions x, y, and z is conserved. An important subset of collisions takes place in a plane. The game of billiards is a familiar example involving multiple collisions of objects moving on a two-dimensional surface. For such two-dimensional collisions, we obtain two component equations for conservation of momentum:

sw

Before the collision S

.a

v1i

m1

w

m2

w

a

m1v1ix 1 m 2v 2ix 5 m1v1fx 1 m 2v 2fx

w

After the collision

v1f

v1f sin θ

θ φ v2f sin φ

m1v1iy 1 m 2v 2iy 5 m1v1fy 1 m 2v 2fy

S

v1f cos θ v2f cos φ S

v2f

b

Figure 9.11 An elastic, glancing collision between two particles.

where the three subscripts on the velocity components in these equations represent, respectively, the identification of the object (1, 2), initial and final values (i, f ), and the velocity component (x, y). Let us consider a specific two-dimensional problem in which particle 1 of mass m1 collides with particle 2 of mass m2 initially at rest as in Figure 9.11. After the collision (Fig. 9.11b), particle 1 moves at an angle u with respect to the horizontal and particle 2 moves at an angle f with respect to the horizontal. This event is called a glancing collision. Applying the law of conservation of momentum in component form and noting that the initial y component of the momentum of the two-particle system is zero gives

Dpx 5 0

S pix 5 pfx

S m1v1i 5 m1v1f cos u 1 m 2v 2f cos f

(9.25)

Dpy 5 0

S piy 5 pf y

S

0 5 m1v1f sin u 2 m 2v 2f sin f

(9.26)

9.5 Collisions in Two Dimensions 265

where the minus sign in Equation 9.26 is included because after the collision particle 2 has a y component of velocity that is downward. (The symbols v in these particular equations are speeds, not velocity components. The direction of the component vector is indicated explicitly with plus or minus signs.) We now have two independent equations. As long as no more than two of the seven quantities in Equations 9.25 and 9.26 are unknown, we can solve the problem. If the collision is elastic, we can also use Equation 9.17 (conservation of kinetic energy) with v 2i 5 0:

Ki 5 Kf

S

1 2 2 m 1v 1i

5 12m 1v 1f 2 1 12m 2v 2f 2

(9.27)

om

Knowing the initial speed of particle 1 and both masses, we are left with four unknowns (v1f , v 2f , u, and f). Because we have only three equations, one of the four remaining quantities must be given to determine the motion after the elastic collision from conservation principles alone. If the collision is inelastic, kinetic energy is not conserved and Equation 9.27 does not apply.

ee b

Problem-Solving Strategy Two-Dimensional Collisions

ly

.c

Pitfall Prevention 9.4

The following procedure is recommended when dealing with problems involving collisions between two particles in two dimensions.

tion 9.20, relating the initial and final relative velocities of two colliding objects, is only valid for one-dimensional elastic collisions. Do not use this equation when analyzing two-dimensional collisions.

s. w

1. Conceptualize. Imagine the collisions occurring and predict the approximate

Don’t Use Equation 9.20 Equa-

ic

directions in which the particles will move after the collision. Set up a coordinate system and define your velocities in terms of that system. It is convenient to have the x axis coincide with one of the initial velocities. Sketch the coordinate system, draw and label all velocity vectors, and include all the given information. as elastic, inelastic, or perfectly inelastic.

ys

2. Categorize. Is the system of particles truly isolated? If so, categorize the collision

ar ph

3. Analyze. Write expressions for the x and y components of the momentum of each

w

w

w

.a

sw

object before and after the collision. Remember to include the appropriate signs for the components of the velocity vectors and pay careful attention to signs throughout the calculation. S Apply the isolated system model for momentum Dp 5 0. When applied in each direction, this equation will generally reduce to pix 5 pfx and piy 5 pf y, where each of these terms refer to the sum of the momenta of all objects in the system. Write expressions for the total momentum in the x direction before and after the collision and equate the two. Repeat this procedure for the total momentum in the y direction. Proceed to solve the momentum equations for the unknown quantities. If the collision is inelastic, kinetic energy is not conserved and additional information is probably required. If the collision is perfectly inelastic, the final velocities of the two objects are equal. If the collision is elastic, kinetic energy is conserved and you can equate the total kinetic energy of the system before the collision to the total kinetic energy after the collision, providing an additional relationship between the velocity magnitudes.

4. Finalize. Once you have determined your result, check to see if your answers are consistent with the mental and pictorial representations and that your results are realistic.

Example 9.8

Collision at an Intersection AM

A 1 500-kg car traveling east with a speed of 25.0 m/s collides at an intersection with a 2 500-kg truck traveling north at a speed of 20.0 m/s as shown in Figure 9.12 on page 266. Find the direction and magnitude of the velocity of the wreckage after the collision, assuming the vehicles stick together after the collision.

continued

266 Chapter 9

Linear Momentum and Collisions

▸ 9.8 c o n t i n u e d S o l u ti o n

Conceptualize Figure 9.12 should help you conceptualize the situation before and after the collision. Let us choose east to be along the positive x direction and north to be along the positive y direction.

u

20.0jˆ m/s

S

o p

Apply the isolated system model for momentum in the y direction:

Dpy 5 0

S

o p

xi

.c

ly

Figure 9.12 (Example 9.8) An eastbound car colliding with a northbound truck.

ee b

Dpx 5 0

x

5

o p

5

o p

xf

S (1) m1v1i 5 (m1 1 m 2)vf cos u

S (2) m 2v 2i 5 (m1 1 m 2)vf sin u

s. w

Apply the isolated system model for momentum in the x direction:

vf

25.0iˆ m/s

Analyze Before the collision, the only object having momentum in the x direction is the car. Therefore, the magnitude of the total initial momentum of the system (car plus truck) in the x direction is that of only the car. Similarly, the total initial momentum of the system in the y direction is that of the truck. After the collision, let us assume the wreckage moves at an angle u with respect to the x axis with speed vf .

S

om

Categorize Because we consider moments immediately before and immediately after the collision as defining our time interval, we ignore the small effect that friction would have on the wheels of the vehicles and model the two vehicles as an isolated system in terms of momentum. We also ignore the vehicles’ sizes and model them as particles. The collision is perfectly inelastic because the car and the truck stick together after the collision.

y

yi

yf

m 2v 2i sin u 5 5 tan u m 1v 1i cos u

ic

Divide Equation (2) by Equation (1):

Use Equation (2) to find the value of vf and substitute numerical values:

1 2 500 kg 2 1 20.0 m/s 2 m 2v 2i b 5 tan21 c d 5 53.18 m 1v 1i 1 1 500 kg 2 1 25.0 m/s 2

ys

u 5 tan21 a

ar ph

Solve for u and substitute numerical values:

vf 5

1 2 500 kg 2 1 20.0 m/s 2 m 2v 2i 5 5 15.6 m/s 1 m 1 1 m 2 2 sin u 1 1 500 kg 1 2 500 kg 2 sin 53.18

.a

sw

Finalize Notice that the angle u is qualitatively in agreement with Figure 9.12. Also notice that the final speed of the combination is less than the initial speeds of the two cars. This result is consistent with the kinetic energy of the system being reduced in an inelastic collision. It might help if you draw the momentum vectors of each vehicle before the collision and the two vehicles together after the collision.

Proton–Proton Collision AM

w

w

Example 9.9

w

A proton collides elastically with another proton that is initially at rest. The incoming proton has an initial speed of 3.50 3 105 m/s and makes a glancing collision with the second proton as in Figure 9.11. (At close separations, the protons exert a repulsive electrostatic force on each other.) After the collision, one proton moves off at an angle of 37.08 to the original direction of motion and the second deflects at an angle of f to the same axis. Find the final speeds of the two protons and the angle f. S o l u ti o n

Conceptualize This collision is like that shown in Figure 9.11, which will help you conceptualize the behavior of the system. We define the x axis to be along the direction of the velocity vector of the initially moving proton.

Categorize The pair of protons form an isolated system. Both momentum and kinetic energy of the system are conserved in this glancing elastic collision.

9.6 The Center of Mass 267

▸ 9.9 c o n t i n u e d Analyze Using the isolated system model for both momentum and energy for a two-dimensional elastic collision, set up the mathematical representation with Equations 9.25 through 9.27:

(1)

v1i 5 v1f cos u 1 v 2f cos f

(2)

0 5 v1f sin u 2 v 2f sin f

Rearrange Equations (1) and (2):

v 2f cos f 5 v1i 2 v1f cos u

(3) v1i2 5 v1f 2 1 v 2f 2 v 2f sin f 5 v1f sin u v 2f 2 cos2 f 1 v 2f 2 sin2 f 5 v1i2 2 2v1iv1f cos u 1 v1f 2 cos2 u 1 v1f 2 sin2 u

Incorporate that the sum of the squares of sine and cosine for any angle is equal to 1:

(4) v 2f 2 5 v1i2 2 2v1iv1f cos u 1 v1f 2

Substitute Equation (4) into Equation (3):

v1f 2 1 (v1i2 2 2v1iv1f cos u 1 v1f 2 ) 5 v1i2

.c

om

Square these two equations and add them:

ly

(5) v1f 2 2 v1iv1f cos u 5 0

ee b

One possible solution of Equation (5) is v1f 5 0, which corresponds to a head-on, one-dimensional collision in which the first proton stops and the second continues with the same speed in the same direction. That is not the solution we want. v1f 5 v1i cos u 5 (3.50 3 105 m/s) cos 37.08 5 2.80 3 105 m/s

Use Equation (3) to find v 2f :

v 2f 5 "v 1i 2 2 v 1f 2 5 " 1 3.50 3 105 m/s 2 2 2 1 2.80 3 105 m/s 2 2

s. w

Divide both sides of Equation (5) by v1f and solve for the remaining factor of v1f :

5 2.11 3 105 m/s

(2) f 5 sin21 a

ys

ic

Use Equation (2) to find f:

5 53.08

v 1f sin u v 2f

b 5 sin21 B

1 2.80 3 105 m/s 2 sin 37.08 R 1 2.11 3 105 m/s 2

ar ph

Finalize It is interesting that u 1 f 5 908. This result is not accidental. Whenever two objects of equal mass collide elastically in a glancing collision and one of them is initially at rest, their final velocities are perpendicular to each other.

sw

9.6 The Center of Mass

w

w

w

.a

In this section, we describe the overall motion of a system in terms of a special point called the center of mass of the system. The system can be either a small number of particles or an extended, continuous object, such as a gymnast leaping through the air. We shall see that the translational motion of the center of mass of the system is the same as if all the mass of the system were concentrated at that point. That is, the system moves as if the net external force were applied to a single particle located at the center of mass. This model, the particle model, was introduced in Chapter 2. This behavior is independent of other motion, such as rotation or vibration of the system or deformation of the system (for instance, when a gymnast folds her body). Consider a system consisting of a pair of particles that have different masses and are connected by a light, rigid rod (Fig. 9.13 on page 268). The position of the center of mass of a system can be described as being the average position of the system’s mass. The center of mass of the system is located somewhere on the line joining the two particles and is closer to the particle having the larger mass. If a single force is applied at a point on the rod above the center of mass, the system rotates clockwise (see Fig. 9.13a). If the force is applied at a point on the rod below the center of mass, the system rotates counterclockwise (see Fig. 9.13b). If the force

Linear Momentum and Collisions

is applied at the center of mass, the system moves in the direction of the force without rotating (see Fig. 9.13c). The center of mass of an object can be located with this procedure. The center of mass of the pair of particles described in Figure 9.14 is located on the x axis and lies somewhere between the particles. Its x coordinate is given by m 1x 1 1 m 2x 2 x CM ; (9.28) m1 1 m2

The system rotates clockwise when a force is applied above the center of mass.

CM

The system rotates counterclockwise when a force is applied below the center of mass.

om

For example, if x 1 5 0, x 2 5 d, and m 2 5 2m 1, we find that x CM 5 23d. That is, the center of mass lies closer to the more massive particle. If the two masses are equal, the center of mass lies midway between the particles. We can extend this concept to a system of many particles with masses mi in three dimensions. The x coordinate of the center of mass of n particles is defined to be

a

a m ix i a m ix i m 1 x 1 1 m 2 x 2 1 m 3 x 3 1 c 1 m nx n 1 i i 5 5 5 a m ix i m 1 1 m 2 1 m 3 1 c1 m n M M i a mi i (9.29)

ly

CM

x CM ;

.c

268 Chapter 9

yCM ;

1 m i yi and Ma i

z CM ;

s. w

The system moves in the direction of the force without rotating when a force is applied at the center of mass.

ee b

where xi is the x coordinate of the ith particle and the total mass is M ; oi mi where the sum runs over all n particles. The y and z coordinates of the center of mass are similarly defined by the equations

b

1 m i zi Ma i

(9.30)

ic

The center of mass can be located in three dimensions by its position vector S r CM. The components of this vector are x CM, y CM, and z CM, defined in Equations 9.29 and 9.30. Therefore, 1 1 1 S r CM 5 x CM ^i 1 yCM j^ 1 z CM k^ 5 a m i x i ^i 1 a m i yi j^ 1 a m i z i k^ M i M i M i

c

Figure 9.13 A force is applied

to a system of two particles of unequal mass connected by a light, rigid rod.

m2

x

w

m1

w

CM x2

r CM ;

1 m iS r i Ma i

(9.31)

S

r i ; x i ^i 1 yi j^ 1 zi k^

Although locating the center of mass for an extended, continuous object is somewhat more cumbersome than locating the center of mass of a small number of particles, the basic ideas we have discussed still apply. Think of an extended object as a system containing a large number of small mass elements such as the cube in Figure 9.15. Because the separation between elements is very small, the object can be considered to have a continuous mass distribution. By dividing the object into elements of mass Dmi with coordinates xi , yi , z i , we see that the x coordinate of the center of mass is approximately

.a

x CM

S

where S r i is the position vector of the ith particle, defined by

sw

y

x1

ar ph

ys

CM

w

Figure 9.14 The center of mass of two particles of unequal mass on the x axis is located at x CM, a point between the particles, closer to the one having the larger mass.

x CM <

1 x i Dm i M a i

with similar expressions for y CM and z CM. If we let the number of elements n approach infinity, the size of each element approaches zero and x CM is given precisely. In this limit, we replace the sum by an integral and Dmi by the differential element dm:

x CM 5 lim Dm i S

0

1 1 x i Dm i 5 x dm a M i M3

Likewise, for y CM and z CM we obtain

y CM 5

1 y dm M3

and

z CM 5

1 z dm M3

(9.32)

(9.33)

9.6 The Center of Mass 269

We can express the vector position of the center of mass of an extended object in the form S

r CM 5

1 S r dm M3

An extended object can be considered to be a distribution of small elements of mass mi .

(9.34)

mi CM S

ri

S

rCM

x

z

om

which is equivalent to the three expressions given by Equations 9.32 and 9.33. The center of mass of any symmetric object of uniform density lies on an axis of symmetry and on any plane of symmetry. For example, the center of mass of a uniform rod lies in the rod, midway between its ends. The center of mass of a sphere or a cube lies at its geometric center. Because an extended object is a continuous distribution of mass, each small mass element is acted upon by the gravitational force. The net effect of all these forces is equivalent to the effect of a single force M S g acting through a special point, called the center of gravity. If S g is constant over the mass distribution, the center of gravity coincides with the center of mass. If an extended object is pivoted at its center of gravity, it balances in any orientation. The center of gravity of an irregularly shaped object such as a wrench can be determined by suspending the object first from one point and then from another. In Figure 9.16, a wrench is hung from point A and a vertical line AB (which can be established with a plumb bob) is drawn when the wrench has stopped swinging. The wrench is then hung from point C, and a second vertical line CD is drawn. The center of gravity is halfway through the thickness of the wrench, under the intersection of these two lines. In general, if the wrench is hung freely from any point, the vertical line through this point must pass through the center of gravity.

y

Figure 9.15 The center of mass

ly

.c

is located at the vector position S r CM, which has coordinates x CM, y CM, and z CM.

s. w

ee b

The wrench is hung freely first from point A and then from point C.

A

Q uick Quiz 9.7 A baseball bat of uniform density is cut at the location of its center of mass as shown in Figure 9.17. Which piece has the smaller mass? (a) the piece on the right (b) the piece on the left (c) both pieces have the same mass (d) impossible to determine

ic

B

Figure 9.17 (Quick

The intersection of the two lines AB and CD locates the center of gravity.

Quiz 9.7) A baseball bat cut at the location of its center of mass.

sw

ar ph

ys

C

A

B

D

Figure 9.16 A n experimental

w

.a

technique for determining the center of gravity of a wrench.

w

Example 9.10 The Center of Mass of Three Particles

w

A system consists of three particles located as shown in Figure 9.18. Find the center of mass of the system. The masses of the particles are m1 5 m 2 5 1.0 kg and m 3 5 2.0 kg.

masses. Your intuition should tell you that the center of mass is located somewhere in the region between the blue particle and the pair of tan particles as shown in the figure.

3 2 m3

S o l u ti o n

Conceptualize Figure 9.18 shows the three

y (m)

Figure 9.18 (Example 9.10) Two particles are located on the x axis, and a single particle is located on the y axis as shown. The vector indicates the location of the system’s center of mass.

1 S

rCM

0

m1

m2

1

2

3

x (m)

Categorize We categorize this example as a substitution problem because we will be using the equations for the center of mass developed in this section. continued

Linear Momentum and Collisions

Use the defining equations for the coordinates of the center of mass and notice that z CM 5 0:

x CM 5 5 y CM 5 5

m 1x 1 1 m 2x 2 1 m 3x 3 1 m i xi 5 Ma m1 1 m2 1 m3 i 1 1.0 kg 2 1 1.0 m 2 1 1 1.0 kg 2 1 2.0 m 2 1 1 2.0 kg 2 1 0 2 1.0 kg 1 1.0 kg 1 2.0 kg

1 1.0 kg 2 1 0 2 1 1 1.0 kg 2 1 0 2 1 1 2.0 kg 2 1 2.0 m 2 4.0 kg

4.0 kg

5 0.75 m

5

4.0 kg

ee b

Example 9.11 The Center of Mass of a Rod

(A) Show that the center of mass of a rod of mass M and length L lies midway between its ends, assuming the rod has a uniform mass per unit length.

5 1.0 m

y

s. w

S o l u ti o n

x

ic

x dx

Categorize We categorize this example as an analysis problem because we need

ys

Figure 9.19 (Example 9.11) The geometry used to find the center of mass of a uniform rod.

to divide the rod into small mass elements to perform the integration in Equation 9.32.

ar ph

dm = l dx L

Conceptualize The rod is shown aligned along the x axis in Figure 9.19, so y CM 5

z CM 5 0. What is your prediction of the value of x CM?

4.0 kg # m

.c

r CM ; x CM ^i 1 y CM ^j 5 1 0.75 ^i 1 1.0 ^j 2 m

3.0 kg # m

m 1y1 1 m 2y2 1 m 3y3 1 m i yi 5 Ma m1 1 m2 1 m3 i

S

Write the position vector of the center of mass:

5

om

▸ 9.10 c o n t i n u e d

ly

270 Chapter 9

Analyze The mass per unit length (this quantity is called the linear mass density) can be written as l 5 M/L for the uniform rod. If the rod is divided into elements of length dx, the mass of each element is dm 5 l dx.

Substitute l 5 M/L:

x CM 5

sw

x CM 5

.a

1 1 l x 2 L lL2 ` 5 3 x dm 5 3 x l dx 5 M M 0 M 2 0 2M L

Use Equation 9.32 to find an expression for x CM:

1 2L

w

One can also use symmetry arguments to obtain the same result.

L2 M a b5 2M L

w

w

(B) S uppose a rod is nonuniform such that its mass per unit length varies linearly with x according to the expression l 5 ax, where a is a constant. Find the x coordinate of the center of mass as a fraction of L. S o l u ti o n

Conceptualize Because the mass per unit length is not constant in this case but is proportional to x, elements of the rod to the right are more massive than elements near the left end of the rod. Categorize This problem is categorized similarly to part (A), with the added twist that the linear mass density is not constant.

Analyze In this case, we replace dm in Equation 9.32 by l dx, where l 5 ax.

1 1 1 x dm 5 x l dx 5 x ax dx M 3 M 30 M 30 L

Use Equation 9.32 to find an expression for x CM:

x CM 5

a aL3 2 3 x dx 5 M 0 3M L

5

L

9.6 The Center of Mass 271

▸ 9.11 c o n t i n u e d M 5 3 dm 5 3 l dx 5 3 ax dx 5 L

Find the total mass of the rod:

L

0

Substitute M into the expression for x CM:

x CM 5

0

aL3 5 3aL2/2

aL2 2

2 3L

Finalize Notice that the center of mass in part (B) is farther to the right than that in part (A). That result is reasonable because the elements of the rod become more massive as one moves to the right along the rod in part (B).

om

.c

Example 9.12 The Center of Mass of a Right Triangle

ee b

ly

You have been asked to hang a metal sign from a single vertical string. The sign has the triangular shape shown in Figure 9.20a. The bottom of the sign is to be parallel to the ground. At what distance from the left end of the sign should you attach the support string? S o l u ti o n

s. w

Conceptualize Figure 9.20a shows the sign hanging from the string. The string must be attached at a point directly above the center of gravity of the sign, which is the same as its center of mass because it is in a uniform gravitational field.

Joe’s Cheese Shop a y

dm

ic

Categorize A s in the case of Example 9.11, we categorize this example as an analysis

ys

problem because it is necessary to identify infinitesimal mass elements of the sign to perform the integration in Equation 9.32.

ar ph

Analyze We assume the triangular sign has a uniform density and total mass M.

w

.a

sw

Because the sign is a continuous distribution of mass, we must use the integral expression in Equation 9.32 to find the x coordinate of the center of mass. We divide the triangle into narrow strips of width dx and height y as shown in Figure 9.20b, where y is the height of the hypotenuse of the triangle above the x axis for a given value of x. The mass of each strip is the product of the volume of the strip and the density r of the material from which the sign is made: dm 5 ryt dx, where t is the thickness of the metal sign. The density of the material is the total mass of the sign divided by its total volume (area of the triangle times thickness).

w

Evaluate dm:

w

Use Equation 9.32 to find the x coordinate of the center of mass:

c

b

y dx O

x

x

a b

Figure 9.20 (Example 9.12) (a) A triangular sign to be hung from a single string. (b) Geometric construction for locating the center of mass.

2My M dx byt dx 5 ab abt 2

dm 5 ryt dx 5 a 1 (1) x CM 5

a a 2My 1 1 2 x dm 5 x dx 5 xy dx M 3 M 30 ab ab 30

To proceed further and evaluate the integral, we must express y in terms of x. The line representing the hypotenuse of the triangle in Figure 9.20b has a slope of b/a and passes through the origin, so the equation of this line is y 5 (b/a)x. Substitute for y in Equation (1):

2 b 2 2 x3 a 2 3 x a a xbdx 5 2 3 x dx 5 2 c d ab 0 a 0 a 3 0 a

x CM 5

a

5 23a

Therefore, the string must be attached to the sign at a distance two-thirds of the length of the bottom edge from the left end.

continued

272 Chapter 9

Linear Momentum and Collisions

▸ 9.12 c o n t i n u e d Finalize This answer is identical to that in part (B) of Example 9.11. For the triangular sign, the linear increase in height y with position x means that elements in the sign increase in mass linearly along the x axis, just like the linear increase in mass density in Example 9.11. We could also find the y coordinate of the center of mass of the sign, but that is not needed to determine where the string should be attached. You might try cutting a right triangle out of cardboard and hanging it from a string so that the long base is horizontal. Does the string need to be attached at 23a?

om

9.7 Systems of Many Particles

Velocity of the center of mass of a system of particles

S

v CM 5

ee b

ly

.c

Consider a system of two or more particles for which we have identified the center of mass. We can begin to understand the physical significance and utility of the center of mass concept by taking the time derivative of the position vector for the center of mass given by Equation 9.31. From Section 4.1, we know that the time derivative of a position vector is by definition the velocity vector. Assuming M remains constant for a system of particles—that is, no particles enter or leave the system—we obtain the following expression for the velocity of the center of mass of the system: dS r CM dS ri 1 1 5 a mi 5 a m iS v i dt M i dt M i

(9.35)

Total momentum of a system of particles

s. w

where S v i is the velocity of the ith particle. Rearranging Equation 9.35 gives MS v CM 5 a m iS vi 5 aS pi 5 S p tot

i

(9.36)

i

Acceleration of the center of mass of a system of particles

ar ph

ys

ic

Therefore, the total linear momentum of the system equals the total mass multiplied by the velocity of the center of mass. In other words, the total linear momentum of the system is equal to that of a single particle of mass M moving with a velocity S v CM. Differentiating Equation 9.35 with respect to time, we obtain the acceleration of the center of mass of the system: S

a CM 5

dS v CM dS vi 1 1 5 a mi 5 a m iS a i dt M i dt M i

(9.37)

sw

Rearranging this expression and using Newton’s second law gives

w

w

w

.a

Newton’s second law for a system of particles

MS a CM 5 a m iS a i 5 a F i S

i

(9.38)

i

S

where F i is the net force on particle i. The forces on any particle in the system may include both external forces (from outside the system) and internal forces (from within the system). By Newton’s third law, however, the internal force exerted by particle 1 on particle 2, for example, is equal in magnitude and opposite in direction to the internal force exerted by particle 2 on particle 1. Therefore, when we sum over all internal force vectors in Equation 9.38, they cancel in pairs and we find that the net force on the system is caused only by external forces. We can then write Equation 9.38 in the form

S (9.39) a F ext 5 M a CM That is, the net external force on a system of particles equals the total mass of the system multiplied by the acceleration of the center of mass. Comparing Equation 9.39 with Newton’s second law for a single particle, we see that the particle model we have used in several chapters can be described in terms of the center of mass:

S

The center of mass of a system of particles having combined mass M moves like an equivalent particle of mass M would move under the influence of the net external force on the system.

9.7 Systems of Many Particles 273

Let us integrate Equation 9.39 over a finite time interval:

dS v CM dt 5 M 3 d S v CM 5 M DS v CM 3 a F ext dt 5 3 M a CM dt 5 3 M dt S

S

Notice that this equation can be written as S

S Dp tot 5 I

(9.40)

S

S

om

where I is the impulse imparted to the system by external forces and p tot is the momentum of the system. Equation 9.40 is the generalization of the impulse– momentum theorem for a particle (Eq. 9.13) to a system of many particles. It is also the mathematical representation of the nonisolated system (momentum) model for a system of many particles. Finally, if the net external force on a system is zero so that the system is isolated, it follows from Equation 9.39 that

WW Impulse–momentum theorem for a system of particles

.c

dS v CM 50 dt Therefore, the isolated system model for momentum for a system of many particles is described by S Dp tot 5 0

p tot 5 constant MS v CM 5 S

(9.41)

1 when a F ext 5 0 2 S

s. w

which can be rewritten as

ee b

ly

MS a CM 5 M

(9.42)

sw

ar ph

ys

ic

That is, the total linear momentum of a system of particles is conserved if no net external force is acting on the system. It follows that for an isolated system of particles, both the total momentum and the velocity of the center of mass are constant in time. This statement is a generalization of the isolated system (momentum) model for a many-particle system. Suppose the center of mass of an isolated system consisting of two or more members is at rest. The center of mass of the system remains at rest if there is no net force on the system. For example, consider a system of a swimmer standing on a raft, with the system initially at rest. When the swimmer dives horizontally off the raft, the raft moves in the direction opposite that of the swimmer and the center of mass of the system remains at rest (if we neglect friction between raft and water). Furthermore, the linear momentum of the diver is equal in magnitude to that of the raft, but opposite in direction.

w

w

w

.a

Q uick Quiz 9.8 A cruise ship is moving at constant speed through the water. The vacationers on the ship are eager to arrive at their next destination. They decide to try to speed up the cruise ship by gathering at the bow (the front) and running together toward the stern (the back) of the ship. (i) While they are running toward the stern, is the speed of the ship (a) higher than it was before, (b) unchanged, (c) lower than it was before, or (d) impossible to determine? (ii) The vacationers stop running when they reach the stern of the ship. After they have all stopped running, is the speed of the ship (a) higher than it was before they started running, (b) unchanged from what it was before they started running, (c) lower than it was before they started running, or (d) impossible to determine?

Conceptual Example 9.13 Exploding Projectile A projectile fired into the air suddenly explodes into several fragments (Fig. 9.21 on page 274). (A) W hat can be said about the motion of the center of mass of the system made up of all the fragments after the explosion?

continued

274 Chapter 9

Linear Momentum and Collisions

▸ 9.13 c o n t i n u e d S o l u ti o n

Neglecting air resistance, the only external force on the projectile is the gravitational force. Therefore, if the projectile did not explode, it would continue to move along the parabolic path indicated by the dashed line in Figure 9.21. Because the forces caused by the explosion are internal, they do not affect the motion of the center of mass of the system (the fragments). Therefore, after the explosion, the center of mass of the fragments follows the same parabolic path the projectile would have followed if no explosion had occurred.

R

(B) If the projectile did not explode, it would land at a distance R from its launch point. Suppose the projectile explodes and splits into two pieces of equal mass. One piece lands at a distance 2R to the right of the launch point. Where does the other piece land?

om

.c

ly

S o l u ti o n

Figure 9.21 (Conceptual Example 9.13) When a projectile explodes into several fragments, the center of mass of the system made up of all the fragments follows the same parabolic path the projectile would have taken had there been no explosion.

ee b

As discussed in part (A), the center of mass of the two-piece system lands at a distance R from the launch point. One of the pieces lands at a farther distance R from the landing point (or a distance 2R from the launch point), to the right in Figure 9.21. Because the two pieces have the same mass, the other piece must land a distance R to the left of the landing point in Figure 9.21, which places this piece right back at the launch point!

ic

Example 9.14 The Exploding Rocket AM

s. w

ar ph

ys

A rocket is fired vertically upward. At the instant it reaches an altitude of 1 000 m and a speed of vi 5 300 m/s, it explodes into three fragments having equal mass. One fragment moves upward with a speed of v1 5 450 m/s following the explosion. The second fragment has a speed of v 2 5 240 m/s and is moving east right after the explosion. What is the velocity of the third fragment immediately after the explosion? S o l u ti o n

Conceptualize Picture the explosion in your mind, with one piece going upward and a second piece moving horizon-

sw

tally toward the east. Do you have an intuitive feeling about the direction in which the third piece moves?

w

w

w

.a

Categorize This example is a two-dimensional problem because we have two fragments moving in perpendicular directions after the explosion as well as a third fragment moving in an unknown direction in the plane defined by the velocity vectors of the other two fragments. We assume the time interval of the explosion is very short, so we use the impulse approximation in which we ignore the gravitational force and air resistance. Because the forces of the explosion are internal to the system (the rocket), the rocket is an isolated system in terms of momentum. Therefore, the total momentum S p i of the rocket immediately before the explosion must equal the total momentum S p f of the fragments immediately after the explosion. Analyze Because the three fragments have equal mass, the mass of each fragment is M/3, where M is the total mass of the rocket. We will let S v 3 represent the unknown velocity of the third fragment. Use the isolated system (momentum) model to equate the initial and final momenta of the system and express the momenta in terms of masses and velocities:

S Dp 50

Solve for S v 3:

S

Substitute the numerical values:

S

S

pi 5 S pf

S

MS vi 5

MS MS MS v 1 v 1 v 3 1 3 2 3 3

vi 2 S v1 2 S v2 v 3 5 3S

S

v 3 5 3 1 300 ^j m/s 2 2 1 450 ^j m/s 2 2 1 240 ^i m/s 2 5 1 2240 i^ 1 450 ^j 2 m/s

Finalize Notice that this event is the reverse of a perfectly inelastic collision. There is one object before the collision and three objects afterward. Imagine running a movie of the event backward: the three objects would come together and become a single object. In a perfectly inelastic collision, the kinetic energy of the system decreases. If you were

9.8 Deformable Systems 275

▸ 9.14 c o n t i n u e d to calculate the kinetic energy before and after the event in this example, you would find that the kinetic energy of the system increases. (Try it!) This increase in kinetic energy comes from the potential energy stored in whatever fuel exploded to cause the breakup of the rocket.

9.8 Deformable Systems

o T

S DK 1 DU 5 0

ar ph

DE system 5

ys

ic

s. w

ee b

ly

.c

om

So far in our discussion of mechanics, we have analyzed the motion of particles or nondeformable systems that can be modeled as particles. The discussion in Section 9.7 can be applied to an analysis of the motion of deformable systems. For example, suppose you stand on a skateboard and push off a wall, setting yourself in motion away from the wall. Your body has deformed during this event: your arms were bent before the event, and they straightened out while you pushed off the wall. How would we describe this event? The force from the wall on your hands moves through no displacement; the force is always located at the interface between the wall and your hands. Therefore, the force does no work on the system, which is you and your skateboard. Pushing off the wall, however, does indeed result in a change in the kinetic energy of the system. If you try to use the work–kinetic energy theorem, W 5 DK, to describe this event, you will notice that the left side of the equation is zero but the right side is not zero. The work–kinetic energy theorem is not valid for this event and is often not valid for systems that are deformable. To analyze the motion of deformable systems, we appeal to Equation 8.2, the conservation of energy equation, and Equation 9.40, the impulse–momentum theorem. For the example of you pushing off the wall on your skateboard, identifying the system as you and the skateboard, Equation 8.2 gives

S

w

.a

sw

where DK is the change in kinetic energy, which is related to the increased speed of the system, and DU is the decrease in potential energy stored in the body from previous meals. This equation tells us that the system transformed potential energy into kinetic energy by virtue of the muscular exertion necessary to push off the wall. Notice that the system is isolated in terms of energy but nonisolated in terms of momentum. Applying Equation 9.40 to the system in this situation gives us S S Dp tot 5 I S m D v 5 3 F wall dt S

S

w

w

where F wall is the force exerted by the wall on your hands, m is the mass of you and the skateboard, and DS v is the change in the velocity of the system during the event. To evaluate the right side of this equation, we would need to know how the force from the wall varies in time. In general, this process might be complicated. In the case of constant forces, or well-behaved forces, however, the integral on the right side of the equation can be evaluated.

Example 9.15 Pushing on a Spring3 AM As shown in Figure 9.22a (page 276), two blocks are at rest on a frictionless, level table. Both blocks have the same mass m, and they are connected by a spring of negligible mass. The separation distance of the blocks when the spring is relaxed is L. During a time interval Dt, a constant force of magnitude F is applied horizontally to the left block, 3Example

9.15 was inspired in part by C. E. Mungan, “A primer on work–energy relationships for introductory physics,” The Physics Teacher 43:10, 2005.

continued

276 Chapter 9

Linear Momentum and Collisions

▸ 9.15 c o n t i n u e d L

moving it through a distance x 1 as shown in Figure 9.22b. During this time interval, the right block moves through a distance x 2. At the end of this time interval, the force F is removed.

m

a

(A) Find the resulting speed S v CM of the center of mass of the system.

m

x1

S o l u ti o n

F b

Conceptualize Imagine what happens as you push on the left block. It begins to move to the right in Figure 9.22, and the spring begins to compress. As a result, the spring pushes to the right on the right block, which begins to move to the right. At any given time, the blocks are generally moving with different velocities. As the center of mass of the system moves to the right with a constant speed after the force is removed, the two blocks oscillate back and forth with respect to the center of mass.

x2 m

m

.c

om

Figure 9.22 (Example 9.15) (a) Two blocks of equal mass are connected by a spring. (b) The left block is pushed with a constant force of magnitude F and moves a distance x 1 during some time interval. During this same time interval, the right block moves through a distance x 2.

ee b

ly

Categorize We apply three analysis models in this problem: the deformable system of two blocks and a spring is modeled as a nonisolated system in terms of energy because work is being done on it by the applied force. It is also modeled as a nonisolated system in terms of momentum because of the force acting on the system during a time interval. Because the applied force on the system is constant, the acceleration of its center of mass is constant and the center of mass is modeled as a particle under constant acceleration.

s. w

Analyze Using the nonisolated system (momentum) model, we apply the impulse–momentum theorem to the system of two blocks, recognizing that the force F is constant during the time interval Dt while the force is applied. Write Equation 9.40 for the system:

1 2m 2 1 v CM 2 0 2 5 F Dt

S

ic

Dpx 5 Ix

ys

(1) 2mv CM 5 F Dt

Dt 5

Because the center of mass is modeled as a particle under constant acceleration, the average velocity of the center of mass is the average of the initial velocity, which is zero, and the final velocity v CM:

Dt 5

.a

sw

ar ph

During the time interval Dt, the center of mass of the system moves a distance 12 1 x 1 1 x 2 2 . Use this fact to express the time interval in terms of v CM,avg:

w

w

Solve for v CM:

w

Substitute this expression into Equation (1):

1 2 1 x1

1 x2 2

v CM,avg

1 2 1 x1 1 x2 2 1 2 1 0 1 v CM 2

2mv CM 5 F

v CM 5

Å

F

5

1 x1 1 x2 2 v CM

1 x1 1 x2 2 v CM 1 x1 1 x2 2 2m

(B) Find the total energy of the system associated with vibration relative to its center of mass after the force F is removed. S o l u ti o n

Analyze The vibrational energy is all the energy of the system other than the kinetic energy associated with translational motion of the center of mass. To find the vibrational energy, we apply the conservation of energy equation. The kinetic energy of the system can be expressed as K 5 K CM 1 K vib, where K vib is the kinetic energy of the blocks relative to the center of mass due to their vibration. The potential energy of the system is U vib, which is the potential energy stored in the spring when the separation of the blocks is some value other than L. From the nonisolated system (energy) model, express Equation 8.2 for this system:

(2) DK CM 1 DK vib 1 DU vib 5 W

9.9 Rocket Propulsion 277

▸ 9.15 c o n t i n u e d DK CM 1 DE vib 5 W

The initial values of the kinetic energy of the center of mass and the vibrational energy of the system are zero. Use this fact and substitute for the work done on the system by the force F :

K CM 1 E vib 5 W 5 Fx 1

Solve for the vibrational energy and use the result from part (A):

E vib 5 Fx 1 2 K CM 5 Fx 1 2 12 1 2m 2 v CM 2 5 F

1 x1 2 x2 2 2

om

Express Equation (2) in an alternate form, noting that K vib 1 U vib 5 E vib:

.c

ly

9.9 Rocket Propulsion

The force from a nitrogenpropelled hand-controlled device allows an astronaut to move about freely in space without restrictive tethers, using the thrust force from the expelled nitrogen.

sw

ar ph

ys

ic

s. w

When ordinary vehicles such as cars are propelled, the driving force for the motion is friction. In the case of the car, the driving force is the force exerted by the road on the car. We can model the car as a nonisolated system in terms of momentum. An impulse is applied to the car from the roadway, and the result is a change in the momentum of the car as described by Equation 9.40. A rocket moving in space, however, has no road to push against. The rocket is an isolated system in terms of momentum. Therefore, the source of the propulsion of a rocket must be something other than an external force. The operation of a rocket depends on the law of conservation of linear momentum as applied to an isolated system, where the system is the rocket plus its ejected fuel. Rocket propulsion can be understood by first considering our archer standing on frictionless ice in Example 9.1. Imagine the archer fires several arrows horizontally. For each arrow fired, the archer receives a compensating momentum in the opposite direction. As more arrows are fired, the archer moves faster and faster across the ice. In addition to this analysis in terms of momentum, we can also understand this phenomenon in terms of Newton’s second and third laws. Every time the bow pushes an arrow forward, the arrow pushes the bow (and the archer) backward, and these forces result in an acceleration of the archer. In a similar manner, as a rocket moves in free space, its linear momentum changes when some of its mass is ejected in the form of exhaust gases. Because the gases are given momentum when they are ejected out of the engine, the rocket receives a compensating momentum in the opposite direction. Therefore, the rocket is accelerated as a result of the “push,” or thrust, from the exhaust gases. In free space, the center of mass of the system (rocket plus expelled gases) moves uniformly, independent of the propulsion process.4 Suppose at some time t the magnitude of the momentum of a rocket plus its fuel is (M 1 Dm)v, where v is the speed of the rocket relative to the Earth (Fig. 9.23a). Over a short time interval Dt, the rocket ejects fuel of mass Dm. At the end of this interval, the rocket’s mass is M and its speed is v 1 Dv, where Dv is the change in speed of the rocket (Fig. 9.23b). If the fuel is ejected with a speed ve relative to

Courtesy of NASA

ee b

Finalize Neither of the two answers in this example depends on the spring length, the spring constant, or the time interval. Notice also that the magnitude x 1 of the displacement of the point of application of the applied force is different from the magnitude 12 1 x 1 1 x 2 2 of the displacement of the center of mass of the system. This difference reminds us that the displacement in the definition of work (Eq. 7.1) is that of the point of application of the force.

S

w

w

w

.a

v

4The rocket and the archer represent cases of the reverse of a perfectly inelastic collision: momentum is conserved, but the kinetic energy of the rocket–exhaust gas system increases (at the expense of chemical potential energy in the fuel), as does the kinetic energy of the archer–arrow system (at the expense of potential energy from the archer’s previous meals).

M m S

S

pi (M m)v

a

m

M

S

S

v v

b

Figure 9.23 Rocket propulsion. (a) The initial mass of the rocket plus all its fuel is M 1 Dm at a time t, and its speed is v. (b) At a time t 1 Dt, the rocket’s mass has been reduced to M and an amount of fuel Dm has been ejected. The rocket’s speed increases by an amount Dv.

278 Chapter 9

Linear Momentum and Collisions

the rocket (the subscript e stands for exhaust, and ve is usually called the exhaust speed), the velocity of the fuel relative to the Earth is v 2 ve . Because the system of the rocket and the ejected fuel is isolated, we apply the isolated system model for momentum and obtain Dp 5 0 S pi 5 p f S 1 M 1 Dm 2 v 5 M 1 v 1 Dv 2 1 Dm 1 v 2 v e 2

Simplifying this expression gives

M Dv 5 v e Dm

M dv 5 ve dm 5 2ve dM

om

If we now take the limit as Dt goes to zero, we let Dv S dv and Dm S dm. Furthermore, the increase in the exhaust mass dm corresponds to an equal decrease in the rocket mass, so dm 5 2dM. Notice that dM is negative because it represents a decrease in mass, so 2dM is a positive number. Using this fact gives (9.43)

vi

Expression for rocket propulsion

Mf

Mi

v f 2 v i 5 v e lna

s. w

dM M

ee b

3 dv 5 2v e 3 vf

ly

.c

Now divide the equation by M and integrate, taking the initial mass of the rocket plus fuel to be Mi and the final mass of the rocket plus its remaining fuel to be Mf . The result is

Mi b Mf

(9.44)

sw

ar ph

ys

ic

which is the basic expression for rocket propulsion. First, Equation 9.44 tells us that the increase in rocket speed is proportional to the exhaust speed ve of the ejected gases. Therefore, the exhaust speed should be very high. Second, the increase in rocket speed is proportional to the natural logarithm of the ratio Mi /Mf . Therefore, this ratio should be as large as possible; that is, the mass of the rocket without its fuel should be as small as possible and the rocket should carry as much fuel as possible. The thrust on the rocket is the force exerted on it by the ejected exhaust gases. We obtain the following expression for the thrust from Newton’s second law and Equation 9.43: Thrust 5 M

dv dM 5 ` ve ` dt dt

(9.45)

w

.a

This expression shows that the thrust increases as the exhaust speed increases and as the rate of change of mass (called the burn rate) increases.

w

Example 9.16 Fighting a Fire

w

Two firefighters must apply a total force of 600 N to steady a hose that is discharging water at the rate of 3 600 L/min. Estimate the speed of the water as it exits the nozzle. S o l u ti o n

Conceptualize A s the water leaves the hose, it acts in a way similar to the gases being ejected from a rocket engine. As a result, a force (thrust) acts on the firefighters in a direction opposite the direction of motion of the water. In this case, we want the end of the hose to be modeled as a particle in equilibrium rather than to accelerate as in the case of the rocket. Consequently, the firefighters must apply a force of magnitude equal to the thrust in the opposite direction to keep the end of the hose stationary. Categorize This example is a substitution problem in which we use given values in an equation derived in this section. The water exits at 3 600 L/min, which is 60 L/s. Knowing that 1 L of water has a mass of 1 kg, we estimate that about 60 kg of water leaves the nozzle each second.

Summary 279

▸ 9.16 c o n t i n u e d Thrust 5 `v e

Use Equation 9.45 for the thrust:

ve 5

Solve for the exhaust speed:

ve 5

Substitute numerical values:

Thrust 0 dM /dt 0

dM ` dt

600 N 5 10 m/s 60 kg/s

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Example 9.17 A Rocket in Space

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A rocket moving in space, far from all other objects, has a speed of 3.0 3 103 m/s relative to the Earth. Its engines are turned on, and fuel is ejected in a direction opposite the rocket’s motion at a speed of 5.0 3 103 m/s relative to the rocket.

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(A) W hat is the speed of the rocket relative to the Earth once the rocket’s mass is reduced to half its mass before ignition? S o l u ti o n

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Conceptualize Figure 9.23 shows the situation in this problem. From the discussion in this section and scenes from science fiction movies, we can easily imagine the rocket accelerating to a higher speed as the engine operates.

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Categorize This problem is a substitution problem in which we use given values in the equations derived in this section. v f 5 v i 1 v e lna

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Solve Equation 9.44 for the final velocity and substitute the known values:

Mi b Mf

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5 3.0 3 103 m/s 1 1 5.0 3 103 m/s 2 ln a 5 6.5 3 103 m/s

Mi b 0.50M i

(B) W hat is the thrust on the rocket if it burns fuel at the rate of 50 kg/s?

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S o l u ti o n

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Use Equation 9.45, noting that dM/dt 5 50 kg/s: dM ` 5 1 5.0 3 103 m/s 2 1 50 kg/s 2 5 2.5 3 105 N dt

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Thrust 5 `v e

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Summary Definitions

The impulse imparted to a particle by a net force S g F is equal to the time integral of the force:

The linear momentum S p of a particle of mass m S moving with a velocity v is

S

v p ; mS

(9.2)

I ; 3 a F dt

S

tf

S

(9.9)

ti

continued

280 Chapter 9

Linear Momentum and Collisions

An inelastic collision is one for which the total kinetic energy of the system of colliding particles is not conserved. A perfectly inelastic collision is one in which the colliding particles stick together after the collision. An elastic collision is one in which the kinetic energy of the system is conserved.

The position vector of the center of mass of a system of particles is defined as S

r CM ;

1 m iS r i M a i

(9.31)

where M 5 Si mi is the total mass of the system and S r i is the position vector of the ith particle.

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Concepts and Principles

1 S r dm M3

(9.34)

v CM 5

1 m iS v i Ma i

(9.35)

Impulse

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System boundary

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Analysis Models for Problem Solving

where a CM is the acceleration of the center of mass and the sum is over all external forces. The center of mass moves like an imaginary particle of mass M under the influence of the resultant external force on the system.

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The total momentum of a system of particles equals the total mass multiplied by the velocity of the center of mass.

(9.39)

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S

S a F ext 5 M a CM S

S

The velocity of the center of mass for a system of particles is

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r CM 5

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S

Newton’s second law applied to a system of particles is

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The position vector of the center of mass of an extended object can be obtained from the integral expression

System boundary Momentum

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Momentum

If no external forces act on the system, the total momentum of the system is constant.

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The change in the total momentum of the system is equal to the total impulse on the system.

Nonisolated System (Momentum). If a system interacts with its environment in the sense that there is an external force on the system, the behavior of the system is described by the impulse–momentum theorem:

S

S Dp tot 5 I

(9.40)

Isolated System (Momentum). The total momentum of an isolated system (no external forces) is conserved regardless of the nature of the forces between the members of the system: ￼

S Dp tot 5 0

(9.41)

The system may be isolated in terms of momentum but nonisolated in terms of energy, as in the case of inelastic collisions.

Objective Questions

Objective Questions

281

1. denotes answer available in Student Solutions Manual/Study Guide

8. The kinetic energy of an object is increased by a factor of 4. By what factor is the magnitude of its momentum changed? (a) 16 (b) 8 (c) 4 (d) 2 (e) 1

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9. If two particles have equal momenta, are their kinetic energies equal? (a) yes, always (b) no, never (c) no, except when their speeds are the same (d) yes, as long as they move along parallel lines

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10. If two particles have equal kinetic energies, are their momenta equal? (a) yes, always (b) no, never (c) yes, as long as their masses are equal (d) yes, if both their masses and directions of motion are the same (e) yes, as long as they move along parallel lines 11. A 10.0-g bullet is fired into a 200-g block of wood at rest on a horizontal surface. After impact, the block slides 8.00 m before coming to rest. If the coefficient of friction between the block and the surface is 0.400, what is the speed of the bullet before impact? (a) 106 m/s (b) 166 m/s (c) 226 m/s (d) 286 m/s (e) none of those answers is correct

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2. A boxcar at a rail yard is set into motion at the top of a hump. The car rolls down quietly and without friction onto a straight, level track where it couples with a flatcar of smaller mass, originally at rest, so that the two cars then roll together without friction. Consider the two cars as a system from the moment of release of the boxcar until both are rolling together. Answer the following questions yes or no. (a) Is mechanical energy of the system conserved? (b) Is momentum of the system conserved? Next, consider only the process of the boxcar gaining speed as it rolls down the hump. For the boxcar and the Earth as a system, (c) is mechanical energy conserved? (d) Is momentum conserved? Finally, consider the two cars as a system as the boxcar is slowing down in the coupling process. (e) Is mechanical energy of this system conserved? (f) Is momentum of this system conserved?

7. The momentum of an object is increased by a factor of 4 in magnitude. By what factor is its kinetic energy changed? (a) 16 (b) 8 (c) 4 (d) 2 (e) 1

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1. You are standing on a saucer-shaped sled at rest in the middle of a frictionless ice rink. Your lab partner throws you a heavy Frisbee. You take different actions in successive experimental trials. Rank the following situations according to your final speed from largest to smallest. If your final speed is the same in two cases, give them equal rank. (a) You catch the Frisbee and hold onto it. (b) You catch the Frisbee and throw it back to your partner. (c) You bobble the catch, just touching the Frisbee so that it continues in its original direction more slowly. (d) You catch the Frisbee and throw it so that it moves vertically upward above your head. (e) You catch the Frisbee and set it down so that it remains at rest on the ice.

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12. Two particles of different mass start from rest. The same net force acts on both of them as they move over equal distances. How do their final kinetic energies compare? (a) The particle of larger mass has more kinetic energy. (b) The particle of smaller mass has more kinetic energy. (c) The particles have equal kinetic energies. (d) Either particle might have more kinetic energy.

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3. A massive tractor is rolling down a country road. In a perfectly inelastic collision, a small sports car runs into the machine from behind. (i) Which vehicle experiences a change in momentum of larger magnitude? (a) The car does. (b) The tractor does. (c) Their momentum changes are the same size. (d) It could be either vehicle. (ii) Which vehicle experiences a larger change in kinetic energy? (a) The car does. (b) The tractor does. (c) Their kinetic energy changes are the same size. (d) It could be either vehicle.

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4. A 2-kg object moving to the right with a speed of 4 m/s makes a head-on, elastic collision with a 1-kg object that is initially at rest. The velocity of the 1-kg object after the collision is (a) greater than 4 m/s, (b) less than 4 m/s, (c) equal to 4 m/s, (d) zero, or (e) impossible to say based on the information provided. 5. A 5-kg cart moving to the right with a speed of 6 m/s collides with a concrete wall and rebounds with a speed of 2 m/s. What is the change in momentum of the cart? (a) 0 (b) 40 kg ? m/s (c) 240 kg ? m/s (d) 230 kg ? m/s (e) 210 kg ? m/s 6. A 57.0-g tennis ball is traveling straight at a player at 21.0 m/s. The player volleys the ball straight back at 25.0 m/s. If the ball remains in contact with the racket for 0.060 0 s, what average force acts on the ball? (a) 22.6 N (b) 32.5 N (c) 43.7 N (d) 72.1 N (e) 102 N

13. Two particles of different mass start from rest. The same net force acts on both of them as they move over equal distances. How do the magnitudes of their final momenta compare? (a) The particle of larger mass has more momentum. (b) The particle of smaller mass has more momentum. (c) The particles have equal momenta. (d) Either particle might have more momentum. 14. A basketball is tossed up into the air, falls freely, and bounces from the wooden floor. From the moment after the player releases it until the ball reaches the top of its bounce, what is the smallest system for which momentum is conserved? (a) the ball (b) the ball plus player (c) the ball plus floor (d) the ball plus the Earth (e) momentum is not conserved for any system 15. A 3-kg object moving to the right on a frictionless, horizontal surface with a speed of 2 m/s collides headon and sticks to a 2-kg object that is initially moving to the left with a speed of 4 m/s. After the collision, which statement is true? (a) The kinetic energy of the system is 20 J. (b) The momentum of the system is 14 kg ? m/s. (c) The kinetic energy of the system is greater than 5 J but less than 20 J. (d) The momentum of the system is 22 kg ? m/s. (e) The momentum of the system is less than the momentum of the system before the collision.

Linear Momentum and Collisions

Conceptual Questions

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18. A head-on, elastic collision occurs between two billiard balls of equal mass. If a red ball is traveling to the right with speed v and a blue ball is traveling to the left with speed 3v before the collision, what statement is true concerning their velocities subsequent to the collision? Neglect any effects of spin. (a) The red ball travels to the left with speed v, while the blue ball travels to the right with speed 3v. (b) The red ball travels to the left with speed v, while the blue ball continues to move to the left with a speed 2v. (c) The red ball travels to the left with speed 3v, while the blue ball travels to the right with speed v. (d) Their final velocities cannot be determined because momentum is not conserved in the collision. (e) The velocities cannot be determined without knowing the mass of each ball.

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17. A car of mass m traveling at speed v crashes into the rear of a truck of mass 2m that is at rest and in neutral at an intersection. If the collision is perfectly inelastic,

what is the speed of the combined car and truck after the collision? (a) v (b) v/2 (c) v/3 (d) 2v (e) None of those answers is correct.

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16. A ball is suspended by a string that is tied to a fixed point above a wooden block standu L ing on end. The ball is pulled m back as shown in Figure OQ9.16 and released. In trial A, the ball rebounds elastically from the block. In trial B, two-sided tape causes the ball to stick to the block. In which Figure OQ9.16 case is the ball more likely to knock the block over? (a) It is more likely in trial A. (b) It is more likely in trial B. (c) It makes no difference. (d) It could be either case, depending on other factors.

1. denotes answer available in Student Solutions Manual/Study Guide

7. Two students hold a large bed sheet vertically between them. A third student, who happens to be the star pitcher on the school baseball team, throws a raw egg at the center of the sheet. Explain why the egg does not break when it hits the sheet, regardless of its initial speed.

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1. An airbag in an automobile inflates when a collision occurs, which protects the passenger from serious injury (see the photo on page 254). Why does the airbag soften the blow? Discuss the physics involved in this dramatic photograph.

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282 Chapter 9

8. A juggler juggles three balls in a continuous cycle. Any one ball is in contact with one of his hands for one fifth of the time. (a) Describe the motion of the center of mass of the three balls. (b) What average force does the juggler exert on one ball while he is touching it?

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2. In golf, novice players are often advised to be sure to “follow through” with their swing. Why does this advice make the ball travel a longer distance? If a shot is taken near the green, very little follow-through is required. Why?

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3. An open box slides across a frictionless, icy surface of a frozen lake. What happens to the speed of the box as water from a rain shower falls vertically downward into the box? Explain.

9. (a) Does the center of mass of a rocket in free space accelerate? Explain. (b) Can the speed of a rocket exceed the exhaust speed of the fuel? Explain. 10. On the subject of the following positions, state your own view and argue to support it. (a) The best theory of motion is that force causes acceleration. (b) The true measure of a force’s effectiveness is the work it does, and the best theory of motion is that work done on an object changes its energy. (c) The true measure of a force’s effect is impulse, and the best theory of motion is that impulse imparted to an object changes its momentum.

5. You are standing perfectly still and then take a step forward. Before the step, your momentum was zero, but afterward you have some momentum. Is the principle of conservation of momentum violated in this case? Explain your answer.

11. Does a larger net force exerted on an object always produce a larger change in the momentum of the object compared with a smaller net force? Explain.

6. A sharpshooter fires a rifle while standing with the butt of the gun against her shoulder. If the forward momentum of a bullet is the same as the backward momentum of the gun, why isn’t it as dangerous to be hit by the gun as by the bullet?

13. A bomb, initially at rest, explodes into several pieces. (a) Is linear momentum of the system (the bomb before the explosion, the pieces after the explosion) conserved? Explain. (b) Is kinetic energy of the system conserved? Explain.

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4. While in motion, a pitched baseball carries kinetic energy and momentum. (a) Can we say that it carries a force that it can exert on any object it strikes? (b) Can the baseball deliver more kinetic energy to the bat and batter than the ball carries initially? (c) Can the baseball deliver to the bat and batter more momentum than the ball carries initially? Explain each of your answers.

12. Does a larger net force always produce a larger change in kinetic energy than a smaller net force? Explain.

Problems

283

Problems The problems found in this chapter may be assigned online in Enhanced WebAssign

1. straightforward; 2. intermediate; 3. challenging 1. full solution available in the Student Solutions Manual/Study Guide

AMT Analysis Model tutorial available in

Enhanced WebAssign

GP Guided Problem M Master It tutorial available in Enhanced WebAssign W Watch It video solution available in Enhanced WebAssign

BIO Q/C

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Section 9.1 Linear Momentum

energy of the boy–girl system? (c) Is the momentum of the boy–girl system conserved in the pushing-apart process? If so, explain how that is possible considering (d) there are large forces acting and (e) there is no motion beforehand and plenty of motion afterward.

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a person’s heart in one stroke. This information can be obtained by means of a ballistocardiograph. The instrument works as follows. The subject lies on a horizontal pallet floating on a film of air. Friction on the pallet is negligible. Initially, the momentum of the system is zero. When the heart beats, it expels a mass m of blood into the aorta with speed v, and the body and platform move in the opposite direction with speed V. The blood velocity can be determined independently (e.g., by observing the Doppler shift of ultrasound). Assume that it is 50.0 cm/s in one typical trial. The mass of the subject plus the pallet is 54.0 kg. The pallet moves 6.00 3 10 –5 m in 0.160 s after one heartbeat. Calculate the mass of blood that leaves the heart. Assume that the mass of blood is negligible compared with the total mass of the person. (This simplified example illustrates the principle of ballistocardiography, but in practice a more sophisticated model of heart function is used.)

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3. At one instant, a 17.5-kg sled is moving over a horizontal surface of snow at 3.50 m/s. After 8.75 s has elapsed, the sled stops. Use a momentum approach to find the average friction force acting on the sled while it was moving. 4. A 3.00-kg particle has a velocity of 1 3.00 ^i 2 4.00 ^j 2 m/s. (a) Find its x and y components of momentum. (b) Find the magnitude and direction of its momentum.

9. In research in cardiology and exercise physiology, it is

BIO often important to know the mass of blood pumped by

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2. An object has a kinetic energy of 275 J and a momentum of magnitude 25.0 kg ? m/s. Find the speed and mass of the object.

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1. A particle of mass m moves with momentum of magniS tude p. (a) Show that the kinetic energy of the particle is K 5 p 2/2m. (b) Express the magnitude of the particle’s momentum in terms of its kinetic energy and mass.

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5. A baseball approaches home plate at a speed of 45.0 m/s, moving horizontally just before being hit by a bat. The batter hits a pop-up such that after hitting the bat, the baseball is moving at 55.0 m/s straight up. The ball has a mass of 145 g and is in contact with the bat for 2.00 ms. What is the average vector force the ball exerts on the bat during their interaction?

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Section 9.2 Analysis Model: Isolated System (Momentum)

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6. A 45.0-kg girl is standing on a 150-kg plank. Both are M originally at rest on a frozen lake that constitutes a frictionless, flat surface. The girl begins to walk along the plank at a constant velocity of 1.50 ^i m/s relative to the plank. (a) What is the velocity of the plank relative to the ice surface? (b) What is the girl’s velocity relative to the ice surface?

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7. A girl of mass mg is standing on a plank of mass mp. Both S are originally at rest on a frozen lake that constitutes a frictionless, flat surface. The girl begins to walk along the plank at a constant velocity vgp to the right relative to the plank. (The subscript gp denotes the girl relative to plank.) (a) What is the velocity vpi of the plank relative to the surface of the ice? (b) What is the girl’s velocity vgi relative to the ice surface? 8. A 65.0-kg boy and his 40.0-kg sister, both wearing roller

Q/C blades, face each other at rest. The girl pushes the boy

hard, sending him backward with velocity 2.90 m/s toward the west. Ignore friction. (a) Describe the subsequent motion of the girl. (b) How much potential energy in the girl’s body is converted into mechanical

10. When you jump straight up as high as you can, what is the order of magnitude of the maximum recoil speed that you give to the Earth? Model the Earth as a perfectly solid object. In your solution, state the physical quantities you take as data and the values you measure or estimate for them. 11. Two blocks of masses m and 3m m W 3m are placed on a frictionQ/C less, horizontal surface. A Before light spring is attached to the a more massive block, and the S blocks are pushed together v 2.00 m/s with the spring between them (Fig. P9.11). A cord 3m m initially holding the blocks together is burned; after that happens, the block of mass After b 3m moves to the right with a speed of 2.00 m/s. (a) What Figure P9.11 is the velocity of the block of mass m? (b) Find the system’s original elastic potential energy, taking m 5 0.350 kg. (c) Is the original energy

Linear Momentum and Collisions

Consider this man in a collision in which he is in one of two identical cars each traveling toward the other at 60.0 mi/h relative to the ground. The car in which he rides is brought to rest in 0.10 s. (a) Find the magnitude of the average force needed to hold onto the child. (b) Based on your result to part (a), is the man’s claim valid? (c) What does the answer to this problem say about laws requiring the use of proper safety devices such as seat belts and special toddler seats? F (N) 20 000 15 000

Fmax = 18 000 N

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10 000 5 000 0

1

2

Figure P9.13

t (ms)

20. Review. A force platform is a tool used to analyze the performance of athletes by measuring the vertical force the athlete exerts on the ground as a function of time. Starting from rest, a 65.0-kg athlete jumps down onto the platform from a height of 0.600 m. While she is in contact with the platform during the time interval 0 , t , 0.800 s, the force she exerts on it is described by the function

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19. The magnitude of the net F (N) 4 force exerted in the x direction on a 2.50-kg particle 3 varies in time as shown in 2 Figure P9.19. Find (a) the 1 impulse of the force over the 5.00-s time interval, t (s) 0 1 2 3 4 5 (b) the final velocity the particle attains if it is origiFigure P9.19 nally at rest, (c) its final velocity if its original velocity is 22.00 ^i m/s, and (d) the average force exerted on the particle for the time interval between 0 and 5.00 s.

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13. An estimated force– W time curve for a baseball struck by a bat is shown in Figure P9.13. From this curve, determine (a) the magnitude of the impulse delivered to the ball and (b) the average force exerted on the ball.

with the ball traveling horizontally at 40.0 m/s in the opposite direction. (a) What is the impulse delivered to the ball by the tennis racket? (b) Some work is done on the system of the ball and some energy appears in the ball as an increase in internal energy during the collision between the ball and the racket. What is the sum W 2 DE int for the ball?

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12. A man claims that he can hold onto a 12.0-kg child in a

Q/C head-on collision as long as he has his seat belt on.

18. A tennis player receives a shot with the ball (0.060 0 kg)

AMT traveling horizontally at 20.0 m/s and returns the shot

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Section 9.3 Analysis Model: Nonisolated System (Momentum)

(c) what is the acceleration of the car? Express the acceleration as a multiple of the acceleration due to gravity.

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in the spring or in the cord? (d) Explain your answer to part (c). (e) Is the momentum of the system conserved in the bursting-apart process? Explain how that is possible considering (f) there are large forces acting and (g) there is no motion beforehand and plenty of motion afterward?

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284 Chapter 9

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14. Review. After a 0.300-kg rubber ball is dropped from a height of 1.75 m, it bounces off a concrete floor and rebounds to a height of 1.50 m. (a) Determine the magnitude and direction of the impulse delivered to the ball by the floor. (b) Estimate the time the ball is in contact with the floor and use this estimate to calculate the average force the floor exerts on the ball.

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15. A glider of mass m is free to slide along a horizontal

Q/C air track. It is pushed against a launcher at one end S of the track. Model the launcher as a light spring of

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force constant k compressed by a distance x. The glider is released from rest. (a) Show that the glider attains a speed of v 5 x(k/m)1/2. (b) Show that the magnitude of the impulse imparted to the glider is given by the expression I 5 x(km)1/2. (c) Is more work done on a cart with a large or a small mass?

16. In a slow-pitch softball game, a 0.200-kg softball crosses the plate at 15.0 m/s at an angle of 45.0° below the horizontal. The batter hits the ball toward center field, giving it a velocity of 40.0 m/s at 30.0° above the horizontal. (a) Determine the impulse delivered to the ball. (b) If the force on the ball increases linearly for 4.00 ms, holds constant for 20.0 ms, and then decreases linearly to zero in another 4.00 ms, what is the maximum force on the ball? 17. The front 1.20 m of a 1 400-kg car is designed as a M “crumple zone” that collapses to absorb the shock of a collision. If a car traveling 25.0 m/s stops uniformly in 1.20 m, (a) how long does the collision last, (b) what is the magnitude of the average force on the car, and

F 5 9 200t 2 11 500t 2

where F is in newtons and t is in seconds. (a) What impulse did the athlete receive from the platform? (b) With what speed did she reach the platform? (c) With what speed did she leave it? (d) To what height did she jump upon leaving the platform? 21. Water falls without splashing at a rate of 0.250 L/s from a height of 2.60 m into a 0.750-kg bucket on a scale. If the bucket is originally empty, what does the scale read in newtons 3.00 s after water starts to accumulate in it? Section 9.4 Collisions in One Dimension 22. A 1 200-kg car traveling initially at v Ci 5 25.0 m/s in an Q/C easterly direction crashes into the back of a 9 000-kg truck moving in the same direction at v Ti 5 20.0 m/s (Fig. P9.22). The velocity of the car immediately after the collision is v Cf 5 18.0 m/s to the east. (a) What is the velocity of the truck immediately after the colliS

vCi

S

vCf

S

vTi

Before

S

vTf

After

Figure P9.22

285

Problems

slides 7.50 m before coming to rest. If the coefficient of friction between the block and the surface is 0.650, what was the speed of the clay immediately before impact?

32. A wad of sticky clay of mass m is hurled horizontally at a S wooden block of mass M initially at rest on a horizontal surface. The clay sticks to the block. After impact, the block slides a distance d before coming to rest. If the coefficient of friction between the block and the surface is m, what was the speed of the clay immediately before impact?

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26. Four railroad cars, each of mass 2.50 3 104 kg, are coupled together and coasting along horizontal tracks at speed vi toward the south. A very strong but foolish movie actor, riding on the second car, uncouples the front car and gives it a big push, increasing its speed to 4.00 m/s southward. The remaining three cars continue moving south, now at 2.00 m/s. (a) Find the initial speed of the four cars. (b) By how much did the potential energy within the body of the actor change? (c) State the relationship between the process described here and the process in Problem 25.

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25. A railroad car of mass 2.50 3 104 kg is moving with a speed of 4.00 m/s. It collides and couples with three other coupled railroad cars, each of the same mass as the single car and moving in the same direction with an initial speed of 2.00 m/s. (a) What is the speed of the four cars after the collision? (b) How much mechanical energy is lost in the collision?

31. A 12.0-g wad of sticky clay is hurled horizontally at a

AMT 100-g wooden block initially at rest on a horizontal surM face. The clay sticks to the block. After impact, the block

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24. A car of mass m moving at a speed v1 collides and couS ples with the back of a truck of mass 2m moving initially in the same direction as the car at a lower speed v 2. (a) What is the speed vf of the two vehicles immediately after the collision? (b) What is the change in kinetic energy of the car–truck system in the collision?

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23. A 10.0-g bullet is fired into a stationary block of wood W having mass m 5 5.00 kg. The bullet imbeds into the block. The speed of the bullet-plus-wood combination immediately after the collision is 0.600 m/s. What was the original speed of the bullet?

30. As shown in Figure P9.30, a S bullet of mass m and speed v passes completely through a pendulum bob of mass M. The m bullet emerges with a speed M of v/2. The pendulum bob is S S v v/2 suspended by a stiff rod (not a string) of length , and negliFigure P9.30 gible mass. What is the minimum value of v such that the pendulum bob will barely swing through a complete vertical circle?

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sion? (b) What is the change in mechanical energy of the car–truck system in the collision? (c) Account for this change in mechanical energy.

33. Two blocks are free to slide along the frictionless,

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AMT wooden track shown in Figure P9.33. The block of W mass m1 5 5.00 kg is released from the position shown,

m1

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27. A neutron in a nuclear reactor makes an elastic, headM on collision with the nucleus of a carbon atom initially at rest. (a) What fraction of the neutron’s kinetic energy is transferred to the carbon nucleus? (b) The initial kinetic energy of the neutron is 1.60 3 10213 J. Find its final kinetic energy and the kinetic energy of the carbon nucleus after the collision. (The mass of the carbon nucleus is nearly 12.0 times the mass of the neutron.)

at height h 5 5.00 m above the flat part of the track. Protruding from its front end is the north pole of a strong magnet, which repels the north pole of an identical magnet embedded in the back end of the block of mass m 2 5 10.0 kg, initially at rest. The two blocks never touch. Calculate the maximum height to which m1 rises after the elastic collision.

m2

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28. A 7.00-g bullet, when fired from a gun into a 1.00-kg block of wood held in a vise, penetrates the block to a depth of 8.00 cm. This block of wood is next placed on a frictionless horizontal surface, and a second 7.00-g bullet is fired from the gun into the block. To what depth will the bullet penetrate the block in this case? 29. A tennis ball of mass 57.0 g is held M just above a basketball of mass 590 g. With their centers vertically aligned, both balls are released from rest at the same time, to fall through a distance of 1.20 m, as shown in Figure P9.29. (a) Find the magnitude of the Figure P9.29 downward velocity with which the basketball reaches the ground. (b) Assume that an elastic collision with the ground instantaneously reverses the velocity of the basketball while the tennis ball is still moving down. Next, the two balls meet in an elastic collision. To what height does the tennis ball rebound?

Figure P9.33 3 4. (a) Three carts of masses m1 5 4.00 kg, m 2 5 10.0 kg,

Q/C and m 3 5 3.00 kg move on a frictionless, horizontal

track with speeds of v1 5 5.00 m/s to the right, v 2 5 3.00 m/s to the right, and v 3 5 4.00 m/s to the left as shown in Figure P9.34. Velcro couplers make the carts stick together after colliding. Find the final velocity of the train of three carts. (b) What If? Does your answer in part (a) require that all the carts collide and stick v1 m1

v2

v3 m2

Figure P9.34

m3

Linear Momentum and Collisions

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43. An unstable atomic nucleus of mass 17.0 3 10227 kg iniM tially at rest disintegrates into three particles. One of the particles, of mass 5.00 3 10227 kg, moves in the y direction with a speed of 6.00 3 106 m/s. Another particle, of mass 8.40 3 10227 kg, moves in the x direction with a speed of 4.00 3 106 m/s. Find (a) the velocity of the third particle and (b) the total kinetic energy increase in the process.

.c

4 4. The mass of the blue puck in Figure P9.44 is 20.0% greater 30.0˚ than the mass of the green puck. Before colliding, the 30.0˚ pucks approach each other with momenta of equal magnitudes and opposite directions, Figure P9.44 and the green puck has an initial speed of 10.0 m/s. Find the speeds the pucks have after the collision if half the kinetic energy of the system becomes internal energy during the collision. Section 9.6 The Center of Mass 45. Four objects are situated along the y axis as follows: a W 2.00-kg object is at 13.00 m, a 3.00-kg object is at 12.50 m, a 2.50-kg object is at the origin, and a 4.00-kg object is at 20.500 m. Where is the center of mass of these objects?

ic

35. A 0.300-kg puck, initially at rest on a horizontal, frictionless surface, is struck by a 0.200-kg puck moving initially along the x axis with a speed of 2.00 m/s. After the collision, the 0.200-kg puck has a speed of 1.00 m/s at an angle of u 5 53.0° to the positive x axis (see Figure 9.11). (a) Determine the velocity of the 0.300-kg puck after the collision. (b) Find the fraction of kinetic energy transferred away or transformed to other forms of energy in the collision. 36. Two automobiles of equal mass approach an intersection. One vehicle is traveling with speed 13.0 m/s toward the east, and the other is traveling north with speed v 2i . Neither driver sees the other. The vehicles collide in the intersection and stick together, leaving parallel skid marks at an angle of 55.08 north of east. The speed limit for both roads is 35 mi/h, and the driver of the northward-moving vehicle claims he was within the speed limit when the collision occurred. Is he telling the truth? Explain your reasoning. 37. An object of mass 3.00 kg, moving with an initial velocW ity of 5.00 ^i m/s, collides with and sticks to an object of mass 2.00 kg with an initial velocity of 23.00 ^j m/s. Find the final velocity of the composite object.

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Section 9.5 Collisions in Two Dimensions

constitutes a perfectly inelastic collision. (b) Calculate the velocity of the players immediately after the tackle. (c) Determine the mechanical energy that disappears as a result of the collision. Account for the missing energy.

ee b

together at the same moment? What if they collide in a different order?

s. w

286 Chapter 9

ar ph

ys

38. Two shuffleboard disks of equal mass, one orange and W the other yellow, are involved in an elastic, glancing collision. The yellow disk is initially at rest and is struck by the orange disk moving with a speed of 5.00 m/s. After the collision, the orange disk moves along a direction that makes an angle of 37.08 with its initial direction of motion. The velocities of the two disks are perpendicular after the collision. Determine the final speed of each disk.

w

w

.a

sw

39. Two shuffleboard disks of equal mass, one orange and S the other yellow, are involved in an elastic, glancing collision. The yellow disk is initially at rest and is struck by the orange disk moving with a speed vi . After the collision, the orange disk moves along a direction that makes an angle u with its initial direction of motion. The velocities of the two disks are perpendicular after the collision. Determine the final speed of each disk.

w

40. A proton, moving with a velocity of v i ^i, collides elasS tically with another proton that is initially at rest. Assuming that the two protons have equal speeds after the collision, find (a) the speed of each proton after the collision in terms of vi and (b) the direction of the velocity vectors after the collision. 41. A billiard ball moving at 5.00 m/s strikes a stationary M ball of the same mass. After the collision, the first ball moves at 4.33 m/s at an angle of 30.08 with respect to the original line of motion. Assuming an elastic collision (and ignoring friction and rotational motion), find the struck ball’s velocity after the collision. 42. A 90.0-kg fullback running east with a speed of 5.00 m/s W is tackled by a 95.0-kg opponent running north with a Q/C speed of 3.00 m/s. (a) Explain why the successful tackle

46. The mass of the Earth is 5.97 3 1024 kg, and the mass of the Moon is 7.35 3 1022 kg. The distance of separation, measured between their centers, is 3.84 3 108 m. Locate the center of mass of the Earth–Moon system as measured from the center of the Earth. 47. Explorers in the jungle find an ancient monument in the shape of a large isosceles triangle as shown in Figure P9.47. The monument is made from tens of thousands of small stone blocks of density 3 800 kg/m3. The monument is 15.7 m high and 64.8 m wide at its base and is everywhere 3.60 m thick from front to back. Before the monument was built many years ago, all the stone blocks lay on the ground. How much work did laborers do on the blocks to put them in position while building the entire monument? Note: The gravitational potential energy of an object–Earth system is given by Ug 5 Mg y CM, where M is the total mass of the object and y CM is the elevation of its center of mass above the chosen reference level.

15.7 m 64.8 m

3.60 m

Figure P9.47

287

Problems

30

Section 9.8 Deformable Systems

l 5 50.0 1 20.0x

10

20

30

x (cm)

Figure P9.48

where x is the distance from one end, measured in meters, and l is in grams/meter. (a) What is the mass of the rod? (b) How far from the x 5 0 end is its center of mass? O

0.100 nm 53°

0.100 nm

53°

H

H

Figure P9.50

s. w

50. A water molecule consists of an oxygen atom with two hydrogen atoms bound to it (Fig. P9.50). The angle between the two bonds is 106°. If the bonds are 0.100 nm long, where is the center of mass of the molecule?

56. For a technology project, a stuQ/C dent has built a vehicle, of total mass 6.00 kg, that moves itself. As shown in Figure P9.56, it runs on four light wheels. A reel is attached to one of the axles, and a cord originally wound on the reel goes up over a pulley attached to the vehicle to support an elevated load. After the vehicle is released from rest, the load descends very slowly, unwinding the cord to turn Figure P9.56 the axle and make the vehicle move forward (to the left in Fig. P9.56). Friction is negligible in the pulley and axle bearings. The wheels do not slip on the floor. The reel has been constructed with a conical shape so that the load descends at a constant low speed while the vehicle moves horizontally across the floor with constant acceleration, reaching a final velocity of 3.00 ^i m/s. (a) Does the floor impart impulse to the vehicle? If so, how much? (b) Does the floor do work on the vehicle? If so, how much? (c) Does it make sense to say that the final momentum of the vehicle came from the floor? If not, where did it come from? (d) Does it make sense to say that the final kinetic energy of the vehicle came from the floor? If not, where did it come from? (e) Can we say that one particular force causes the forward acceleration of the vehicle? What does cause it?

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10

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20

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49. A rod of length 30.0 cm has linear density (mass per length) given by

after the collision. (b) Find the velocity of their center of mass before and after the collision.

y (cm)

ee b

48. A uniform piece of sheet W metal is shaped as shown in Figure P9.48. Compute the x and y coordinates of the center of mass of the piece.

Section 9.7 Systems of Many Particles

ys

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51. A 2.00-kg particle has a velocity 1 2.00 ^i 2 3.00 ^j 2 m/s, W and a 3.00-kg particle has a velocity 1 1.00 ^i 1 6.00 ^j 2 m/s. Find (a) the velocity of the center of mass and (b) the total momentum of the system.

.a

sw

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52. Consider a system of two particles in the xy plane: m 1 5 2.00 kg is at the location S r 1 5 1 1.00i^ 1 2.00j^ 2 m and has a velocity of 1 3.00i^ 1 0.500j^ 2 m/s; m 2 5 3.00 kg is at S r 2 5 124.00i^ 2 3.00j^ 2 m and has velocity 1 3.00i^ 2 ^ 2.00j 2 m/s. (a) Plot these particles on a grid or graph paper. Draw their position vectors and show their velocities. (b) Find the position of the center of mass of the system and mark it on the grid. (c) Determine the velocity of the center of mass and also show it on the diagram. (d) What is the total linear momentum of the system?

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53. Romeo (77.0 kg) entertains Juliet (55.0 kg) by playM ing his guitar from the rear of their boat at rest in still water, 2.70 m away from Juliet, who is in the front of the boat. After the serenade, Juliet carefully moves to the rear of the boat (away from shore) to plant a kiss on Romeo’s cheek. How far does the 80.0-kg boat move toward the shore it is facing? 54. The vector position of a 3.50-g particle moving in the xy plane varies in time according to S r 1 5 1 3 ^i 1 3 ^j 2 t 1 r is in centimeters. At 2 ^jt 2, where t is in seconds and S the same time, the vector position of a 5.50 g particle varies as S r 2 5 3 ^i 2 2 ^it 2 2 6 ^jt. At t 5 2.50 s, determine (a) the vector position of the center of mass, (b) the linear momentum of the system, (c) the velocity of the center of mass, (d) the acceleration of the center of mass, and (e) the net force exerted on the two-particle system. 55. A ball of mass 0.200 kg with a velocity of 1.50 ^i m/s meets a ball of mass 0.300 kg with a velocity of 20.400 ^i m/s in a head-on, elastic collision. (a) Find their velocities

57. A particle is suspended from a post on top of a cart by

Q/C a light string of length L as shown in Figure P9.57a. S The cart and particle are initially moving to the right

at constant speed vi , with the string vertical. The cart suddenly comes to rest when it runs into and sticks to a bumper as shown in Figure P9.57b. The suspended particle swings through an angle u. (a) Show that the original speed of the cart can be computed from v i 5 !2gL 1 1 2 cos u 2 . (b) If the bumper is still exerting a horizontal force on the cart when the hanging particle is at its maximum angle forward from the vertical, at what moment does the bumper stop exerting a horizontal force? S

vi L

a

u

b

Figure P9.57 58. A 60.0-kg person bends his knees and then jumps

Q/C straight up. After his feet leave the floor, his motion is

Linear Momentum and Collisions

d CM F

ic

m t0

tt b

Figure P9.59 Section 9.9 Rocket Propulsion

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a

w

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60. A model rocket engine has an average thrust of 5.26 N. It has an initial mass of 25.5 g, which includes fuel mass of 12.7 g. The duration of its burn is 1.90 s. (a) What is the average exhaust speed of the engine? (b) This engine is placed in a rocket body of mass 53.5 g. What is the final velocity of the rocket if it were to be fired from rest in outer space by an astronaut on a spacewalk? Assume the fuel burns at a constant rate.

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61. A garden hose is held as shown in Figure P9.61. The hose is originally full of motionless water. What additional force is necessary to hold the nozzle stationary after the water flow is turned on if the discharge rate is 0.600 kg/s with a speed of 25.0 m/s?

kt b Mi

(b) Make a graph of the velocity of the rocket as a function of time for times running from 0 to 132 s. (c) Show that the acceleration of the rocket is

F

d

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v 1 t 2 5 2v e lna1 2

v

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,

s. w

m

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59. Figure P9.59a shows an overhead view of the initial S configuration of two pucks of mass m on frictionless ice. The pucks are tied together with a string of length , and negligible mass. At time t 5 0, a constant force of magnitude F begins to pull to the right on the center point of the string. At time t, the moving pucks strike each other and stick together. At this time, the force has moved through a distance d, and the pucks have attained a speed v (Fig. P9.59b). (a) What is v in terms of F, d, ,, and m? (b) How much of the energy transferred into the system by work done by the force has been transformed to internal energy?

pad on the Earth, taking the vehicle’s initial mass as 3.00 3 106 kg. 63. A rocket for use in deep space is to be capable of Q/C boosting a total load (payload plus rocket frame and engine) of 3.00 metric tons to a speed of 10 000 m/s. (a) It has an engine and fuel designed to produce an exhaust speed of 2 000 m/s. How much fuel plus oxidizer is required? (b) If a different fuel and engine design could give an exhaust speed of 5 000 m/s, what amount of fuel and oxidizer would be required for the same task? (c) Noting that the exhaust speed in part (b) is 2.50 times higher than that in part (a), explain why the required fuel mass is not simply smaller by a factor of 2.50. 6 4. A rocket has total mass Mi 5 360 kg, including Mf 5 330 kg of fuel and oxidizer. In interstellar space, it starts from rest at the position x 5 0, turns on its engine at time t 5 0, and puts out exhaust with relative speed ve 5 1 500 m/s at the constant rate k 5 2.50 kg/s. The fuel will last for a burn time of T b 5 Mf /k 5 330 kg/(2.5 kg/s) 5 132 s. (a) Show that during the burn the velocity of the rocket as a function of time is given by

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unaffected by air resistance and his center of mass rises by a maximum of 15.0 cm. Model the floor as completely solid and motionless. (a) Does the floor impart impulse to the person? (b) Does the floor do work on the person? (c) With what momentum does the person leave the floor? (d) Does it make sense to say that this momentum came from the floor? Explain. (e) With what kinetic energy does the person leave the floor? (f) Does it make sense to say that this energy came from the floor? Explain.

ee b

288 Chapter 9

Figure P9.61

62. Review. The first stage of a Saturn V space vehicle consumed fuel and oxidizer at the rate of 1.50 3 104 kg/s with an exhaust speed of 2.60 3 103 m/s. (a) Calculate the thrust produced by this engine. (b) Find the acceleration the vehicle had just as it lifted off the launch

a1t2 5

kv e Mi 2 kt

(d) Graph the acceleration as a function of time. (e) Show that the position of the rocket is x 1 t 2 5 ve a

Mi kt 2 tb ln a1 2 b 1 ve t k Mi

(f) Graph the position during the burn as a function of time. Additional Problems 65. A ball of mass m is thrown straight up into the air with S an initial speed vi . Find the momentum of the ball (a) at its maximum height and (b) halfway to its maximum height. 66. An amateur skater of mass M is trapped in the middle Q/C of an ice rink and is unable to return to the side where S there is no ice. Every motion she makes causes her to slip on the ice and remain in the same spot. She decides to try to return to safety by throwing her gloves of mass m in the direction opposite the safe side. (a) She throws the gloves as hard as she can, and they leave her hand v gloves. Explain whether or with a horizontal velocity S not she moves. If she does move, calculate her velocity S v girl relative to the Earth after she throws the gloves. (b) Discuss her motion from the point of view of the forces acting on her. 67. A 3.00-kg steel ball strikes a wall with a speed of 10.0 m/s M at an angle of u 5 60.08 with the surface. It bounces off with the same speed and angle (Fig. P9.67). If the

289

Problems ball is in contact with the wall for 0.200 s, what is the average force exerted by the wall on the ball?

y

4.00 m/s

60.0 kg

u

x

120 kg

Figure P9.69

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70. A cannon is rigidly attached to a carriage, which can 45.0° move along horizontal rails but is connected to a post by a large spring, initially unstretched and with force constant k 5 Figure P9.70 2.00 3 104 N/m, as shown in Figure P9.70. The cannon fires a 200-kg projectile at a velocity of 125 m/s directed 45.0° above the horizontal. (a) Assuming that the mass of the cannon and its carriage is 5 000 kg, find the recoil speed of the cannon. (b) Determine the maximum extension of the spring. (c) Find the maximum force the spring exerts on the carriage. (d) Consider the system consisting of the cannon, carriage, and projectile. Is the momentum of this system conserved during the firing? Why or why not?

vi m2

m1 a

b

h

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vf

ys

ic

s. w

68. (a) Figure P9.68 shows u Q/C three points in the S operation of the ballistic pendulum disFigure P9.67 cussed in Example 9.6 (and shown in Fig. 9.9b). The projectile approaches the pendulum in Figure P9.68a. Figure P9.68b shows the situation just after the projectile is captured in the pendulum. In Figure P9.68c, the pendulum arm has swung upward and come to rest at a height h above its initial position. Prove that the ratio of the kinetic energy of the projectile–pendulum system immediately after the collision to the kinetic energy immediately before is m1/(m1 1 m 2). (b) What is the ratio of the momentum of the system immediately after the collision to the momentum immediately before? (c) A student believes that such a large decrease in mechanical energy must be accompanied by at least a small decrease in momentum. How would you convince this student of the truth?

c

Figure P9.68 Problems 68 and 86. (a) A metal ball

sw

moves toward the pendulum. (b) The ball is captured by the pendulum. (c) The ball–pendulum combination swings up through a height h before coming to rest.

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69. Review. A 60.0-kg person running at an initial speed of 4.00 m/s jumps onto a 120-kg cart initially at rest (Fig. P9.69). The person slides on the cart’s top surface and finally comes to rest relative to the cart. The coefficient of kinetic friction between the person and the cart is 0.400. Friction between the cart and ground can be ignored. (a) Find the final velocity of the person and cart relative to the ground. (b) Find the friction force acting on the person while he is sliding across the top surface of the cart. (c) How long does the friction force act on the person? (d) Find the change in momentum of the person and the change in momentum of the cart. (e) Determine the displacement of the person relative to the ground while he is sliding on the cart. (f) Determine the displacement of the cart relative to the ground while the person is sliding. (g) Find the change in kinetic energy of the person. (h) Find the change in kinetic energy of the cart. (i) Explain why the answers to (g) and (h) differ. (What kind of collision is this one, and what accounts for the loss of mechanical energy?)

71. A

1.25-kg

wooden

Q/C block rests on a table

M over a large hole as in Figure P9.71. A 5.00-g bullet with an iniS tial velocity vi is fired vi m upward into the bottom of the block and Figure P9.71 remains in the block Problems 71 and 72. after the collision. The block and bullet rise to a maximum height of 22.0 cm. (a) Describe how you would find the initial velocity of the bullet using ideas you have learned in this chapter. (b) Calculate the initial velocity of the bullet from the information provided.

72. A wooden block of mass M rests on a table over a large

Q/C hole as in Figure 9.71. A bullet of mass m with an iniS tial velocity of vi is fired upward into the bottom of

the block and remains in the block after the collision. The block and bullet rise to a maximum height of h. (a) Describe how you would find the initial velocity of the bullet using ideas you have learned in this chapter. (b) Find an expression for the initial velocity of the bullet.

73. Two particles with masses m and 3m are moving toward each other along the x axis with the same initial speeds vi . The particle with mass m is traveling to the left, and particle with mass 3m is traveling to the right. They

290 Chapter 9

Linear Momentum and Collisions

undergo a head-on elastic collision, and each rebounds along the same line as it approached. Find the final speeds of the particles. 74. Pursued by ferocious wolves, you are in a sleigh with no horses, gliding without friction across an ice-covered lake. You take an action described by the equations

meet on the level portion of the track, they undergo a head-on, elastic collision. Determine the maximum heights to which m1 and m 2 rise on the curved portion of the track after the collision. 78. Review. A metal cannonball of mass m rests next to a

Q/C tree at the very edge of a cliff 36.0 m above the surface

of the ocean. In an effort to knock the cannonball off the cliff, some children tie one end of a rope around a stone of mass 80.0 kg and the other end to a tree limb just above the cannonball. They tighten the rope so that the stone just clears the ground and hangs next to the cannonball. The children manage to swing the stone back until it is held at rest 1.80 m above the ground. The children release the stone, which then swings down and makes a head-on, elastic collision with the cannonball, projecting it horizontally off the cliff. The cannonball lands in the ocean a horizontal distance R away from its initial position. (a) Find the horizontal component R of the cannonball’s displacement as it depends on m. (b) What is the maximum possible value for R, and (c) to what value of m does it correspond? (d) For the stone–cannonball–Earth system, is mechanical energy conserved throughout the process? Is this principle sufficient to solve the entire problem? Explain. (e) What if? Show that R does not depend on the value of the gravitational acceleration. Is this result remarkable? State how one might make sense of it.

1 270 kg 2 1 7.50 m/s 2 ^i 5 1 15.0 kg 2 12v 1f ^i 2 1 1 255 kg 2 1 v 2f ^i 2 v 1f 1 v 2f 5 8.00 m/s

S

v1

k

Figure P9.75

w

w

w

.a

sw

76. Why is the following situation impossible? An astronaut, together with the equipment he carries, has a mass of 150 kg. He is taking a space walk outside his spacecraft, which is drifting through space with a constant velocity. The astronaut accidentally pushes against the spacecraft and begins moving away at 20.0 m/s, relative to the spacecraft, without a tether. To return, he takes equipment off his space suit and throws it in the direction away from the spacecraft. Because of his bulky space suit, he can throw equipment at a maximum speed of 5.00 m/s relative to himself. After throwing enough equipment, he starts moving back to the spacecraft and is able to grab onto it and climb inside. 77. Two blocks of masses m1 5 2.00 kg and m 2 5 4.00 kg are released from rest at a height of h 5 5.00 m on a frictionless track as shown in Figure P9.77. When they m1

m2

h

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ee b

79. A 0.400-kg blue bead A slides on a frictionless, curved wire, starting from rest at point A in h Figure P9.79, where h 5 B 1.50 m. At point B, the blue bead collides elasFigure P9.79 tically with a 0.600-kg green bead at rest. Find the maximum height the green bead rises as it moves up the wire.

ys

m2

ar ph

m1

v2

ic

S

s. w

75. Two gliders are set in motion on a horizontal air track. S A spring of force constant k is attached to the back end of the second glider. As shown in Figure P9.75, the first glider, of mass m1, moves to the right with speed v1, and the second glider, of mass m 2, moves more slowly to the right with speed v 2. When m1 collides with the spring attached to m 2, the spring compresses by a distance x max, and the gliders then move apart again. In terms of v1, v 2, m1, m 2, and k, find (a) the speed v at maximum compression, (b) the maximum compression x max, and (c) the velocity of each glider after m1 has lost contact with the spring.

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(a) Complete the statement of the problem, giving the data and identifying the unknowns. (b) Find the values of v1f and v 2f . (c) Find the amount of energy that has been transformed from potential energy stored in your body to kinetic energy of the system.

8 0. A small block of mass m1 5 0.500 kg is released from W rest at the top of a frictionless, curve-shaped wedge of mass m 2 5 3.00 kg, which sits on a frictionless, horizontal surface as shown in Figure P9.80a. When the block leaves the wedge, its velocity is measured to be 4.00 m/s to the right as shown in Figure P9.80b. (a) What is the velocity of the wedge after the block reaches the horizontal surface? (b) What is the height h of the wedge? m1

h

m2

S

v2

m2

h a

Figure P9.77

b

Figure P9.80

S

vf

Problems

81. Review. A bullet of mass m 5 8.00 g is fired into a block M of mass M 5 250 g that is initially at rest at the edge of a table of height h 5 1.00 m (Fig. P9.81). The bullet remains in the block, and after the impact the block lands d 5 2.00 m from the bottom of the table. Determine the initial speed of the bullet. m

291

tile mass m1 5 68.8 g, and pendulum mass m 2 5 263 g. (a) Determine the initial speed v1A of the projectile. (b) The second part of her experiment is to obtain v1A by firing the same projectile horizontally (with the pendulum removed from the path) and measuring its final horizontal position x and distance of fall y (Fig. P9.86). What numerical value does she obtain for v1A based on her measured values of x 5 257 cm and y 5 85.3 cm? (c) What factors might account for the difference in this value compared with that obtained in part (a)?

M

h

S

v1A

om

d

Figure P9.81 Problems 81 and 82.

y

ly

x

ee b

Figure P9.86

87. Review. A light spring of force constant 3.85 N/m is compressed by 8.00 cm and held between a 0.250-kg block on the left and a 0.500-kg block on the right. Both blocks are at rest on a horizontal surface. The blocks are released simultaneously so that the spring tends to push them apart. Find the maximum velocity each block attains if the coefficient of kinetic friction between each block and the surface is (a) 0, (b) 0.100, and (c) 0.462. Assume the coefficient of static friction is greater than the coefficient of kinetic friction in every case.

w

w

w

.a

sw

ar ph

ys

ic

s. w

83. A 0.500-kg sphere moving with a velocity given by 1 2.00 i^ 2 3.00 ^j 1 1.00k^ 2 m/s strikes another sphere of mass 1.50 kg moving with an initial velocity of 1 21.00 ^i 1 2.00 ^j 2 3.00k^ 2 m/s. (a) The velocity of the 0.500-kg sphere after the collision is 1 21.00 i^ 1 3.00 ^j 2 8.00k^ 2 m/s. Find the final velocity of the 1.50-kg sphere and identify the kind of collision (elastic, inelastic, or perfectly inelastic). (b) Now assume the velocity of the 0.500-kg sphere after the collision is (20.250 i^ 1 0.750 ^j 2 2.00k^ ) m/s. Find the final velocity of the 1.50-kg sphere and identify the kind of collision. (c) What If? Take the velocity of the 0.500-kg sphere after the collision as 1 21.00 ^i 1 3.00 ^j 1 a k^ 2 m/s. Find the value of a and the velocity of the 1.50-kg sphere after an elastic collision. 8 4. A 75.0-kg firefighter slides down a pole while a constant friction force of 300 N retards her motion. A horizontal 20.0-kg platform is supported by a spring at the bottom of the pole to cushion the fall. The firefighter starts from rest 4.00 m above the platform, and the spring constant is 4 000 N/m. Find (a) the firefighter’s speed just before she collides with the platform and (b) the maximum distance the spring is compressed. Assume the friction force acts during the entire motion. 85. George of the Jungle, with mass m, swings on a light vine hanging from a stationary tree branch. A second vine of equal length hangs from the same point, and a gorilla of larger mass M swings in the opposite direction on it. Both vines are horizontal when the primates start from rest at the same moment. George and the gorilla meet at the lowest point of their swings. Each is afraid that one vine will break, so they grab each other and hang on. They swing upward together, reaching a point where the vines make an angle of 35.08 with the vertical. Find the value of the ratio m/M. 86. Review. A student performs a ballistic pendulum Q/C experiment using an apparatus similar to that discussed in Example 9.6 and shown in Figure P9.68. She obtains the following average data: h 5 8.68 cm, projec-

.c

82. Review. A bullet of mass m is fired into a block of mass S M initially at rest at the edge of a frictionless table of height h (Fig. P9.81). The bullet remains in the block, and after impact the block lands a distance d from the bottom of the table. Determine the initial speed of the bullet.

8 8. Consider as a system the Sun with the Earth in a circular orbit around it. Find the magnitude of the change in the velocity of the Sun relative to the center of mass of the system over a six-month period. Ignore the influence of other celestial objects. You may obtain the necessary astronomical data from the endpapers of the book. 89. A 5.00-g bullet mov-

AMT ing with an initial M speed of vi 5 400 m/s

vi

is fired into and passes through a 1.00-kg vf d block as shown in Figure P9.89. The block, initially at rest on a frictionless, horizontal surface, is connected Figure P9.89 to a spring with force constant 900 N/m. The block moves d 5 5.00 cm to the right after impact before being brought to rest by the spring. Find (a) the speed at which the bullet emerges from the block and (b) the amount of initial kinetic energy of the bullet that is converted into internal energy in the bullet– block system during the collision.

90. Review. There are (one can say) three coequal theoGP ries of motion for a single particle: Newton’s second Q/C law, stating that the total force on the particle causes its

Linear Momentum and Collisions

9 4. Sand from a stationary hopper falls onto a moving

Q/C conveyor belt at the rate of 5.00 kg/s as shown in

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Figure P9.94. The conveyor belt is supported by frictionless rollers and moves at a constant speed of v 5 0.750 m/s under the action of a constant horizontal S external force F ext supplied by the motor that drives the belt. Find (a) the sand’s rate of change of momentum in the horizontal direction, (b) the force of friction exerted by the belt on the sand, (c) the external S S force F ext , (d) the work done by F ext in 1 s, and (e) the kinetic energy acquired by the falling sand each second due to the change in its horizontal motion. (f) Why are the answers to parts (d) and (e) different?

ic

91. A 2.00-g particle moving at 8.00 m/s makes a perfectly M elastic head-on collision with a resting 1.00-g object. (a) Find the speed of each particle after the collision. (b) Find the speed of each particle after the collision if the stationary particle has a mass of 10.0 g. (c) Find the final kinetic energy of the incident 2.00-g particle in the situations described in parts (a) and (b). In which case does the incident particle lose more kinetic energy?

vi . Particle m is traveling to the left, and particle 3m is traveling to the right. They undergo an elastic glancing collision such that particle m is moving in the negative y direction after the collision at a right angle from its initial direction. (a) Find the final speeds of the two particles in terms of vi . (b) What is the angle u at which the particle 3m is scattered?

ee b

acceleration; the work–kinetic energy theorem, stating that the total work on the particle causes its change in kinetic energy; and the impulse–momentum theorem, stating that the total impulse on the particle causes its change in momentum. In this problem, you compare predictions of the three theories in one particular case. A 3.00-kg object has velocity 7.00 ^j m/s. Then, a constant net force 12.0 ^i N acts on the object for 5.00 s. (a) Calculate the object’s final velocity, using the impulse–momentum theorem. (b) Calculate its accelerS a 5 1S v 2 /Dt. (c) Calculate its accelvf 2 ation from S S i S eration from a 5 g F /m . (d) Find the object’s vector vi t 1 12 S aSt 2. (e) Find the work displacement from DS r 5S r . (f) Find the done on the object from W 5 F ? DS vf ? S vf . (g) Find the final kinetic energy from 12mv f2 5 12mS final kinetic energy from 12mv i 2 1 W. (h) State the result of comparing the answers to parts (b) and (c), and the answers to parts (f) and (g).

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292 Chapter 9

Challenge Problems

S

Fext

Figure P9.94

Mark Dadswell/Getty Images

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ar ph

ys

92. In the 1968 Olympic games, University of Oregon jumper Dick Fosbury introduced a new technique of high jumping called the “Fosbury flop.” It contributed to raising the world record by about 30 cm and is currently used by nearly every world-class jumper. In this technique, the jumper goes over the bar face-up while arching her back as much as possible as shown in Figure P9.92a. This action places her center of mass outside her body, below her back. As her body goes over the bar, her center of mass passes below the bar. Because a given energy input implies a certain elevation for her center of mass, the action of arching her back means that her body is higher than if her back were straight. As a model, consider the jumper as a thin uniform rod of length L. When the rod is straight, its center of mass is at its center. Now bend the rod in a circular arc so that it subtends an angle of 90.08 at the center of the arc as shown in Figure P9.92b. In this configuration, how far outside the rod is the center of mass?

v

u b

a

Figure P9.92 93. Two particles with masses m and 3m are moving toward each other along the x axis with the same initial speeds

9 5. On a horizontal air track, a glider of mass m carries S a G-shaped post. The post supports a small dense sphere, also of mass m, hanging just above the top of the glider on a cord of length L. The glider and sphere are initially at rest with the cord vertical. (Figure P9.57 shows a cart and a sphere similarly connected.) A constant horizontal force of magnitude F is applied to the glider, moving it through displacement x 1; then the force is removed. During the time interval when the force is applied, the sphere moves through a displacement with horizontal component x 2. (a) Find the horizontal component of the velocity of the center of mass of the glider–sphere system when the force is removed. (b) After the force is removed, the glider continues to move on the track and the sphere swings back and forth, both without friction. Find an expression for the largest angle the cord makes with the vertical. 9 6. Review. A chain of length L S and total mass M is released from rest with its lower end just touching the top of a table as shown in Figure P9.96a. Find the force exerted by the table on the chain after the chain has fallen through a distance x as shown in Figure P9.96b. (Assume each link comes to rest the instant it reaches the table.)

x

L Lx

a

b

Figure P9.96

10

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Rotation of a Rigid Object About a Fixed Axis

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10.1 Angular Position, Velocity, and Acceleration

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When an extended object such as a wheel rotates about its axis, the motion cannot be analyzed by modeling the object as a particle because at any given time different parts of the object have different linear velocities and linear accelerations. We can, however, analyze the motion of an extended object by modeling it as a system of many particles, each of which has its own linear velocity and linear acceleration as discussed in Section 9.7. In dealing with a rotating object, analysis is greatly simplified by assuming the object is rigid. A rigid object is one that is nondeformable; that is, the relative locations of all particles of which the object is composed remain constant. All real objects are deformable to some extent; our rigid-object model, however, is useful in many situations in which deformation is negligible. We have developed analysis models based on particles and systems. In this chapter, we introduce another class of analysis models based on the rigid-object model.

10.1 Angular Position, Velocity, and Acceleration

10.2 Analysis Model: Rigid Object Under Constant Angular Acceleration 10.3 Angular and Translational Quantities 10.4 Torque 10.5 Analysis Model: Rigid Object Under a Net Torque 10.6 Calculation of Moments of Inertia 10.7 Rotational Kinetic Energy 10.8 Energy Considerations in Rotational Motion 10.9 Rolling Motion of a Rigid Object

The Malaysian pastime of gasing involves the spinning of tops that can have masses up to 5 kg. Professional spinners can spin their tops so that they might rotate for more than an hour before stopping. We will study the rotational motion of objects such as these tops in this chapter. (Courtesy Tourism Malaysia)

We will develop our understanding of rotational motion in a manner parallel to that used for translational motion in previous chapters. We began in Chapter 2 by

293

294 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

defining kinematic variables: position, velocity, and acceleration. We do the same here for rotational motion. Figure 10.1 illustrates an overhead view of a rotating compact disc, or CD. The disc rotates about a fixed axis perpendicular to the plane of the figure and passing through the center of the disc at O. A small element of the disc modeled as a particle at P is at a fixed distance r from the origin and rotates about it in a circle of radius r. (In fact, every element of the disc undergoes circular motion about O.) It is convenient to represent the position of P with its polar coordinates (r, u), where r is the distance from the origin to P and u is measured counterclockwise from some reference line fixed in space as shown in Figure 10.1a. In this representation, the angle u changes in time while r remains constant. As the particle moves along the circle from the reference line, which is at angle u 5 0, it moves through an arc of length s as in Figure 10.1b. The arc length s is related to the angle u through the relationship

To define angular position for the disc, a fixed reference line is chosen. A particle at P is located at a distance r from the rotation axis through O.

r

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Reference line

a

P r u O

s

s u5 r

(10.1a) (10.1b)

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b

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Figure 10.1 A compact disc rotating about a fixed axis through O perpendicular to the plane of the figure.

ys

u 1 rad 2 5

p u 1 deg 2 1808

For example, 608 equals p/3 rad and 458 equals p/4 rad. Because the disc in Figure 10.1 is a rigid object, as the particle moves through an angle u from the reference line, every other particle on the object rotates through the same angle u. Therefore, we can associate the angle u with the entire rigid object as well as with an individual particle, which allows us to define the angular position of a rigid object in its rotational motion. We choose a reference line on the object, such as a line connecting O and a chosen particle on the object. The angular position of the rigid object is the angle u between this reference line on the object and the fixed reference line in space, which is often chosen as the x axis. Such identification is similar to the way we define the position of an object in translational motion as the distance x between the object and the reference position, which is the origin, x 5 0. Therefore, the angle u plays the same role in rotational motion that the position x does in translational motion. As the particle in question on our rigid object travels from position A to position B in a time interval Dt as in Figure 10.2, the reference line fixed to the object sweeps out an angle Du 5 uf 2 ui . This quantity Du is defined as the angular displacement of the rigid object:

ar ph

Pitfall Prevention 10.1

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Remember the Radian In rotational equations, you must use angles expressed in radians. Don’t fall into the trap of using angles measured in degrees in rotational equations.

w

B, t f

r

w

A,ti

uf

ui O

s 5 r u

Because u is the ratio of an arc length and the radius of the circle, it is a pure number. Usually, however, we give u the artificial unit radian (rad), where one radian is the angle subtended by an arc length equal to the radius of the arc. Because the circumference of a circle is 2pr, it follows from Equation 10.1b that 3608 corresponds to an angle of (2pr/r) rad 5 2p rad. Hence, 1 rad 5 3608/2p < 57.38. To convert an angle in degrees to an angle in radians, we use that p rad 5 1808, so

Reference line

y

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As the disc rotates, a particle at P moves through an arc length s on a circular path of radius r. The angular position of P is u.

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P

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O

Du ; uf 2 ui

x

Figure 10.2 A particle on a rotating rigid object moves from A to B along the arc of a circle. In the time interval Dt 5 tf 2 ti , the radial line of length r moves through an angular displacement Du 5 uf 2 ui .

Average angular speed

The rate at which this angular displacement occurs can vary. If the rigid object spins rapidly, this displacement can occur in a short time interval. If it rotates slowly, this displacement occurs in a longer time interval. These different rotation rates can be quantified by defining the average angular speed vavg (Greek letter omega) as the ratio of the angular displacement of a rigid object to the time interval Dt during which the displacement occurs:

v avg ;

uf 2 ui tf 2 ti

5

Du Dt

(10.2)

10.1 Angular Position, Velocity, and Acceleration 295

In analogy to translational speed, the instantaneous angular speed v is defined as the limit of the average angular speed as Dt approaches zero: v ; lim

Dt S 0

Du du 5 Dt dt

(10.3)

WW Instantaneous angular speed

Angular speed has units of radians per second (rad/s), which can be written as s21 because radians are not dimensional. We take v to be positive when u is increasing (counterclockwise motion in Fig. 10.2) and negative when u is decreasing (clockwise motion in Fig. 10.2).

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Q uick Quiz 10.1 A rigid object rotates in a counterclockwise sense around a fixed axis. Each of the following pairs of quantities represents an initial angular position and a final angular position of the rigid object. (i) Which of the sets can only occur if the rigid object rotates through more than 1808? (a) 3 rad, 6 rad (b) 21 rad, 1 rad (c) 1 rad, 5 rad (ii) Suppose the change in angular position for each of these pairs of values occurs in 1 s. Which choice represents the lowest average angular speed?

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If the instantaneous angular speed of an object changes from vi to vf in the time interval Dt, the object has an angular acceleration. The average angular acceleration aavg (Greek letter alpha) of a rotating rigid object is defined as the ratio of the change in the angular speed to the time interval Dt during which the change in the angular speed occurs: vf 2 vi Dv aavg ; 5 (10.4) tf 2 ti Dt

WW Average angular acceleration

a ; lim

0

Dv dv 5 Dt dt

(10.5)

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Dt

S

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In analogy to translational acceleration, the instantaneous angular acceleration is defined as the limit of the average angular acceleration as Dt approaches zero:

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Angular acceleration has units of radians per second squared (rad/s2), or simply s22. Notice that a is positive when a rigid object rotating counterclockwise is speeding up or when a rigid object rotating clockwise is slowing down during some time interval. When a rigid object is rotating about a fixed axis, every particle on the object rotates through the same angle in a given time interval and has the same angular speed and the same angular acceleration. Therefore, like the angular position u, the quantities v and a characterize the rotational motion of the entire rigid object as well as individual particles in the object. Angular position (u), angular speed (v), and angular acceleration (a) are analogous to translational position (x), translational speed (v), and translational acceleration (a). The variables u, v, and a differ dimensionally from the variables x, v, and a only by a factor having the unit of length. (See Section 10.3.) We have not specified any direction for angular speed and angular acceleration. Strictly speaking, v and a are the magnitudes of the angular velocity and the anguS and S a, respectively, and they should always be positive. lar acceleration vectors1 v Because we are considering rotation about a fixed axis, however, we can use nonvector notation and indicate the vectors’ directions by assigning a positive or negative sign to v and a as discussed earlier with regard to Equations 10.3 and 10.5. For rotation about a fixed axis, the only direction that uniquely specifies the rotational S motion is the direction along the axis of rotation. Therefore, the directions of v S and a are along this axis. If a particle rotates in the xy plane as in Figure 10.2, the 1Although

we do not verify it here, the instantaneous angular velocity and instantaneous angular acceleration are vector quantities, but the corresponding average values are not because angular displacements do not add as vector quantities for finite rotations.

WW Instantaneous angular acceleration

Pitfall Prevention 10.2 Specify Your Axis In solving rotation problems, you must specify an axis of rotation. This new feature does not exist in our study of translational motion. The choice is arbitrary, but once you make it, you must maintain that choice consistently throughout the problem. In some problems, the physical situation suggests a natural axis, such as one along the axle of an automobile wheel. In other problems, there may not be an obvious choice, and you must exercise judgment.

296 Chapter 10

Rotation of a Rigid Object About a Fixed Axis S direction of v for the particle is out of the plane of the diagram when the rotation is counterclockwise and into the plane of the diagram when the rotation is clockwise. To illustrate this convention, it is convenient to use the right-hand rule demonstrated in Figure 10.3. When the four fingers of the right hand are wrapped in the S direction of rotation, the extended right thumb points in the direction of v . The S S S direction of a follows from its definition a ; dv /dt. It is in the same direction as S S if the angular speed is increasing in time, and it is antiparallel to v if the angular v speed is decreasing in time.

S

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Figure 10.3 The right-hand rule for determining the direction of the angular velocity vector.

In our study of translational motion, after introducing the kinematic variables, we considered the special case of a particle under constant acceleration. We follow the same procedure here for a rigid object under constant angular acceleration. Imagine a rigid object such as the CD in Figure 10.1 rotates about a fixed axis and has a constant angular acceleration. In parallel with our analysis model of the particle under constant acceleration, we generate a new analysis model for rotational motion called the rigid object under constant angular acceleration. We develop kinematic relationships for this model in this section. Writing Equation 10.5 in the form dv 5 a dt and integrating from ti 5 0 to tf 5 t gives

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10.2 Analysis Model: Rigid Object Under Constant Angular Acceleration

vf 5 vi 1 at (for constant a)

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Rotational kinematic equations

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where vi is the angular speed of the rigid object at time t 5 0. Equation 10.6 allows us to find the angular speed vf of the object at any later time t. Substituting Equation 10.6 into Equation 10.3 and integrating once more, we obtain u f 5 u i 1 v i t 1 12 at 2

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vf 2 5 vi2 1 2a(uf 2 ui ) (for constant a)

(10.7)

(10.8)

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This equation allows us to find the angular speed vf of the rigid object for any value of its angular position uf . If we eliminate a between Equations 10.6 and 10.7, we obtain u f 5 u i 1 12 1 v i 1 v f 2 t

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1 for constant a 2

where ui is the angular position of the rigid object at time t 5 0. Equation 10.7 allows us to find the angular position uf of the object at any later time t. Eliminating t from Equations 10.6 and 10.7 gives

Pitfall Prevention 10.3 Just Like Translation? Equations 10.6 to 10.9 and Table 10.1 might suggest that rotational kinematics is just like translational kinematics. That is almost true, with two key differences. (1) In rotational kinematics, you must specify a rotation axis (per Pitfall Prevention 10.2). (2) In rotational motion, the object keeps returning to its original orientation; therefore, you may be asked for the number of revolutions made by a rigid object. This concept has no analog in translational motion.

(10.6)

1 for constant a 2

(10.9)

Notice that these kinematic expressions for the rigid object under constant angular acceleration are of the same mathematical form as those for a particle under constant acceleration (Chapter 2). They can be generated from the equations for translational motion by making the substitutions x S u, v S v, and a S a. Table 10.1 compares the kinematic equations for the rigid object under constant angular acceleration and particle under constant acceleration models. Q uick Quiz 10.2 Consider again the pairs of angular positions for the rigid object in Quick Quiz 10.1. If the object starts from rest at the initial angular position, moves counterclockwise with constant angular acceleration, and arrives at the final angular position with the same angular speed in all three cases, for which choice is the angular acceleration the highest?

Table 10.1

Kinematic Equations for Rotational and Translational Motion

Rigid Object Under Constant Angular Acceleration

vf 5 vi 1 at uf 5 ui 1 vit 1 12 at 2 vf 2 5 vi2 1 2a(uf 2 ui) uf 5 ui 1 12 (vi 1 vf)t

(10.6) (10.7) (10.8) (10.9)

Particle Under Constant Acceleration

vf 5 vi 1 at xf 5 xi 1 vit 1 12 at 2 vf 2 5 vi2 1 2a(xf 2 xi) xf 5 xi 1 12 (vi 1 vf)t

(2.13) (2.16) (2.17) (2.15)

10.2 Analysis Model: Rigid Object Under Constant Angular Acceleration

297

Analysis Model Rigid Object Under Constant Angular Acceleration Examples:

Imagine an object that undergoes a spinning motion such that its angular acceleration is constant. The equations describing its angular position and angular speed are analogous to those for the particle under constant acceleration model:

vf 2 5 vi2 1 2a(uf 2 ui ) uf 5 ui 1

1 2 1 vi

1 v f 2 t

(10.7)

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u f 5 u i 1 v it 1

1 2 2 at

(10.8) (10.9)

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(10.6)

vf 5 vi 1 at

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a constant

• during its spin cycle, the tub of a clothes washer begins from rest and accelerates up to its final spin speed • a workshop grinding wheel is turned off and comes to rest under the action of a constant friction force in the bearings of the wheel • a gyroscope is powered up and approaches its operating speed (Chapter 11) • the crankshaft of a diesel engine changes to a higher angular speed (Chapter 22)

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Example 10.1 Rotating Wheel AM

A wheel rotates with a constant angular acceleration of 3.50 rad/s2.

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(A) I f the angular speed of the wheel is 2.00 rad/s at ti 5 0, through what angular displacement does the wheel rotate in 2.00 s? S o l u ti o n

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Conceptualize Look again at Figure 10.1. Imagine that the compact disc rotates with its angular speed increasing at a constant rate. You start your stopwatch when the disc is rotating at 2.00 rad/s. This mental image is a model for the motion of the wheel in this example.

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Categorize The phrase “with a constant angular acceleration” tells us to apply the rigid object under constant angular acceleration model to the wheel. Analyze From the rigid object under constant angular

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acceleration model, choose Equation 10.7 and rearrange it so that it expresses the angular displacement of the wheel:

Du 5 (2.00 rad/s)(2.00 s) 1 12(3.50 rad/s2)(2.00 s)2 5 11.0 rad 5 (11.0 rad)(1808/p rad) 5 6308

.a

Substitute the known values to find the angular displacement at t 5 2.00 s:

Du 5 u f 2 u i 5 v i t 1 12 at 2

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(B) Through how many revolutions has the wheel turned during this time interval?

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Multiply the angular displacement found in part (A) by a conversion factor to find the number of revolutions:

Du 5 6308a

1 rev b 5 1.75 rev 3608

(C) W hat is the angular speed of the wheel at t 5 2.00 s? S o l u ti o n

Use Equation 10.6 from the rigid object under constant angular acceleration model to find the angular speed at t 5 2.00 s:

vf 5 vi 1 at 5 2.00 rad/s 1 (3.50 rad/s2)(2.00 s) 5 9.00 rad/s

Finalize We could also obtain this result using Equation 10.8 and the results of part (A). (Try it!) uppose a particle moves along a straight line with a constant acceleration of 3.50 m/s2. If the velocity of S the particle is 2.00 m/s at ti 5 0, through what displacement does the particle move in 2.00 s? What is the velocity of the continued particle at t 5 2.00 s? W h at I f ?

298 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

▸ 10.1 c o n t i n u e d Answer Notice that these questions are translational analogs to parts (A) and (C) of the original problem. The mathematical solution follows exactly the same form. For the displacement, from the particle under constant acceleration model,

Dx 5 x f 2 x i 5 v it 1 12at 2 5 1 2.00 m/s 2 1 2.00 s 2 1 12 1 3.50 m/s2 2 1 2.00 s 2 2 5 11.0 m

and for the velocity,

vf 5 vi 1 at 5 2.00 m/s 1 (3.50 m/s2)(2.00 s) 5 9.00 m/s

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There is no translational analog to part (B) because translational motion under constant acceleration is not repetitive.

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10.3 Angular and Translational Quantities S

In this section, we derive some useful relationships between the angular speed and acceleration of a rotating rigid object and the translational speed and acceleration of a point in the object. To do so, we must keep in mind that when a rigid object rotates about a fixed axis as in Figure 10.4, every particle of the object moves in a circle whose center is on the axis of rotation. v Because point P in Figure 10.4 moves in a circle, the translational velocity vector S is always tangent to the circular path and hence is called tangential velocity. The magnitude of the tangential velocity of the point P is by definition the tangential speed v 5 ds/dt, where s is the distance traveled by this point measured along the circular path. Recalling that s 5 r u (Eq. 10.1a) and noting that r is constant, we obtain

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P s

r u

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Figure 10.4 As a rigid object

v5

ar ph

ys

rotates about the fixed axis (the z axis) through O, the point P v that is has a tangential velocity S always tangent to the circular path of radius r.

ds du 5r dt dt

Because d u/dt 5 v (see Eq. 10.3), it follows that v 5 r v

(10.10)

As we saw in Equation 4.17, the tangential speed of a point on a rotating rigid object equals the perpendicular distance of that point from the axis of rotation multiplied by the angular speed. Therefore, although every point on the rigid object has the same angular speed, not every point has the same tangential speed because r is not the same for all points on the object. Equation 10.10 shows that the tangential speed of a point on the rotating object increases as one moves outward from the center of rotation, as we would intuitively expect. For example, the outer end of a swinging golf club moves much faster than a point near the handle. We can relate the angular acceleration of the rotating rigid object to the tangential acceleration of the point P by taking the time derivative of v :

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Relation between tangential velocity and angular velocity

at 5 Relation between tangential acceleration and angular acceleration

dv dv 5r dt dt

at 5 ra

(10.11)

That is, the tangential component of the translational acceleration of a point on a rotating rigid object equals the point’s perpendicular distance from the axis of rotation multiplied by the angular acceleration. In Section 4.4, we found that a point moving in a circular path undergoes a radial acceleration ar directed toward the center of rotation and whose magnitude is that of the centripetal acceleration v 2/r (Fig. 10.5). Because v 5 r v for a point

10.3 Angular and Translational Quantities 299

P on a rotating object, we can express the centripetal acceleration at that point in terms of angular speed as we did in Equation 4.18:

ac 5

v2 5 rv 2 r

The total acceleration S S S of point P is a at ar

(10.12)

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at

The total acceleration vector at the point is S a 5S at 1 S a r , where the magnitude S S of a r is the centripetal acceleration ac . Because a is a vector having a radial and a at the point P on the rotating rigid a tangential component, the magnitude of S object is a 5 "a t 2 1 a r2 5 "r 2 a 2 1 r 2 v4 5 r "a2 1 v4

a

ar

(10.13)

x

O

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P

S

Figure 10.5 A s a rigid object rotates about a fixed axis (the z axis) through O, the point P experiences a tangential component of translational acceleration at and a radial component of translational acceleration ar .

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Q uick Quiz 10.3 Ethan and Joseph are riding on a merry-go-round. Ethan rides on a horse at the outer rim of the circular platform, twice as far from the center of the circular platform as Joseph, who rides on an inner horse. (i) When the merry-go-round is rotating at a constant angular speed, what is Ethan’s angular speed? (a) twice Joseph’s (b) the same as Joseph’s (c) half of Joseph’s (d) impossible to determine (ii) When the merry-go-round is rotating at a constant angular speed, describe Ethan’s tangential speed from the same list of choices.

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Example 10.2 CD Player AM

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On a compact disc (Fig. 10.6), audio information is stored digitally in a series of pits and flat areas on the surface of the disc. The alternations between pits and flat areas on the surface represent binary ones and zeros to be read by the CD player and converted back to sound waves. The pits and flat areas are detected by a system consisting of a laser and lenses. The length of a string of ones and zeros representing one piece of information is the same everywhere on the disc, whether the information is near the center of the disc or near its outer edge. So that this length of ones and zeros always passes by the laser–lens system in the same time interval, the tangential speed of the disc surface at the location of the lens must be constant. According to Equation 10.10, the angular speed must therefore vary as the laser–lens system moves radially along the disc. In a typical CD player, the constant speed of the surface at the point of the laser– lens system is 1.3 m/s.

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Conceptualize Figure 10.6 shows a photograph of a compact disc. Trace your finger around the circle marked “23 mm” and mentally estimate the time interval to go around the circle once. Now trace your finger around the circle marked “58 mm,” moving your finger across the surface of the page at the same speed as you did when tracing the smaller circle. Notice how much longer in time it takes your finger to go around the larger circle. If your finger represents the laser reading the disc, you can see that the disc rotates once in a longer time interval when the laser reads the information in the outer circle. Therefore, the disc must rotate more slowly when the laser is reading information from this part of the disc. Categorize This part of the example is categorized as a simple substitution problem. In later parts, we will need to identify analysis models. Use Equation 10.10 to find the angular speed that gives the required tangential speed at the position of the inner track:

vi 5

23 mm © Cengage Learning/George Semple

S o l u ti o n

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ind the angular speed of the disc in revolutions per minute when information is being read from the innermost (A) F first track (r 5 23 mm) and the outermost final track (r 5 58 mm).

58 mm

Figure 10.6 (Example 10.2) A compact disc.

1.3 m/s v 5 5 57 rad/s ri 2.3 3 1022 m

5 1 57 rad/s 2 a

60 s 1 rev ba b 5 5.4 3 102 rev/min 2p rad 1 min

continued

300 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

▸ 10.2 c o n t i n u e d Do the same for the outer track:

vf 5

1.3 m/s v 5 5 22 rad/s 5 2.1 3 102 rev/min rf 5.8 3 1022 m

The CD player adjusts the angular speed v of the disc within this range so that information moves past the objective lens at a constant rate. (B) T he maximum playing time of a standard music disc is 74 min and 33 s. How many revolutions does the disc make during that time? S o l u ti o n

om

Categorize From part (A), the angular speed decreases as the disc plays. Let us assume it decreases steadily, with a constant. We can then apply the rigid object under constant angular acceleration model to the disc. Analyze If t 5 0 is the instant the disc begins rotating, with angular speed of 57 rad/s, the final value of the time t is Du 5 u f 2 u i 5 12 1 v i 1 v f 2 t

Convert this angular displacement to revolutions:

Du 5 1 1.8 3 105 rad 2 a

ly

Use Equation 10.9 to find the angular displacement of the disc at t 5 4 473 s:

.c

(74 min)(60 s/min) 1 33 s 5 4 473 s. We are looking for the angular displacement Du during this time interval.

ee b

5 12 1 57 rad/s 1 22 rad/s 2 1 4 473 s 2 5 1.8 3 105 rad 1 rev b 5 2.8 3 104 rev 2p rad

s. w

(C) What is the angular acceleration of the compact disc over the 4 473-s time interval? S o l u ti o n

ar ph

ys

ic

Categorize We again model the disc as a rigid object under constant angular acceleration. In this case, Equation 10.6 gives the value of the constant angular acceleration. Another approach is to use Equation 10.4 to find the average angular acceleration. In this case, we are not assuming the angular acceleration is constant. The answer is the same from both equations; only the interpretation of the result is different. Analyze Use Equation 10.6 to find the angular acceleration:

a5

vf 2 vi t

5

22 rad/s 2 57 rad/s 5 27.6 3 10 23 rad/s2 4 473 s

sw

Finalize The disc experiences a very gradual decrease in its rotation rate, as expected from the long time interval required for the angular speed to change from the initial value to the final value. In reality, the angular acceleration of the disc is not constant. Problem 90 allows you to explore the actual time behavior of the angular acceleration.

w

.a

w

w

The component F sin f tends to rotate the wrench about an axis through O.

F sin f

S

F

r f O

S

d

r

F cos f

f

Figure 10.7 The force

Line of action

S

F has a greater rotating tendency about an axis through O as F increases and as the moment arm d increases.

10.4 Torque In our study of translational motion, after investigating the description of motion, we studied the cause of changes in motion: force. We follow the same plan here: What is the cause of changes in rotational motion? Imagine trying to rotate a door by applying a force of magnitude F perpendicular to the door surface near the hinges and then at various distances from the hinges. You will achieve a more rapid rate of rotation for the door by applying the force near the doorknob than by applying it near the hinges. When a force is exerted on a rigid object pivoted about an axis, the object tends to rotate about that axis. The tendency of a force to rotate an object about some axis is measured by a quantity called torque S t (Greek letter tau). Torque is a vector, but we will consider only its magnitude here; we will explore its vector nature in Chapter 11. Consider the wrench in Figure 10.7 that we wish to rotate around an axis that is perpendicular to the page and passes through the center of the bolt. The applied

10.4 Torque 301 S

force F acts at an angle fSto the horizontal. We define the magnitude of the torque associated with the force F around the axis passing through O by the expression t ; rF sin f 5 Fd

Pitfall Prevention 10.4 Torque Depends on Your Choice of Axis There is no unique value

(10.14)

of the torque on an object. Its value depends on your choice of rotation axis.

S

WW Moment arm

om

where r is the distance between the rotation axis and the point of application of F , and d is the perpendicular distance from the rotation axis to the line of action of S F. (The line of action of a force is an imaginary line extending out both ends of the S vector representing the force. The dashed line extending from the tail of F in Fig. S 10.7 is part of the line of action of F.) From the right triangle in Figure 10.7 that has the wrench as its hypotenuse, Swe see that d 5 r sin f. The quantity d is called the moment arm (or lever arm) of F. S In Figure 10.7, the only component of F that tends to cause rotation of the wrench around an axis through O is F sin f, the component perpendicular to a line drawn from the rotation axis to the point of application of the force. The horizontal component F cos f, because its line of action passes through O, has no tendency to produce rotation about an axis passing through O. From the definition of torque, the rotating tendency increases as F increases and as d increases, which explains why it is easier to rotate a door if we push at the doorknob rather than at a point close to the hinges. We also want to apply our push as closely perpendicular to the door as we can so that f is close to 908. Pushing sideways on the doorknob (f 5 0) will not cause the door to rotate. If two or more forces act on a rigid object as in Figure 10.8, each tends to proS duce rotation about the axis through O. In this example, F tends to rotate the 2 S object clockwise and F1 tends to rotate it counterclockwise. We use the convention that the sign of the torque resulting from a force is positive if the turning tendency of the force is counterclockwise and negative if the turning tendency is clockwise. S For Example, in Figure 10.8, the torque resulting from F , which has a moment arm 1 S d1, is positive and equal to 1F 1d1; the torque from F2 is negative and equal to 2F 2d 2. Hence, the net torque about an axis through O is

S

ly

.c

F1

d1

ys

ic

s. w

ee b

O d2

S

F2

Figure 10.8 The force

S

F 1 tends to rotate the object counterclockwise about an axis through O, and S F 2 tends to rotate it clockwise.

ar ph

o t 5 t1 1 t2 5 F 1d1 2 F 2d2

.a

sw

Torque should not be confused with force. Forces can cause a change in translational motion as described by Newton’s second law. Forces can also cause a change in rotational motion, but the effectiveness of the forces in causing this change depends on both the magnitudes of the forces and the moment arms of the forces, in the combination we call torque. Torque has units of force times length—newton meters (N ? m) in SI units—and should be reported in these units. Do not confuse torque and work, which have the same units but are very different concepts.

w

w

Q uick Quiz 10.4 (i) If you are trying to loosen a stubborn screw from a piece of wood with a screwdriver and fail, should you find a screwdriver for which the handle is (a) longer or (b) fatter? (ii) If you are trying to loosen a stubborn bolt from a piece of metal with a wrench and fail, should you find a wrench for which the handle is (a) longer or (b) fatter?

y

w

S

T1 R1

Example 10.3 The Net Torque on a Cylinder

R2

A one-piece cylinder is shaped as shown in Figure 10.9, with a core section protruding from the larger drum. The cylinder is free to rotate about the central z axis shown in the drawing. A rope wrapped around the drum, which has radius R 1, S exerts a force T 1 to the right on the cylinder. A rope wrapped around the core, S which has radius R 2, exerts a force T2 downward on the cylinder. (A) W hat is the net torque acting on the cylinder about the rotation axis (which is the z axis in Fig. 10.9)?

continued

O

x

z S

T2

Figure 10.9 (Example 10.3) A solid cylinder pivoted about the z axis S through O. The moment arm of T1 is S R 1, and the moment arm of T2 is R 2 .

302 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

▸ 10.3 c o n t i n u e d S o l u ti o n S

Conceptualize Imagine that the cylinder in Figure 10.9 is a shaft in a machine. The force T 1 could be applied by a S

drive belt wrapped around the drum. The force T 2 could be applied by a friction brake at the surface of the core.

Categorize This example is a substitution problem in which we evaluate the net torque using Equation 10.14. S The torque due to T 1 about the rotation axis is 2R 1T1. (The sign is negative because the torque tends to produce S clockwise rotation.) The torque due to T 2 is 1R 2T2. (The sign is positive because the torque tends to produce counterclockwise rotation of the cylinder.)

o t 5 t

Evaluate the net torque about the rotation axis:

1

1 t2 5 R 2T2 2 R 1T1

.c

om

As a quick check, notice that if the two forces are of equal magnitude, the net torque is negative because R 1 . R 2. StartS ing from rest with both forces of equal magnitude acting on it, the cylinder would rotate clockwise because T 1 would S be more effective at turning it than would T 2 .

ly

(B) S uppose T1 5 5.0 N, R 1 5 1.0 m, T2 5 15 N, and R 2 5 0.50 m. What is the net torque about the rotation axis, and which way does the cylinder rotate starting from rest?

ee b

S o l u ti o n

o t 5 (0.50 m)(15 N) 2 (1.0 m)(5.0 N) 5 2.5 N ? m

Substitute the given values:

Because this net torque is positive, the cylinder begins to rotate in the counterclockwise direction.

s. w

10.5 Analysis Model: Rigid Object Under a Net Torque

ic

The tangential force on the particle results in a torque on the particle about an axis through the center of the circle.

ys

In Chapter 5, we learned that a net force on an object causes an acceleration of the object and that the acceleration is proportional to the net force. These facts are the basis of the particle under a net force model whose mathematical representation is Newton’s second law. In this section, we show the rotational analog of Newton’s second law: the angular acceleration of a rigid object rotating about a fixed axis is proportional to the net torque acting about that axis. Before discussing the more complex case of rigid-object rotation, however, it is instructive first to discuss the case of a particle moving in a circular path about some fixed point under the influence of an external force. Consider a particle of mass m rotating in a circle of radius r under the influence S S of a tangential net force g Ft and a radial net force g Fr as shown in Figure 10.10. The radial net force causes the particle to move in the circular path with a centripetal acceleration. The tangential force provides a tangential acceleration S a t , and

S

ar ph

Ft

m S

sw

Fr

.a

r

Figure 10.10 A particle rotating

w

w

w

in a circle under the influence of a S tangential net force g Ft . A radial S net force g Fr also must be present to maintain the circular motion.

o Ft 5Smat

The magnitude of the net torque due to g Ft on the particle about an axis perpendicular to the page through the center of the circle is

o t 5 o Ft r 5 (mat )r Because the tangential acceleration is related to the angular acceleration through the relationship at 5 ra (Eq. 10.11), the net torque can be expressed as

o t 5 (mra)r 5 (mr 2)a

(10.15)

mr 2

Let us denote the quantity with the symbol I for now. We will say more about this quantity below. Using this notation, Equation 10.15 can be written as

o t 5 Ia

(10.16)

That is, the net torque acting on the particle is proportional to its angular acceleration. Notice that o t 5 Ia has the same mathematical form as Newton’s second law of motion, o F 5 ma.

10.5 Analysis Model: Rigid Object Under a Net Torque

Now let us extend this discussion to a rigid object of arbitrary shape rotating about a fixed axis passing through a point O as in Figure 10.11. The object can be regarded as a collection of particles of mass m i . If we impose a Cartesian coordinate system on the object, each particle rotates in a circle about the origin and each has a tangential acceleration ai produced by an external tangential force of magnitude Fi . For any given particle, we know from Newton’s second law that Fi 5 m i ai

303

The particle of mass mi of the rigid object experiences a torque in the same way that the particle in Figure 10.10 does. y

S

Fi

S

mi

S The external torque t i associated with the force Fi acts about the origin and its magnitude is given by ti 5 ri Fi 5 ri m i ai

r

om

Because ai 5 ri a, the expression for ti becomes

O

ti 5 m i ri2a

Figure 10.11 A rigid object rotating about an axis through O. Each particle of mass mi rotates about the axis with the same angular acceleration a.

ee b

ly

.c

Although each particle in the rigid object may have a different translational acceleration ai , they all have the same angular acceleration a. With that in mind, we can add the torques on all of the particles making up the rigid object to obtain the net torque on the object about an axis through O due to all external forces:

x

2 2 a text 5 a ti 5 a m i ri a 5 a a m i ri ba

i

i

i

(10.17)

o text 5 Ia

(10.18)

ic

s. w

where a can be taken outside the summation because it is common to all particles. Calling the quantity in parentheses I, the expression for o text becomes

ar ph

ys

This equation for a rigid object is the same as that found for a particle moving in a circular path (Eq. 10.16). The net torque about the rotation axis is proportional to the angular acceleration of the object, with the proportionality factor being I, a quantity that we have yet to describe fully. Equation 10.18 is the mathematical representation of the analysis model of a rigid object under a net torque, the rotational analog to the particle under a net force. Let us now address the quantity I, defined as follows:

WW Torque on a rigid object is proportional to angular acceleration

I 5 a m i ri 2

sw

(10.19)

i

w

w

w

.a

This quantity is called the moment of inertia of the object, and depends on the masses of the particles making up the object and their distances from the rotation axis. Notice that Equation 10.19 reduces to I 5 mr 2 for a single particle, consistent with our use of the notation I that we used in going from Equation 10.15 to Equation 10.16. Note that moment of inertia has units of kg · m2 in SI units. Equation 10.18 has the same form as Newton’s second law for a system of particles as expressed in Equation 9.39: a Fext 5 M a CM S

S

Consequently, the moment of inertia I must play the same role in rotational motion as the role that mass plays in translational motion: the moment of inertia is the resistance to changes in rotational motion. This resistance depends not only on the mass of the object, but also on how the mass is distributed around the rotation axis. Table 10.2 on page 304 gives the moments of inertia2 for a number of objects about specific axes. The moments of inertia of rigid objects with simple geometry (high symmetry) are relatively easy to calculate provided the rotation axis coincides with an axis of symmetry, as we show in the next section. 2Civil

engineers use moment of inertia to characterize the elastic properties (rigidity) of such structures as loaded beams. Hence, it is often useful even in a nonrotational context.

Pitfall Prevention 10.5 No Single Moment of Inertia There is one major difference between mass and moment of inertia. Mass is an inherent property of an object. The moment of inertia of an object depends on your choice of rotation axis. Therefore, there is no single value of the moment of inertia for an object. There is a minimum value of the moment of inertia, which is that calculated about an axis passing through the center of mass of the object.

304 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

Table 10.2 Moments of Inertia of Homogeneous Rigid Objects with Different Geometries Hollow cylinder 1 M(R12 R22) 2

ICM

R

Solid cylinder or disk 1 ICM MR 2 2

Rectangular plate 1 ICM M(a2 b2) 12

R2

b

ly

.c

R

R1

om

Hoop or thin cylindrical shell ICM MR 2

ee b

a

Long, thin rod with rotation axis through end 1 ML2 I 3

s. w

Long, thin rod with rotation axis through center 1 ICM ML2 12

L

ys

ic

L

Thin spherical shell 2 ICM MR 2 3

R

R

sw

ar ph

Solid sphere 2 ICM MR 2 5

w

w

w

.a

Q uick Quiz 10.5 You turn off your electric drill and find that the time interval for the rotating bit to come to rest due to frictional torque in the drill is Dt. You replace the bit with a larger one that results in a doubling of the moment of inertia of the drill’s entire rotating mechanism. When this larger bit is rotated at the same angular speed as the first and the drill is turned off, the frictional torque remains the same as that for the previous situation. What is the time interval for this second bit to come to rest? (a) 4Dt (b) 2Dt (c) Dt (d) 0.5Dt (e) 0.25Dt (f) impossible to determine

Analysis Model Rigid Object Under a Net Torque Imagine you are analyzing the motion of an object that is free to rotate about a fixed axis. The cause of changes in rotational motion of this object is torque applied to the object and, in parallel to Newton’s second law for translation motion, the torque is equal to the product of the moment of inertia of the object and the angular acceleration:

o t

ext

5 Ia

(10.18)

The torque, the moment of inertia, and the angular acceleration must all be evaluated around the same rotation axis.

a

10.5 Analysis Model: Rigid Object Under a Net Torque

305

Analysis Model Rigid Object Under a Net Torque (continued) Examples: • a bicycle chain around the sprocket of a bicycle causes the rear wheel of the bicycle to rotate • an electric dipole moment in an electric field rotates due to the electric force from the field (Chapter 23) • a magnetic dipole moment in a magnetic field rotates due to the magnetic force from the field (Chapter 30) • the armature of a motor rotates due to the torque exerted by a surrounding magnetic field (Chapter 31)

om

Example 10.4 Rotating Rod AM A uniform rod of length L and mass M is attached at one end to a frictionless pivot and is free to rotate about the pivot in the vertical plane as in Figure 10.12. The rod is released from rest in the horizontal position. What are the initial angular acceleration of the rod and the initial translational acceleration of its right end?

ly

.c

L

ee b

S o l u ti o n

Conceptualize Imagine what happens to the rod in Figure 10.12 when it is released. It rotates clockwise around the pivot at the left end.

s. w

Categorize The rod is categorized as a rigid object under a net torque. The torque is

Pivot S

Mg

Figure 10.12 (Example 10.4) A rod is free to rotate around a pivot at the left end. The gravitational force on the rod acts at its center of mass.

ys

ic

due only to the gravitational force on the rod if the rotation axis is chosen to pass through the pivot in Figure 10.12. We cannot categorize the rod as a rigid object under constant angular acceleration because the torque exerted on the rod and therefore the angular acceleration of the rod vary with its angular position.

Analyze The only force contributing to the torque about an axis through the pivot is the gravitational force M gS

.a

sw

ar ph

exerted on the rod. (The force exerted by the pivot on the rod has zero torque about the pivot because its moment arm is zero.) To compute the torque on the rod, we assume the gravitational force acts at the center of mass of the rod as shown in Figure 10.12. L Write an expression for the magnitude of the net external a text 5 Mg a 2 b torque due to the gravitational force about an axis through the pivot: Mg 1 L/2 2 3g a text Use Equation 10.18 to obtain the angular acceleration of the (1) a 5 5 1 5 2 I 2L 3 ML rod, using the moment of inertia for the rod from Table 10.2: at 5 La 5

3 2g

w

w

Use Equation 10.11 with r 5 L to find the initial translational acceleration of the right end of the rod:

w

Finalize These values are the initial values of the angular and translational accelerations. Once the rod begins to rotate, the gravitational force is no longer perpendicular to the rod and the values of the two accelerations decrease, going to zero at the moment the rod passes through the vertical orientation. W h at I f ? What if we were to place a penny on the end of the rod and then release the rod? Would the penny stay in contact with the rod?

Answer T he result for the initial acceleration of a point on the end of the rod shows that at . g. An unsupported penny falls at acceleration g. So, if we place a penny on the end of the rod and then release the rod, the end of the rod falls faster than the penny does! The penny does not stay in contact with the rod. (Try this with a penny and a meterstick!) The question now is to find the location on the rod from the pivot point, we combine Equation (1) with at which we can place a penny that will stay in contact Equation 10.11: 3g as both begin to fall. To find the translational acceleraat 5 r a 5 r tion of an arbitrary point on the rod at a distance r , L 2L continued

306 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

▸ 10.4 c o n t i n u e d For the penny to stay in contact with the rod, the limiting case is that the translational acceleration must be equal to that due to gravity:

at 5 g 5

3g 2L

r

r 5 23L Therefore, a penny placed closer to the pivot than twothirds of the length of the rod stays in contact with the falling rod, but a penny farther out than this point loses contact.

.c

When a tall smokestack falls over, it often breaks somewhere along its length before it hits the ground as shown in Figure 10.13. Why?

Kevin Spreekmeester/AGE fotostock

Conceptual Example 10.5 Falling Smokestacks and Tumbling Blocks

om

ly

S o l u ti o n

ys

ic

s. w

ee b

As the smokestack rotates around its base, each higher portion of the smokestack falls with a larger tangential acceleration than the portion below it according to Equation 10.11. The angular acceleration increases as the smokestack tips farther. Eventually, higher portions of the smokestack experience an acceleration greater than the acceleration that could result from gravity alone; this situation is similar to that Figure 10.13 (Conceptual described in Example 10.4. That can happen only if these portions are being Example 10.5) A falling smokepulled downward by a force in addition to the gravitational force. The force that stack breaks at some point along causes that to occur is the shear force from lower portions of the smokestack. Evenits length. tually, the shear force that provides this acceleration is greater than the smokestack can withstand, and the smokestack breaks. The same thing happens with a tall tower of children’s toy blocks. Borrow some blocks from a child and build such a tower. Push it over and watch it come apart at some point before it strikes the floor.

ar ph

Example 10.6 Angular Acceleration of a Wheel AM

sw

M

O

w

w

.a

A wheel of radius R, mass M, and moment of inertia I is mounted on a frictionless, horizontal axle as in Figure 10.14. A light cord wrapped around the wheel supports an object of mass m. When the wheel is released, the object accelerates downward, the cord unwraps off the wheel, and the wheel rotates with an angular acceleration. Find expressions for the angular acceleration of the wheel, the translational acceleration of the object, and the tension in the cord.

R S

T

w

S o l u ti o n

S

Conceptualize Imagine that the object is a bucket in an old-fashioned water well. It is tied to a cord that passes around a cylinder equipped with a crank for raising the bucket. After the bucket has been raised, the system is released and the bucket accelerates downward while the cord unwinds off the cylinder.

Categorize We apply two analysis models here. The object is modeled as a particle under a net force. The wheel is modeled as a rigid object under a net torque.

T m

Figure 10.14 (Example 10.6) An object hangs from a cord wrapped around a wheel.

S

mg

Analyze T he magnitude of the torque acting on the wheel about its axis of rotation is t 5 TR, where T is the force exerted by the cord on the rim of the wheel. (The gravitational force exerted by the Earth on the wheel and the

10.6 Calculation of Moments of Inertia 307

▸ 10.6 c o n t i n u e d normal force exerted by the axle on the wheel both pass through the axis of rotation and therefore produce no torque.) From the rigid object under a net torque model, write Equation 10.18:

o t

Solve for a and substitute the net torque:

(1) a 5

Solve for the acceleration a:

y

TR a text 5 I I

5 mg 2 T 5 ma

(2) a 5

mg 2 T m

om

o F

From the particle under a net force model, apply Newton’s second law to the motion of the object, taking the downward direction to be positive:

5 Ia

.c

ext

ee b

ly

Equations (1) and (2) have three unknowns: a, a, and T. Because the object and wheel are connected by a cord that does not slip, the translational acceleration of the suspended object is equal to the tangential acceleration of a point on the wheel’s rim. Therefore, the angular acceleration a of the wheel and the translational acceleration of the object are related by a 5 Ra. Use this fact together with Equations (1) and (2):

s. w

(3) a 5 R a 5

Solve for the tension T :

ic

(4) T 5

(5) a 5

ys

Substitute Equation (4) into Equation (2) and solve for a:

ar ph

Use a 5 Ra and Equation (5) to solve for a:

a5

mg 2 T TR 2 5 m I

mg 1 1 1 mR 2/I 2 g

1 1 1 I/mR 2 2

g a 5 R R 1 1 I/mR 2

Finalize We finalize this problem by imagining the behavior of the system in some extreme limits. W hat if the wheel were to become very massive so that I becomes very large? What happens to the acceleration a of the object and the tension T ?

sw

W h at I f ?

w

.a

Answer If the wheel becomes infinitely massive, we can imagine that the object of mass m will simply hang from the cord without causing the wheel to rotate. We can show that mathematically by taking the limit I S `. Equation (5) then becomes g a5 S 0 1 1 1 I/mR 2 2

w

w

which agrees with our conceptual conclusion that the object will hang at rest. Also, Equation (4) becomes mg T5 S mg 1 1 1 mR 2/I 2

which is consistent because the object simply hangs at rest in equilibrium between the gravitational force and the tension in the string.

10.6 Calculation of Moments of Inertia The moment of inertia of a system of discrete particles can be calculated in a straightforward way with Equation 10.19. We can evaluate the moment of inertia of a continuous rigid object by imagining the object to be divided into many small elements, each of which has mass Dm i . We use the definition I 5 oi ri2 Dmi

308 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

and take the limit of this sum as Dm i S 0. In this limit, the sum becomes an integral over the volume of the object:

I 5 lim a ri2 Dm i 5 3 r 2 dm Dm S 0

Moment of inertia of a rigid object

(10.20)

i

i

It is usually easier to calculate moments of inertia in terms of the volume of the elements rather than their mass, and we can easily make that change by using Equation 1.1, r ; m/V, where r is the density of the object and V is its volume. From this equation, the mass of a small element is dm 5 r dV. Substituting this result into Equation 10.20 gives

om

I 5 3 rr 2 dV

(10.21)

s. w

ee b

ly

.c

If the object is homogeneous, r is constant and the integral can be evaluated for a known geometry. If r is not constant, its variation with position must be known to complete the integration. The density given by r 5 m/V sometimes is referred to as volumetric mass density because it represents mass per unit volume. Often we use other ways of expressing density. For instance, when dealing with a sheet of uniform thickness t, we can define a surface mass density s 5 rt, which represents mass per unit area. Finally, when mass is distributed along a rod of uniform cross-sectional area A, we sometimes use linear mass density l 5 M/L 5 rA, which is the mass per unit length.

Example 10.7 Uniform Rigid Rod

y

y

ys

ic

Calculate the moment of inertia of a uniform thin rod of length L and mass M (Fig. 10.15) about an axis perpendicular to the rod (the y9 axis) and passing through its center of mass.

ar ph

S o l u ti o n

dx

Conceptualize Imagine twirling the rod in Fig-

sw

ure 10.15 with your fingers around its midpoint. If you have a meterstick handy, use it to simulate the spinning of a thin rod and feel the resistance it offers to being spun.

Figure 10.15 (Example 10.7) A uniform rigid rod of length L. The moment of inertia about the y9 axis is less than that about the y axis. The latter axis is examined in Example 10.9.

x O x L

w

.a

Categorize This example is a substitution problem, using the definition of moment of inertia in Equation 10.20. As with any integration problem, the solution involves reducing the integrand to a single variable. The shaded length element dx9 in Figure 10.15 has a mass dm equal to the mass per unit length l multiplied by dx9.

w

w

Express dm in terms of dx9:

Substitute this expression into Equation 10.20, with r 2 5 (x9)2:

dm 5 l dxr 5

M dxr L

I y r 5 3 r 2 dm 5 3

2L/2

5

L/2

M M 1 xr 2 2 dxr dxr 5 L L 32L/2 L/2

1 xr 2 2

M 1 xr 2 5 c d L 3 2L/2 3

Check this result in Table 10.2.

L/2

1 2 12 M L

Example 10.8 Uniform Solid Cylinder A uniform solid cylinder has a radius R, mass M, and length L. Calculate its moment of inertia about its central axis (the z axis in Fig. 10.16).

10.6 Calculation of Moments of Inertia 309

▸ 10.8 c o n t i n u e d S o l u ti o n

z

Conceptualize To simulate this situation, imagine twirling a can of frozen juice around its central axis. Don’t twirl a nonfrozen can of vegetable soup; it is not a rigid object! The liquid is able to move relative to the metal can.

r

dr

R

Categorize This example is a substitution problem, using the defini-

L

tion of moment of inertia. As with Example 10.7, we must reduce the integrand to a single variable. It is convenient to divide the cylinder into many cylindrical shells, each having radius r, thickness dr, and length L as shown in Figure 10.16. The density of the cylinder is r. The volume dV of each shell is its cross-sectional area multiplied by its length: dV 5 L dA 5 L(2pr) dr.

om

Figure 10.16 (Exam-

.c

ple 10.8) Calculating I about the z axis for a uniform solid cylinder.

dm 5 r dV 5 rL(2pr) dr

Substitute this expression into Equation 10.20:

Iz 5 3 r 2 dm 5 3 r 2 3 rL 1 2pr 2 dr 4 5 2prL 3 r 3 dr 5 12 prLR 4

Iz 5 12 pa

Check this result in Table 10.2.

0

M bLR 4 5 pR 2L

1 2 2M R

ic

Substitute this value into the expression for Iz:

R

M M 5 r5 V pR 2L

s. w

Use the total volume pR 2L of the cylinder to express its density:

ee b

ly

Express dm in terms of dr :

What if the length of the cylinder in Figure 10.16 is increased to 2L, while the mass M and radius R are held fixed? How does that change the moment of inertia of the cylinder?

ys

W h at I f ?

ar ph

Answer Notice that the result for the moment of inertia of a cylinder does not depend on L, the length of the cylinder. It applies equally well to a long cylinder and a flat disk having the same mass M and radius R. Therefore, the moment of inertia of the cylinder is not affected by how the mass is distributed along its length.

sw

w

w

w

.a

The calculation of moments of inertia of an object about an arbitrary axis can be cumbersome, even for a highly symmetric object. Fortunately, use of an important theorem, called the parallel-axis theorem, often simplifies the calculation. To generate the parallel-axis theorem, suppose the object in Figure 10.17a on page 310 rotates about the z axis. The moment of inertia does not depend on how the mass is distributed along the z axis; as we found in Example 10.8, the moment of inertia of a cylinder is independent of its length. Imagine collapsing the threedimensional object into a planar object as in Figure 10.17b. In this imaginary process, all mass moves parallel to the z axis until it lies in the xy plane. The coordinates of the object’s center of mass are now x CM, y CM, and z CM 5 0. Let the mass element dm have coordinates (x, y, 0) as shown in the view down the z axis in Figure 10.17c. Because this element is a distance r 5 !x 2 1 y 2 from the z axis, the moment of inertia of the entire object about the z axis is I 5 3 r 2 dm 5 3 1 x 2 1 y 2 2 dm

We can relate the coordinates x, y of the mass element dm to the coordinates of this same element located in a coordinate system having the object’s center of mass as its origin. If the coordinates of the center of mass are x CM, y CM, and z CM 5 0 in the original coordinate system centered on O, we see from Figure 10.17c that

310 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

z Rotation axis

Axis through CM y

y

y

dm x, y

z Rotation axis

Axis through CM y

O

y y

CM yCM

xCM, yCM D

om

x

O

xCM x

x

x

x

.c

x

c

ly

b

a

r

Figure 10.17 (a) An arbitrarily shaped rigid object. The origin of the coordinate system is not at

s. w

ee b

the center of mass of the object. Imagine the object rotating about the z axis. (b) All mass elements of the object are collapsed parallel to the z axis to form a planar object. (c) An arbitrary mass element dm is indicated in blue in this view down the z axis. The parallel axis theorem can be used with the geometry shown to determine the moment of inertia of the original object around the z axis.

the relationships between the unprimed and primed coordinates are x 5 x9 1 x CM, y 5 y9 1 y CM, and z 5 z9 5 0. Therefore,

ic

I 5 3 3 1 xr 1 x CM 2 2 1 1 yr 1 y CM 2 2 4 dm

ar ph

ys

5 3 3 1 xr 2 2 1 1 yr 2 2 4 dm 1 2x CM 3 xr dm 1 2y CM 3 yrdm 1 1 x CM2 1 y CM2 2 3 dm

I 5 I CM 1 MD 2

(10.22)

w

.a

Parallel-axis theorem

sw

The first integral is, by definition, the moment of inertia I CM about an axis that is parallel to the z axis and passes through the center of mass. The second two integrals are zero because, by definition of the center of mass, e x9 dm 5 e y9 dm 5 0. The last integral is simply MD 2 because e dm 5 M and D 2 5 x CM2 1 y CM2. Therefore, we conclude that

w

Example 10.9 Applying the Parallel-Axis Theorem

w

Consider once again the uniform rigid rod of mass M and length L shown in Figure 10.15. Find the moment of inertia of the rod about an axis perpendicular to the rod through one end (the y axis in Fig. 10.15). S o l u ti o n

Conceptualize Imagine twirling the rod around an endpoint rather than the midpoint. If you have a meterstick handy, try it and notice the degree of difficulty in rotating it around the end compared with rotating it around the center.

Categorize This example is a substitution problem, involving the parallel-axis theorem. 1 Intuitively, we expect the moment of inertia to be greater than the result ICM 5 12 ML2 from Example 10.7 because there is mass up to a distance of L away from the rotation axis, whereas the farthest distance in Example 10.7 was only L/2. The distance between the center-of-mass axis and the y axis is D 5 L/2.

10.7 Rotational Kinetic Energy 311

▸ 10.9 c o n t i n u e d I 5 I CM 1 M D 2 5

Use the parallel-axis theorem:

1 2 12 M L

Check this result in Table 10.2.

L 2 1Ma b 5 2

1 2 3M L

10.7 Rotational Kinetic Energy

om

After investigating the role of forces in our study of translational motion, we turned our attention to approaches involving energy. We do the same thing in our current study of rotational motion. In Chapter 7, we defined the kinetic energy of an object as the energy associated with its motion through space. An object rotating about a fixed axis remains stationary in space, so there is no kinetic energy associated with translational motion. The individual particles making up the rotating object, however, are moving through space; they follow circular paths. Consequently, there is kinetic energy associated with rotational motion. Let us consider an object as a system of particles and assume it rotates about a fixed z axis with an angular speed v. Figure 10.18 shows the rotating object and identifies one particle on the object located at a distance r i from the rotation axis. If the mass of the ith particle is mi and its tangential speed is vi , its kinetic energy is

z axis

.c

v

S

s. w

ee b

ly

vi

ic

K i 5 12m iv i 2

ys

To proceed further, recall that although every particle in the rigid object has the same angular speed v, the individual tangential speeds depend on the distance r i from the axis of rotation according to Equation 10.10. The total kinetic energy of the rotating rigid object is the sum of the kinetic energies of the individual particles:

ar ph

K R 5 a K i 5 a 12m iv i 2 5 12 a m iri 2v 2 i

i

i

mi ri O

Figure 10.18 A rigid object rotating about the z axis with angular speed v. The kinetic energy of the particle of mass mi is 1 2 2 m i v i . The total kinetic energy of the object is called its rotational kinetic energy.

We can write this expression in the form

K R 5 12 a a m i ri 2 b v 2

(10.23)

K R 5 12Iv 2

(10.24)

sw

i

w

w

.a

where we have factored v2 from the sum because it is common to every particle. We recognize the quantity in parentheses as the moment of inertia of the object, introduced in Section 10.5. Therefore, Equation 10.23 can be written 1 2 2 Iv

w

Although we commonly refer to the quantity as rotational kinetic energy, it is not a new form of energy. It is ordinary kinetic energy because it is derived from a sum over individual kinetic energies of the particles contained in the rigid object. The mathematical form of the kinetic energy given by Equation 10.24 is convenient when we are dealing with rotational motion, provided we know how to calculate I. Q uick Quiz 10.6 A section of hollow pipe and a solid cylinder have the same radius, mass, and length. They both rotate about their long central axes with the same angular speed. Which object has the higher rotational kinetic energy? (a) The hollow pipe does. (b) The solid cylinder does. (c) They have the same rotational kinetic energy. (d) It is impossible to determine.

WW Rotational kinetic energy

312 Chapter 10

Rotation of a Rigid Object About a Fixed Axis y

Example 10.10 An Unusual Baton

m

Four tiny spheres are fastened to the ends of two rods of negligible mass lying in the xy plane to form an unusual baton (Fig. 10.19). We shall assume the radii of the spheres are small compared with the dimensions of the rods.

y m

b M

a

a

(A) I f the system rotates about the y axis (Fig. 10.19a) with an angular speed v, find the moment of inertia and the rotational kinetic energy of the system about this axis.

M b

M

a

x

x a

b

b

M

m

m

Conceptualize Figure 10.19 is a pictorial representation that helps conceptualize the system of spheres and how it spins. Model the spheres as particles.

.c

om

Solution

a

Categorize This example is a substitution problem because it is a straightforward application of the definitions discussed in this section.

b

ee b

ly

Figure 10.19 (Example 10.10) Four spheres form an unusual baton. (a) The baton is rotated about the y axis. (b) The baton is rotated about the z axis.

Apply Equation 10.19 to the system:

Iy 5 a m i ri 2 5 Ma 2 1 Ma 2 5 2Ma 2

Evaluate the rotational kinetic energy using Equation 10.24:

K R 5 12 Iy v 2 5 12 1 2Ma 2 2 v 2 5 Ma 2v2

s. w

i

ys

ic

That the two spheres of mass m do not enter into this result makes sense because they have no motion about the axis of rotation; hence, they have no rotational kinetic energy. By similar logic, we expect the moment of inertia about the x axis to be Ix 5 2mb 2 with a rotational kinetic energy about that axis of K R 5 mb 2 v2.

ar ph

uppose the system rotates in the xy plane about an axis (the z axis) through the center of the baton (Fig. 10.19b). (B) S Calculate the moment of inertia and rotational kinetic energy about this axis. Solution

Apply Equation 10.19 for this new rotation axis:

sw

Evaluate the rotational kinetic energy using Equation 10.24:

Iz 5 a m i ri 2 5 Ma 2 1 Ma 2 1 mb 2 1 mb 2 5 2Ma 2 1 2mb 2 i

K R 5 12 Iz v 2 5 12 1 2Ma 2 1 2mb 2 2 v 2 5 (Ma 2 1 mb 2)v2

What if the mass M is much larger than m? How do the answers to parts (A) and (B) compare?

w

W h at If ?

w

w

.a

Comparing the results for parts (A) and (B), we conclude that the moment of inertia and therefore the rotational kinetic energy associated with a given angular speed depend on the axis of rotation. In part (B), we expect the result to include all four spheres and distances because all four spheres are rotating in the xy plane. Based on the work–kinetic energy theorem, the smaller rotational kinetic energy in part (A) than in part (B) indicates it would require less work to set the system into rotation about the y axis than about the z axis.

Answer If M .. m, then m can be neglected and the moment of inertia and the rotational kinetic energy in part (B) become Iz 5 2Ma 2 and K R 5 Ma 2v2 which are the same as the answers in part (A). If the masses m of the two tan spheres in Figure 10.19 are negligible, these spheres can be removed from the figure and rotations about the y and z axes are equivalent.

10.8 Energy Considerations in Rotational Motion Having introduced rotational kinetic energy in Section 10.7, let us now see how an energy approach can be useful in solving rotational problems. We begin by considering the relationship between the torque acting on a rigid object and its resulting

10.8 Energy Considerations in Rotational Motion 313

rotational motion so as to generate expressions for power and a rotational analog to the work–kinetic energy theorem. Consider the rigid object pivotedSat O in FigS ure 10.20. Suppose a single external force F is applied at P, where F lies in the S plane of the page. The work done on the object by F as its point of application rotates through an infinitesimal distance ds 5 r d u is

S

F

f

S

dW 5 F ? d S s 5 1 F sin f 2 r du

S

ds

S

O

Figure 10.20 A rigid object rotates about an axis through O under the action of an external S force F applied at P.

(10.25)

dW 5 t du

.c

P

r

om

where F sin f is the tangential component of F, or, in other words, the component S of the force along the displacement. Notice that the radial component vector of F does no work on the object because it is perpendicular to the displacement of S the point of application of F. S Because the magnitude of the torque due to F about an axis through O is defined as rF sin f by Equation 10.14, we can write the work done for the infinitesimal rotation as

du

S

ee b

ly

The rate at which work is being done by F as the object rotates about the fixed axis through the angle du in a time interval dt is dW du 5t dt dt

P5

dW 5 tv dt

(10.26)

ic

s. w

Because dW/dt is the instantaneous power P (see Section 8.5) delivered by the force and du/dt 5 v, this expression reduces to WW Power delivered to a rotating rigid object

sw

ar ph

ys

This equation is analogous to P 5 Fv in the case of translational motion, and Equation 10.25 is analogous to dW 5 Fx dx. In studying translational motion, we have seen that models based on an energy approach can be extremely useful in describing a system’s behavior. From what we learned of translational motion, we expect that when a symmetric object rotates about a fixed axis, the work done by external forces equals the change in the rotational energy of the object. To prove that fact, let us begin with the rigid object under a net torque model, whose mathematical representation is o text 5 Ia. Using the chain rule from calculus, we can express the net torque as

.a

dv dv du dv a text 5 Ia 5 I dt 5 I du dt 5 I du v

w

w

Rearranging this expression and noting that o text du 5 dW gives

o text du 5 dW 5 Iv dv

w

Integrating this expression, we obtain for the work W done by the net external force acting on a rotating system W 5 3 Iv dv 5 12Iv f2 2 12Iv i2 vf

(10.27)

vi

where the angular speed changes from vi to vf . Equation 10.27 is the work–kinetic energy theorem for rotational motion. Similar to the work–kinetic energy theorem in translational motion (Section 7.5), this theorem states that the net work done by external forces in rotating a symmetric rigid object about a fixed axis equals the change in the object’s rotational energy. This theorem is a form of the nonisolated system (energy) model discussed in Chapter 8. Work is done on the system of the rigid object, which represents a transfer of energy across the boundary of the system that appears as an increase in the object’s rotational kinetic energy.

WW Work–kinetic energy theorem for rotational motion

314 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

Table 10.3

Useful Equations in Rotational and Translational Motion

Rotational Motion About a Fixed Axis

Translational Motion

Angular speed v 5 d u/dt Angular acceleration a 5 d v/dt Net torque otext 5 Ia If vf 5 vi 1 at

Translational speed v 5 dx/dt Translational acceleration a 5 dv/dt Net force oF 5 ma If vf 5 vi 1 at

a 5 constant • uf 5 ui 1 vit 1 12 at 2 a 5 constant vf 2 5 vi2 1 2a(uf 2 ui )

1

• xf 5 xi 1 vi t 1 2 at 2 2

2

vf 5 vi 1 2a(xf 2 xi )

Work W 5 3 t d u Work W 5 3 Fx dx xf

uf

xi

om

ui

Kinetic energy K 5 12 mv 2 Power P 5 Fv Linear momentum p 5 mv Net force oF 5 dp/dt

ly

.c

Rotational kinetic energy K R 5 12 Iv2 Power P 5 tv Angular momentum L 5 Iv Net torque ot 5 dL/dt

w

w

.a

sw

ar ph

ys

ic

s. w

ee b

In general, we can combine this theorem with the translational form of the work– kinetic energy theorem from Chapter 7. Therefore, the net work done by external forces on an object is the change in its total kinetic energy, which is the sum of the translational and rotational kinetic energies. For example, when a pitcher throws a baseball, the work done by the pitcher’s hands appears as kinetic energy associated with the ball moving through space as well as rotational kinetic energy associated with the spinning of the ball. In addition to the work–kinetic energy theorem, other energy principles can also be applied to rotational situations. For example, if a system involving rotating objects is isolated and no nonconservative forces act within the system, the isolated system model and the principle of conservation of mechanical energy can be used to analyze the system as in Example 10.11 below. In general, Equation 8.2, the conservation of energy equation, applies to rotational situations, with the recognition that the change in kinetic energy ∆K will include changes in both translational and rotational kinetic energies. Finally, in some situations an energy approach does not provide enough information to solve the problem and it must be combined with a momentum approach. Such a case is illustrated in Example 10.14 in Section 10.9. Table 10.3 lists the various equations we have discussed pertaining to rotational motion together with the analogous expressions for translational motion. Notice the similar mathematical forms of the equations. The last two equations in the lefthand column of Table 10.3, involving angular momentum L, are discussed in Chapter 11 and are included here only for the sake of completeness.

w

Example 10.11 Rotating Rod Revisited AM A uniform rod of length L and mass M is free to rotate on a frictionless pin passing through one end (Fig 10.21). The rod is released from rest in the horizontal position. (A) What is its angular speed when the rod reaches its lowest position?

O L/2 CM

S o l u ti o n

Conceptualize Consider Figure 10.21 and imagine the rod rotating downward through a quarter turn about the pivot at the left end. Also look back at Example 10.8. This physical situation is the same. Categorize A s mentioned in Example 10.4, the angular acceleration of the rod is not constant. Therefore, the kinematic equations for rotation (Section 10.2) can-

Figure 10.21 (Example 10.11) A uniform rigid rod pivoted at O rotates in a vertical plane under the action of the gravitational force.

10.8 Energy Considerations in Rotational Motion 315

▸ 10.11 c o n t i n u e d not be used to solve this example. We categorize the system of the rod and the Earth as an isolated system in terms of energy with no nonconservative forces acting and use the principle of conservation of mechanical energy.

Analyze We choose the configuration in which the rod is hanging straight down as the reference configuration for gravitational potential energy and assign a value of zero for this configuration. When the rod is in the horizontal position, it has no rotational kinetic energy. The potential energy of the system in this configuration relative to the reference configuration is MgL/2 because the center of mass of the rod is at a height L/2 higher than its position in the reference configuration. When the rod reaches its lowest position, the energy of the system is entirely rotational energy 12I v 2, where I is the moment of inertia of the rod about an axis passing through the pivot. DK 1 DU 5 0

Substitute for each of the final and initial energies:

1 12I v 2 2 0 2 1 1 0 2 12MgL 2 5 0 Å I

.c

MgL

MgL

ly

v5

5

Å 13ML2

ee b

Solve for v and use I 5 13ML2 (see Table 10.2) for the rod:

om

Using the isolated system (energy) model, write an appropriate reduction of Equation 8.2:

5

3g

ÅL

S o l u ti o n

v CM 5 r v 5

ic

Use Equation 10.10 and the result from part (A):

s. w

(B) Determine the tangential speed of the center of mass and the tangential speed of the lowest point on the rod when it is in the vertical position.

ys

Because r for the lowest point on the rod is twice what it is for the center of mass, the lowest point has a tangential speed twice that of the center of mass:

L v5 2

v 5 2v CM 5 "3gL

1 2

"3gL

ar ph

Finalize The initial configuration in this example is the same as that in Example 10.4. In Example 10.4, however, we could only find the initial angular acceleration of the rod. Applying an energy approach in the current example allows us to find additional information, the angular speed of the rod at the lowest point. Convince yourself that you could find the angular speed of the rod at any angular position by knowing the location of the center of mass at this position.

.a

sw

W h at I f ? W hat if we want to find the angular speed of the rod when the angle it makes with the horizontal is 45.08? Because this angle is half of 90.08, for which we solved the problem above, is the angular speed at this configuration half the answer in the calculation above, that is, 12 !3g/L?

Answer Imagine the rod in Figure 10.21 at the 45.08 position. Use a pencil or a ruler to represent the rod at this posi-

w

w

w

tion. Notice that the center of mass has dropped through more than half of the distance L/2 in this configuration. Therefore, more than half of the initial gravitational potential energy has been transformed to rotational kinetic energy. So, we should not expect the value of the angular speed to be as simple as proposed above. Note that the center of mass of the rod drops through a distance of 0.500L as the rod reaches the vertical configuration. When the rod is at 45.08 to the horizontal, we can show that the center of mass of the rod drops through a distance of 0.354L. Continuing the calculation, we find that the angular speed of the rod at this configuration is 0.841 !3g/L , (not 12 !3g/L).

Example 10.12 Energy and the Atwood Machine AM Two blocks having different masses m1 and m 2 are connected by a string passing over a pulley as shown in Figure 10.22 on page 316. The pulley has a radius R and moment of inertia I about its axis of rotation. The string does not slip on the pulley, and the system is released from rest. Find the translational speeds of the blocks after block 2 descends through a distance h and find the angular speed of the pulley at this time.

continued

316 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

▸ 10.12 c o n t i n u e d S o l u ti o n

Conceptualize We have already seen examples involving the Atwood machine, so the motion of the objects in Figure 10.22 should be easy to visualize.

R

Categorize Because the string does not slip, the pulley rotates about the axle. We can neglect friction in the axle because the axle’s radius is small relative to that of the pulley. Hence, the frictional torque is much smaller than the net torque applied by the two blocks provided that their masses are significantly different. Consequently, the system consisting of the two blocks, the pulley, and the Earth is an isolated system in terms of energy with no nonconservative forces acting; therefore, the mechanical energy of the system is conserved.

m2

m1

h

ly

.c

Figure 10.22 (Example 10.12) An Atwood machine with a massive pulley.

om

h

Analyze We define the zero configuration for gravitational potential energy as that which exists when the system is

ee b

released. From Figure 10.22, we see that the descent of block 2 is associated with a decrease in system potential energy and that the rise of block 1 represents an increase in potential energy. DK 1 DU 5 0

Substitute for each of the energies:

3 1 12m 1v f 2 1 12m 2v f 2 1 12I v f2 2 2 0 4 1 3 1 m 1gh 2 m 2gh 2 2 0 4 5 0

s. w

Using the isolated system (energy) model, write an appropriate reduction of the conservation of energy equation:

1 2 2 m 1v f

1 12m 2v f 2 1 12 I

ys

ic

Use vf 5 Rvf to substitute for vf :

ar ph

1 2 am 1

Solve for vf :

1 m2 1

(1) vf 5 c

sw

Use vf 5 R vf to solve for vf :

vf 5

vf R

5

vf2 R2

5 m 2gh 2 m 1gh

I bv f 2 5 1 m 2 2 m 1 2 gh R2

2 1 m 2 2 m 1 2 gh

m 1 1 m 2 1 I/R 2

1/2

d

1/2 2 1 m 2 2 m 1 2 gh 1 c 2 d R m 1 1 m 2 1 I/R

w w

w

.a

Finalize Each block can be modeled as a particle under constant acceleration because it experiences a constant net force. Think about what you would need to do to use Equation (1) to find the acceleration of one of the blocks. Then imagine the pulley becoming massless and determine the acceleration of a block. How does this result compare with the result of Example 5.9?

10.9 Rolling Motion of a Rigid Object In this section, we treat the motion of a rigid object rolling along a flat surface. In general, such motion is complex. For example, suppose a cylinder is rolling on a straight path such that the axis of rotation remains parallel to its initial orientation in space. As Figure 10.23 shows, a point on the rim of the cylinder moves in a complex path called a cycloid. We can simplify matters, however, by focusing on the center of mass rather than on a point on the rim of the rolling object. As shown in Figure 10.23, the center of mass moves in a straight line. If an object such as a cylinder rolls without slipping on the surface (called pure rolling motion), a simple relationship exists between its rotational and translational motions. Consider a uniform cylinder of radius R rolling without slipping on a horizontal surface (Fig. 10.24). As the cylinder rotates through an angle u, its center of mass

10.9 Rolling Motion of a Rigid Object 317 The center moves in a straight line (green line).

Figure 10.23 Two points on a

The point on the rim moves in the path called a cycloid (red curve).

rolling object take different paths through space. Henry Leap and Jim Lehman

One light source at the center of a rolling cylinder and another at one point on the rim illustrate the different paths these two points take.

ds du 5R 5 Rv dt dt

v CM 5

(10.28)

.c

om

moves a linear distance s 5 Ru (see Eq. 10.1a). Therefore, the translational speed of the center of mass for pure rolling motion is given by

.a

sw

ar ph

ys

ic

s. w

ee b

ly

where v is the angular speed of the cylinder. Equation 10.28 holds whenever a cylinder or sphere rolls without slipping and is the condition for pure rolling motion. The magnitude of the linear acceleration of the center of mass for pure rolling motion is dv CM dv a CM 5 5R 5 R a (10.29) dt dt where a is the angular acceleration of the cylinder. Imagine that you are moving along with a rolling object at speed v CM, staying in a frame of reference at rest with respect to the center of mass of the object. As you observe the object, you will see the object in pure rotation around its center of mass. Figure 10.25a shows the velocities of points at the top, center, and bottom of the object as observed by you. In addition to these velocities, every point on the object moves in the same direction with speed v CM relative to the surface on which it rolls. Figure 10.25b shows these velocities for a nonrotating object. In the reference frame at rest with respect to the surface, the velocity of a given point on the object is the sum of the velocities shown in Figures 10.25a and 10.25b. Figure 10.25c shows the results of adding these velocities. Notice that the contact point between the surface and object in Figure 10.25c has a translational speed of zero. At this instant, the rolling object is moving in exactly the same way as if the surface were removed and the object were pivoted at point P and spun about an axis passing through P. We can express the total kinetic energy of this imagined spinning object as K 5 12 IP v 2

w

R

u

s

s R u

Figure 10.24 For pure rolling motion, as the cylinder rotates through an angle u its center moves a linear distance s 5 R u.

Pitfall Prevention 10.6 Equation 10.28 Looks Familiar Equation 10.28 looks very similar to Equation 10.10, so be sure to be clear on the difference. Equation 10.10 gives the tangential speed of a point on a rotating object located a distance r from a fixed rotation axis if the object is rotating with angular speed v. Equation 10.28 gives the translational speed of the center of mass of a rolling object of radius R rotating with angular speed v.

(10.30)

w

w

where IP is the moment of inertia about a rotation axis through P.

Pure rotation

Pure translation

v R v

v CM

v0

CM

Combination of translation and rotation

CM

v v CM R v 2v CM

CM

v CM

v v CM

Figure 10.25 The motion of a v Rv a

P

P b

v CM

P c

v 0

rolling object can be modeled as a combination of pure translation and pure rotation.

318 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

Because the motion of the imagined spinning object is the same at this instant as our actual rolling object, Equation 10.30 also gives the kinetic energy of the rolling object. Applying the parallel-axis theorem, we can substitute IP 5 ICM 1 MR 2 into Equation 10.30 to obtain K 5 12 ICM v 2 1 12 MR 2v 2 Using v CM 5 Rv, this equation can be expressed as

om

ee b

vCM

.c

v S

u

The term represents the rotational kinetic energy of the object about its center of mass, and the term 12Mv CM2 represents the kinetic energy the object would have if it were just translating through space without rotating. Therefore, the total kinetic energy of a rolling object is the sum of the rotational kinetic energy about the center of mass and the translational kinetic energy of the center of mass. This statement is consistent with the situation illustrated in Figure 10.25, which shows that the velocity of a point on the object is the sum of the velocity of the center of mass and the tangential velocity around the center of mass. Energy methods can be used to treat a class of problems concerning the rolling motion of an object on a rough incline. For example, consider Figure 10.26, which shows a sphere rolling without slipping after being released from rest at the top of the incline. Accelerated rolling motion is possible only if a friction force is present between the sphere and the incline to produce a net torque about the center of mass. Despite the presence of friction, no loss of mechanical energy occurs because the contact point is at rest relative to the surface at any instant. (On the other hand, if the sphere were to slip, mechanical energy of the sphere– incline–Earth system would decrease due to the nonconservative force of kinetic friction.) In reality, rolling friction causes mechanical energy to transform to internal energy. Rolling friction is due to deformations of the surface and the rolling object. For example, automobile tires flex as they roll on a roadway, representing a transformation of mechanical energy to internal energy. The roadway also deforms a small amount, representing additional rolling friction. In our problem-solving models, we ignore rolling friction unless stated otherwise. Using v CM 5 Rv for pure rolling motion, we can express Equation 10.31 as

ly

R h

(10.31)

1 2 2 ICMv

M

x

K 5 12 ICM v 2 1 12 Mv CM2

Total kinetic energy of a rolling object

Figure 10.26 A sphere roll-

sw

ar ph

ys

ic

s. w

ing down an incline. Mechanical energy of the sphere–Earth system is conserved if no slipping occurs.

.a

K 5 12 ICM a K 5 12 a

ICM 1 Mbv CM2 R2

(10.32)

For the sphere–Earth system in Figure 10.26, we define the zero configuration of gravitational potential energy to be when the sphere is at the bottom of the incline. Therefore, Equation 8.2 gives

w

w

w

v CM 2 b 1 12 Mv CM2 R

DK 1 DU 5 0

c 12 a

ICM 1 Mbv CM2 2 0d 1 1 0 2 Mgh 2 5 0 R2 v CM 5 c

2gh

1 1 1 ICM /MR 2 2

1/2

d

(10.33)

Q uick Quiz 10.7 A ball rolls without slipping down incline A, starting from rest. At the same time, a box starts from rest and slides down incline B, which is identical to incline A except that it is frictionless. Which arrives at the bottom first? (a) The ball arrives first. (b) The box arrives first. (c) Both arrive at the same time. (d) It is impossible to determine.

10.9 Rolling Motion of a Rigid Object 319

Example 10.13 Sphere Rolling Down an Incline AM For the solid sphere shown in Figure 10.26, calculate the translational speed of the center of mass at the bottom of the incline and the magnitude of the translational acceleration of the center of mass. S o l u ti o n

Conceptualize Imagine rolling the sphere down the incline. Compare it in your mind to a book sliding down a frictionless incline. You probably have experience with objects rolling down inclines and may be tempted to think that the sphere would move down the incline faster than the book. You do not, however, have experience with objects sliding down frictionless inclines! So, which object will reach the bottom first? (See Quick Quiz 10.7.)

2gh

1/2

1 1 1 25 MR 2/MR 2 2

ly

sphere from Equation 10.33:

(1) v CM 5 c

d

.c

Analyze Evaluate the speed of the center of mass of the

om

Categorize We model the sphere and the Earth as an isolated system in terms of energy with no nonconservative forces acting. This model is the one that led to Equation 10.33, so we can use that result. 1/2 5 1 10 7 gh 2

ee b

This result is less than !2gh, which is the speed an object would have if it simply slid down the incline without rotating. (Eliminate the rotation by setting ICM 5 0 in Eq. 10.33.) To calculate the translational acceleration of the center of mass, notice that the vertical displacement of the sphere is related to the distance x it moves along the incline through the relationship h 5 x sin u.

Write Equation 2.17 for an object starting from rest and moving through a distance x under constant acceleration:

sin u

v CM2 5 2a CMx

ic

Equate the preceding two expressions to find a CM:

10 7 gx

s. w

v CM 2 5

Use this relationship to rewrite Equation (1):

a CM 5 57g sin u

sw

ar ph

ys

Finalize Both the speed and the acceleration of the center of mass are independent of the mass and the radius of the sphere. That is, all homogeneous solid spheres experience the same speed and acceleration on a given incline. Try to verify this statement experimentally with balls of different sizes, such as a marble and a croquet ball. If we were to repeat the acceleration calculation for a hollow sphere, a solid cylinder, or a hoop, we would obtain similar results in which only the factor in front of g sin u would differ. The constant factors that appear in the expressions for v CM and a CM depend only on the moment of inertia about the center of mass for the specific object. In all cases, the acceleration of the center of mass is less than g sin u, the value the acceleration would have if the incline were frictionless and no rolling occurred.

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Example 10.14 Pulling on a Spool3 AM

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A cylindrically symmetric spool of mass m and radius R sits at rest on a horizontal table with friction (Fig. 10.27). With your hand on a light string wrapped around the axle of radius r, you pull on the spool with a constant horizontal force of magnitude T to the right. As a result, the spool rolls without slipping a distance L along the table with no rolling friction.

L R

S

T

r

(A) Find the final translational speed of the center of mass of the spool. S o l u ti o n

Conceptualize Use Figure 10.27 to visualize the motion of the spool when you pull the string. For the spool to roll through a distance L, notice that your hand on the string must pull through a distance different from L.

3Example

Figure 10.27 (Example 10.14) A spool rests on a horizontal table. A string is wrapped around the axle and is pulled to the right by a hand.

continued

10.14 was inspired in part by C. E. Mungan, “A primer on work–energy relationships for introductory physics,” The Physics Teacher, 43:10, 2005.

320 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

▸ 10.14 c o n t i n u e d Categorize T he spool is a rigid object under a net torque, but the net torque includes that due to the friction force at the bottom of the spool, about which we know nothing. Therefore, an approach based on the rigid object under a net torque model will not be successful. Work is done by your hand on the spool and string, which form a nonisolated system in terms of energy. Let’s see if an approach based on the nonisolated system (energy) model is fruitful. Analyze The only type of energy that changes in the system is the kinetic energy of the spool. There is no rolling friction, so there is no change in internal energy. The only way that energy crosses the system’s boundary is by the work done by your hand on the string. No work is done by the static force of friction on the bottom of the spool (to the left in Fig. 10.27) because the point of application of the force moves through no displacement. (1) W 5 DK 5 DKtrans 1 DK rot

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Write the appropriate reduction of the conservation of energy equation, Equation 8.2:

ee b

r L R

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where W is the work done on the string by your hand. To find this work, we need to find the displacement of your hand during the process. We first find the length of string that has unwound off the spool. If the spool rolls through a distance L, the total angle through which it rotates is u 5 L/R. The axle also rotates through this angle. Use Equation 10.1a to find the total arc length through which the axle turns:

, 5 ru 5

ic

(2) W 5 TL a1 1

ys

Evaluate the work done by your hand on the string:

s. w

This result also gives the length of string pulled off the axle. Your hand will move through this distance plus the distance L through which the spool moves. Therefore, the magnitude of the displacement of the point of application of the force applied by your hand is , 1 L 5 L(1 1 r/R). r b R

TL a1 1

r b 5 12 mv CM2 1 12 Iv 2 R

Apply the nonslip rolling condition v 5 v CM/R :

TL a1 1

v CM2 r b 5 12 mv CM 2 1 12 I 2 R R

Solve for v CM:

(3) v CM 5

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Substitute Equation (2) into Equation (1):

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where I is the moment of inertia of the spool about its center of mass and v CM and v are the final values after the wheel rolls through the distance L.

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(B) Find the value of the friction force f.

2TL 1 1 1 r/R 2 Å m 1 1 1 I/mR 2 2

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S o l u ti o n

Categorize Because the friction force does no work, we cannot evaluate it from an energy approach. We model the spool as a nonisolated system, but this time in terms of momentum. The string applies a force across the boundary of the system, resulting in an impulse on the system. Because the forces on the spool are constant, we can model the spool’s center of mass as a particle under constant acceleration. Analyze Write the impulse–momentum theorem (Eq. 9.40) for the spool:

m(v CM 2 0) 5 (T 2 f )Dt (4) mv CM 5 (T 2 f )Dt

For a particle under constant acceleration starting from rest, Equation 2.14 tells us that the average velocity of the center of mass is half the final velocity. Use Equation 2.2 to find the time interval for the center of mass of the spool to move a distance L from rest to a final speed v CM:

(5) Dt 5

L 2L 5 v CM,avg v CM

Summary 321

▸ 10.14 c o n t i n u e d Substitute Equation (5) into Equation (4):

mv CM 5 1 T 2 f 2

Solve for the friction force f :

f5T2

Substitute v CM from Equation (3):

f5T2

2L v CM

mv CM2 2L

m 2TL 1 1 1 r/R 2 c d 2L m 1 1 1 I/m R 2 2

1 1 1 r/R 2 I 2 mrR d 5 Tc 1 1 1 I/mR 2 2 I 1 mR2

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5T2T

Finalize Notice that we could use the impulse–momentum theorem for the translational motion of the spool while ignoring that the spool is rotating! This fact demonstrates the power of our growing list of approaches to solving problems.

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Summary

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Definitions

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ys

ic

The angular position of a rigid object is defined as the angle u between a reference line attached to the object and a reference line fixed in space. The angular displacement of a particle moving in a circular path or a rigid object rotating about a fixed axis is Du ; uf 2 ui . The instantaneous angular speed of a particle moving in a circular path or of a rigid object rotating about a fixed axis is v;

du dt

(10.3)

The instantaneous angular acceleration of a particle moving in a circular path or of a rigid object rotating about a fixed axis is dv dt

sw a;

(10.5)

t 5 rF sin f 5 Fd (10.14) where f is the angle between the position vector of the point of application of the force and the force vector, and d is the moment arm of the force, which is the perpendicular distance from the rotation axis to the line of action of the force.

The moment of inertia of a system of particles is defined as I ; a m iri2

(10.19)

i

where mi is the mass of the ith particle and ri is its distance from the rotation axis.

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When a rigid object rotates about a fixed axis, every part of the object has the same angular speed and the same angular acceleration.

The magnitude of the torque associated S with a force F acting on an object at a distance r from the rotation axis is

Concepts and Principles

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continued

When a rigid object rotates about a fixed axis, the angular position, angular speed, and angular acceleration are related to the translational position, translational speed, and translational acceleration through the relationships

If a rigid object rotates about a fixed axis with angular speed v, its rotational kinetic energy can be written

K R 5 12I v 2

(10.24)

where I is the moment of inertia of the object about the axis of rotation.

s 5 r u

(10.1a)

v 5 r v

(10.10)

at 5 r a

(10.11)

where r is the distance from the mass element dm to the axis of rotation.

The moment of inertia of a rigid object is I 5 3 r 2 dm

(10.20) continued

322 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

The rate at which work is done by an external force in rotating a rigid object about a fixed axis, or the power delivered, is

(10.26)

P 5 tv

The total kinetic energy of a rigid object rolling on a rough surface without slipping equals the rotational kinetic energy about its center of mass plus the translational kinetic energy of the center of mass:

If work is done on a rigid object and the only result of the work is rotation about a fixed axis, the net work done by external forces in rotating the object equals the change in the rotational kinetic energy of the object: W 5 12 I v f2 2 12 I v i2

(10.27)

K 5 12 ICM v 2 1 12 Mv CM2

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Analysis Models for Problem Solving

(10.6)

u f 5 u i 1 12 1 v i 1 v f 2 t

(10.8)

(10.9)

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o t

ext

5 Ia

(10.18)

This equation is the rotational analog to Newton’s second law in the particle under a net force model.

1. denotes answer available in Student Solutions Manual/Study Guide

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Objective Questions

s. w

vf 2 5 vi2 1 2a(uf 2 ui )

ic

(10.7)

ys

u f 5 u i 1 v it 1

1 2 2 at

a

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a constant

vf 5 vi 1 at

Rigid Object Under a Net Torque. If a rigid object free to rotate about a fixed axis has a net external torque acting on it, the object undergoes an angular acceleration a, where

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Rigid Object Under Constant Angular Acceleration. If a rigid object rotates about a fixed axis under constant angular acceleration, one can apply equations of kinematics that are analogous to those for translational motion of a particle under constant acceleration:

(10.31)

.a

sw

1. A cyclist rides a bicycle with a wheel radius of 0.500 m across campus. A piece of plastic on the front rim makes a clicking sound every time it passes through the fork. If the cyclist counts 320 clicks between her apartment and the cafeteria, how far has she traveled? (a) 0.50 km (b) 0.80 km (c) 1.0 km (d) 1.5 km (e) 1.8 km

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2. Consider an object on a rotating disk a distance r from its center, held in place on the disk by static friction. Which of the following statements is not true concerning this object? (a) If the angular speed is constant, the object must have constant tangential speed. (b) If the angular speed is constant, the object is not accelerated. (c) The object has a tangential acceleration only if the disk has an angular acceleration. (d) If the disk has an angular acceleration, the object has both a centripetal acceleration and a tangential acceleration. (e) The object always has a centripetal acceleration except when the angular speed is zero. 3. A wheel is rotating about a fixed axis with constant angular acceleration 3 rad/s2. At different moments, its angular speed is 22 rad/s, 0, and 12 rad/s. For a point on the rim of the wheel, consider at these moments the magnitude of the tangential component of acceleration and the magnitude of the radial component of acceleration. Rank the following five items from largest to smallest: (a) uat u when v 5 22 rad/s, (b)uar u when

v 5 22 rad/s, (c)uar u when v 5 0, (d) uat u when v 5 2 rad/s, and (e) uar u when v 5 2 rad/s. If two items are equal, show them as equal in your ranking. If a quantity is equal to zero, show that fact in your ranking. 4. A grindstone increases in angular speed from 4.00 rad/s to 12.00 rad/s in 4.00 s. Through what angle does it turn during that time interval if the angular acceleration is constant? (a) 8.00 rad (b) 12.0 rad (c) 16.0 rad (d) 32.0 rad (e) 64.0 rad 5. Suppose a car’s standard tires are replaced with tires 1.30 times larger in diameter. (i) Will the car’s speedometer reading be (a) 1.69 times too high, (b) 1.30 times too high, (c) accurate, (d) 1.30 times too low, (e) 1.69 times too low, or (f) inaccurate by an unpredictable factor? (ii) Will the car’s fuel economy in miles per gallon or km/L appear to be (a) 1.69 times better, (b) 1.30 times better, (c) essentially the same, (d) 1.30 times worse, or (e) 1.69 times worse? 6. Figure OQ10.6 shows a system of four particles joined by light, rigid rods. Assume a 5 b and M is larger than m. About which of the coordinate axes does the system have (i) the smallest and (ii) the largest moment of inertia? (a) the x axis (b) the y axis (c) the z axis. (d) The moment of inertia has the same small value for two axes. (e) The moment of inertia is the same for all three axes.

Conceptual Questions

8. A constant net torque is exerted on an object. Which of the following quantities for the object cannot be constant? Choose all that apply. (a) angular position (b) angular velocity (c) angular acceleration (d) moment of inertia (e) kinetic energy a

M

x

b m

z

Figure OQ10.6

10. A toy airplane hangs from the ceiling at the bottom end of a string. You turn the airplane many times to wind up the string clockwise and release it. The airplane starts to spin counterclockwise, slowly at first and then faster and faster. Take counterclockwise as the positive sense and assume friction is negligible. When the string is entirely unwound, the airplane has its maximum rate of rotation. (i) At this moment, is its angular acceleration (a) positive, (b) negative, or (c) zero? (ii) The airplane continues to spin, winding the string counterclockwise as it slows down. At the moment it momentarily stops, is its angular acceleration (a) positive, (b) negative, or (c) zero?

ys

ic

s. w

7. As shown in Figure OQ10.7, a cord is wrapped onto a cylindrical reel mounted on a fixed, frictionless, horizontal axle. When does the reel have a greater magnitude of angular acceleration? (a) When the cord is pulled down with a constant force of 50 N. (b) When an object of weight 50 N is hung from the cord and released. (c) The angular accelerations in parts (a) and (b) are equal. (d) It is impossible to determine.

9. A basketball rolls across a classroom floor without slipping, with its center of mass moving at a certain speed. A block of ice of the same mass is set sliding across the floor with the same speed along a parallel line. Which object has more (i) kinetic energy and (ii) momentum? (a) The basketball does. (b) The ice does. (c) The two quantities are equal. (iii) The two objects encounter a ramp sloping upward. Which object will travel farther up the ramp? (a) The basketball will. (b) The ice will. (c) They will travel equally far up the ramp.

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M

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b

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m

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y

a

323

Conceptual Questions

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Figure OQ10.7 Objective Question 7 and Conceptual Question 4.

11. A solid aluminum sphere of radius R has moment of inertia I about an axis through its center. Will the moment of inertia about a central axis of a solid aluminum sphere of radius 2R be (a) 2I, (b) 4I, (c) 8I, (d) 16I, or (e) 32I ?

1. denotes answer available in Student Solutions Manual/Study Guide

sw

1. Is it possible to change the translational kinetic energy of an object without changing its rotational energy?

.a

2. Must an object be rotating to have a nonzero moment of inertia?

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3. Suppose just two external forces act on a stationary, rigid object and the two forces are equal in magnitude and opposite in direction. Under what condition does the object start to rotate?

w

4. Explain how you might use the apparatus described in Figure OQ10.7 to determine the moment of inertia of the wheel. Note: If the wheel does not have a uniform mass density, the moment of inertia is not necessarily equal to 12MR 2. 5. Using the results from Example 10.6, how would you calculate the angular speed of the wheel and the linear speed of the hanging object at t 5 2 s, assuming the system is released from rest at t 5 0? 6. Explain why changing the axis of rotation of an object changes its moment of inertia. 7. Suppose you have two eggs, one hard-boiled and the other uncooked. You wish to determine which is the hard-boiled egg without breaking the eggs, which

can be done by spinning the two eggs on the floor and comparing the rotational motions. (a) Which egg spins faster? (b) Which egg rotates more uniformly? (c) Which egg begins spinning again after being stopped and then immediately released? Explain your answers to parts (a), (b), and (c). 8. Suppose you set your textbook sliding across a gymnasium floor with a certain initial speed. It quickly stops moving because of a friction force exerted on it by the floor. Next, you start a basketball rolling with the same initial speed. It keeps rolling from one end of the gym to the other. (a) Why does the basketball roll so far? (b) Does friction significantly affect the basketball’s motion? 9. (a) What is the angular speed of the second hand of S as you an analog clock? (b) What is the direction of v view a clock hanging on a vertical wall? (c) What is the a of the magnitude of the angular acceleration vector S second hand? 10. One blade of a pair of scissors rotates counterclockwise S for the in the xy plane. (a) What is the direction of v S blade? (b) What is the direction of a if the magnitude of the angular velocity is decreasing in time?

324 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

11. If you see an object rotating, is there necessarily a net torque acting on it?

far side and pulled forward horizontally, the tricycle would start to roll forward. (a) Instead, assume a string is attached to the lower pedal on the near side and pulled forward horizontally as shown by A. Will the tricycle start to roll? If so, which way? Answer the same questions if (b) the string is pulled forward and upward as shown by B, (c) if the string is pulled straight down as shown by C, and (d) if the string is pulled forward and downward as shown by D. (e) What If? Suppose the string is instead attached to the rim of the front wheel and pulled upward and backward as shown by E. Which way does the tricycle roll? (f) Explain a pattern of reasoning, based on the figure, that makes it easy to answer questions such as these. What physical quantity must you evaluate?

12. If a small sphere of mass M were placed at the end of the rod in Figure 10.21, would the result for v be greater than, less than, or equal to the value obtained in Example 10.11?

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13. Three objects of uniform density—a solid sphere, a solid cylinder, and a hollow cylinder—are placed at the top of an incline (Fig. CQ10.13). They are all released from rest at the same elevation and roll without slipping. (a) Which object reaches the bottom first? (b) Which reaches it last? Note: The result is independent of the masses and the radii of the objects. (Try this activity at home!)

s. w

Figure CQ10.13

A

C

D

Figure CQ10.15

ic

14. Which of the entries in Table 10.2 applies to finding the moment of inertia (a) of a long, straight sewer pipe rotating about its axis of symmetry? (b) Of an embroidery hoop rotating about an axis through its center and perpendicular to its plane? (c) Of a uniform door turning on its hinges? (d) Of a coin turning about an axis through its center and perpendicular to its faces?

B

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ly

E

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16. A person balances a meterstick in a horizontal position on the extended index fingers of her right and left hands. She slowly brings the two fingers together. The stick remains balanced, and the two fingers always meet at the 50-cm mark regardless of their original positions. (Try it!) Explain why that occurs.

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15. Figure CQ10.15 shows a side view of a child’s tricycle with rubber tires on a horizontal concrete sidewalk. If a string were attached to the upper pedal on the

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Problems

.a

The problems found in this chapter may be assigned online in Enhanced WebAssign

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1. straightforward; 2. intermediate; 3. challenging

1. full solution available in the Student

Enhanced WebAssign

GP Guided Problem M Master It tutorial available in Enhanced WebAssign

W Watch It video solution available in Enhanced WebAssign

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Solutions Manual/Study Guide

AMT Analysis Model tutorial available in

Section 10.1 Angular Position, Velocity, and Acceleration 1. (a) Find the angular speed of the Earth’s rotation about

Q/C its axis. (b) How does this rotation affect the shape of

the Earth?

2. A potter’s wheel moves uniformly from rest to an angular speed of 1.00 rev/s in 30.0 s. (a) Find its average angular acceleration in radians per second per second. (b) Would doubling the angular acceleration during the given period have doubled the final angular speed? 3. During a certain time interval, the angular position W of a swinging door is described by u 5 5.00 1 10.0t 1 2.00t 2, where u is in radians and t is in seconds. Deter-

BIO Q/C S

mine the angular position, angular speed, and angular acceleration of the door (a) at t 5 0 and (b) at t 5 3.00 s. 4. A bar on a hinge starts from rest and rotates with an angular acceleration a 5 10 1 6t, where a is in rad/s2 and t is in seconds. Determine the angle in radians through which the bar turns in the first 4.00 s. Section 10.2 Analysis Model: Rigid Object Under Constant Angular Acceleration 5. A wheel starts from rest and rotates with constant W angular acceleration to reach an angular speed of 12.0 rad/s in 3.00 s. Find (a) the magnitude of the angu-

Problems lar acceleration of the wheel and (b) the angle in radians through which it rotates in this time interval.

were we justified in ignoring this aspect of the Coriolis effect in our previous study of free fall? (d) Suppose the angular speed of the Earth were to decrease due to tidal friction with constant angular acceleration. Would the eastward displacement of the dropped object increase or decrease compared with that in part (b)?

6. A centrifuge in a medical laboratory rotates at an angular speed of 3 600 rev/min. When switched off, it rotates through 50.0 revolutions before coming to rest. Find the constant angular acceleration of the centrifuge.

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s. w

9. A dentist’s drill starts from rest. After 3.20 s of constant angular acceleration, it turns at a rate of 2.51 3 104 rev/min. (a) Find the drill’s angular acceleration. (b) Determine the angle (in radians) through which the drill rotates during this period.

Section 10.3 Angular and Translational Quantities 15. A racing car travels on a circular track of radius 250 m. Assuming the car moves with a constant speed of 45.0 m/s, find (a) its angular speed and (b) the magnitude and direction of its acceleration. 16. Make an order-of-magnitude estimate of the number of revolutions through which a typical automobile tire turns in one year. State the quantities you measure or estimate and their values. 17. A discus thrower (Fig. P4.33, page 104) accelerates a W discus from rest to a speed of 25.0 m/s by whirling it through 1.25 rev. Assume the discus moves on the arc of a circle 1.00 m in radius. (a) Calculate the final angular speed of the discus. (b) Determine the magnitude of the angular acceleration of the discus, assuming it to be constant. (c) Calculate the time interval required for the discus to accelerate from rest to 25.0 m/s. 18. Figure P10.18 shows the drive train of a bicycle that W has wheels 67.3 cm in diameter and pedal cranks Q/C 17.5 cm long. The cyclist pedals at a steady cadence of 76.0 rev/min. The chain engages with a front sprocket 15.2 cm in diameter and a rear sprocket 7.00 cm in diameter. Calculate (a) the speed of a link of the chain relative to the bicycle frame, (b) the angular speed of the bicycle wheels, and (c) the speed of the bicycle relative to the road. (d) What pieces of data, if any, are not necessary for the calculations?

ee b

7. An electric motor rotating a workshop grinding wheel 2 M at 1.00 3 10 rev/min is switched off. Assume the wheel has a constant negative angular acceleration of magnitude 2.00 rad/s2. (a) How long does it take the grinding wheel to stop? (b) Through how many radians has the wheel turned during the time interval found in part (a)? 8. A machine part rotates at an angular speed of Q/C 0.060 rad/s; its speed is then increased to 2.2 rad/s at an angular acceleration of 0.70 rad/s2. (a) Find the angle through which the part rotates before reaching this final speed. (b) If both the initial and final angular speeds are doubled and the angular acceleration remains the same, by what factor is the angular displacement changed? Why?

325

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10. Why is the following situation impossible? Starting from rest, a disk rotates around a fixed axis through an angle of 50.0 rad in a time interval of 10.0 s. The angular acceleration of the disk is constant during the entire motion, and its final angular speed is 8.00 rad/s.

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11. A rotating wheel requires 3.00 s to rotate through

AMT 37.0 revolutions. Its angular speed at the end of the M 3.00-s interval is 98.0 rad/s. What is the constant anguW lar acceleration of the wheel?

Rear sprocket

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.a

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12. The tub of a washer goes into its spin cycle, starting from rest and gaining angular speed steadily for 8.00 s, at which time it is turning at 5.00 rev/s. At this point, the person doing the laundry opens the lid, and a safety switch turns off the washer. The tub smoothly slows to rest in 12.0 s. Through how many revolutions does the tub turn while it is in motion?

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13. A spinning wheel is slowed down by a brake, giving it a constant angular acceleration of 25.60 rad/s2. During a 4.20-s time interval, the wheel rotates through 62.4 rad. What is the angular speed of the wheel at the end of the 4.20-s interval? 14. Review. Consider a tall building located on the Earth’s

Q/C equator. As the Earth rotates, a person on the top floor of S the building moves faster than someone on the ground

with respect to an inertial reference frame because the person on the ground is closer to the Earth’s axis. Consequently, if an object is dropped from the top floor to the ground a distance h below, it lands east of the point vertically below where it was dropped. (a) How far to the east will the object land? Express your answer in terms of h, g, and the angular speed v of the Earth. Ignore air resistance and assume the free-fall acceleration is constant over this range of heights. (b) Evaluate the eastward displacement for h 5 50.0 m. (c) In your judgment,

Pedal crank Front sprocket Chain

Figure P10.18 19. A wheel 2.00 m in diameter lies in a vertical plane and M rotates about its central axis with a constant angular acceleration of 4.00 rad/s2. The wheel starts at rest at t 5 0, and the radius vector of a certain point P on the rim makes an angle of 57.38 with the horizontal at this time. At t 5 2.00 s, find (a) the angular speed of the wheel and, for point P, (b) the tangential speed, (c) the total acceleration, and (d) the angular position. 20. A car accelerates uniformly from rest and reaches a W speed of 22.0 m/s in 9.00 s. Assuming the diameter of a tire is 58.0 cm, (a) find the number of revolutions the tire makes during this motion, assuming that no slipping occurs. (b) What is the final angular speed of a tire in revolutions per second?

326 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

21. A disk 8.00 cm in radius rotates at a constant rate of M 1 200 rev/min about its central axis. Determine (a) its angular speed in radians per second, (b) the tangential speed at a point 3.00 cm from its center, (c) the radial acceleration of a point on the rim, and (d) the total distance a point on the rim moves in 2.00 s.

10.0 N

30.0

22. A straight ladder is leaning against the wall of a house.

12.0 N

Q/C The ladder has rails 4.90 m long, joined by rungs

Figure P10.27

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2 8. The fishing pole in Figure P10.28 makes an angle of W 20.0° with the horizontal. What is the torque exerted by the fish about an axis perpendicular to the page and passing through the angler’s hand if the fish pulls S on the fishing line with a force F 5 100 N at an angle 37.0° below the horizontal? The force is applied at a point 2.00 m from the angler’s hands. m 2.00 20.0

ic

24. A car traveling on a flat (unbanked), circular track S accelerates uniformly from rest with a tangential acceleration of a. The car makes it one-quarter of the way around the circle before it skids off the track. From these data, determine the coefficient of static friction between the car and the track.

9.00 N

ee b

23. A car traveling on a flat (unbanked), circular track W accelerates uniformly from rest with a tangential acceleration of 1.70 m/s2. The car makes it one-quarter of the way around the circle before it skids off the track. From these data, determine the coefficient of static friction between the car and the track.

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u 5 2.50t 2 2 0.600t 3

100 N

Section 10.5 Analysis Model: Rigid Object Under a Net Torque

29. An electric motor turns a flywheel through a drive belt that joins a pulley on the motor and a pulley that is rigidly attached to the flywheel as shown in Figure P10.29. The flywheel is a solid disk with a mass of 80.0 kg and a radius R 5 0.625 m. It turns on a frictionless axle. Its pulley has much smaller mass and a radius of r 5 0.230 m. The tension Tu in the upper (taut) segment of the belt is 135 N, and the flywheel has a clockwise angular acceleration of 1.67 rad/s2. Find the tension in the lower (slack) segment of the belt.

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where u is in radians and t is in seconds. (a) Find the maximum angular speed of the roller. (b) What is the maximum tangential speed of a point on the rim of the roller? (c) At what time t should the driving force be removed from the roller so that the roller does not reverse its direction of rotation? (d) Through how many rotations has the roller turned between t 5 0 and the time found in part (c)?

20.0 37.0

Figure P10.28

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25. In a manufacturing process, a large, cylindrical roller is used to flatten material fed beneath it. The diameter of the roller is 1.00 m, and, while being driven into rotation around a fixed axis, its angular position is expressed as

b

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0.410 m long. Its bottom end is on solid but sloping ground so that the top of the ladder is 0.690 m to the left of where it should be, and the ladder is unsafe to climb. You want to put a flat rock under one foot of the ladder to compensate for the slope of the ground. (a) What should be the thickness of the rock? (b) Does using ideas from this chapter make it easier to explain the solution to part (a)? Explain your answer.

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26. Review. A small object with mass 4.00 kg moves counterclockwise with constant angular speed 1.50 rad/s in a circle of radius 3.00 m centered at the origin. It starts at the point with position vector 3.00 ^i m. It then undergoes an angular displacement of 9.00 rad. (a) What is its new position vector? Use unit-vector notation for all vector answers. (b) In what quadrant is the particle located, and what angle does its position vector make with the positive x axis? (c) What is its velocity? (d) In what direction is it moving? (e) What is its acceleration? (f) Make a sketch of its position, velocity, and acceleration vectors. (g) What total force is exerted on the object? Section 10.4 Torque 27. Find the net torque on the wheel in Figure P10.27 about M the axle through O, taking a 5 10.0 cm and b 5 25.0 cm.

Tu

R r

Figure P10.29 30. A grinding wheel is in the form of a uniform solid disk

AMT of radius 7.00 cm and mass 2.00 kg. It starts from rest W and accelerates uniformly under the action of the con-

stant torque of 0.600 N ? m that the motor exerts on the wheel. (a) How long does the wheel take to reach its final operating speed of 1 200 rev/min? (b) Through how many revolutions does it turn while accelerating?

327

Problems

Object m 2 is resting on the floor, and object m1 is 4.00 m above the floor when it is released from rest. The pulley axis is frictionless. The cord is light, does not stretch, and does not slip on the pulley. (a) Calculate the time interval required for m1 to hit the floor. (b) How would your answer change if the pulley were massless?

32. Review. A block of mass m1 5 2.00 kg and a block of W mass m 2 5 6.00 kg are connected by a massless string over a pulley in the shape of a solid disk having radius R 5 0.250 m and mass M 5 10.0 kg. The fixed, wedgeshaped ramp makes an angle of u 5 30.08 as shown in Figure P10.32. The coefficient of kinetic friction is 0.360 for both blocks. (a) Draw force diagrams of both blocks and of the pulley. Determine (b) the acceleration of the two blocks and (c) the tensions in the string on both sides of the pulley.

37. A potter’s wheel—a thick stone disk of radius 0.500 m W and mass 100 kg—is freely rotating at 50.0 rev/min. The potter can stop the wheel in 6.00 s by pressing a wet rag against the rim and exerting a radially inward force of 70.0 N. Find the effective coefficient of kinetic friction between wheel and rag. Section 10.6 Calculation of Moments of Inertia

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3 8. Imagine that you stand tall and turn about a vertical axis through the top of your head and the point halfway between your ankles. Compute an order-ofmagnitude estimate for the moment of inertia of your body for this rotation. In your solution, state the quantities you measure or estimate and their values.

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M, R

m1

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31. A 150-kg merry-go-round in the shape of a uniform, M solid, horizontal disk of radius 1.50 m is set in motion by wrapping a rope about the rim of the disk and pulling on the rope. What constant force must be exerted on the rope to bring the merry-go-round from rest to an angular speed of 0.500 rev/s in 2.00 s?

ee b

m2

39. A uniform, thin, solid door has height 2.20 m, width

u

Q/C 0.870 m, and mass 23.0 kg. (a) Find its moment of iner-

tia for rotation on its hinges. (b) Is any piece of data unnecessary?

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Figure P10.32

40. Two balls with masses M and m are connected by a S rigid rod of length L and negligible mass as shown in Figure P10.40. For an axis perpendicular to the rod, (a) show that the system has the minimum moment of inertia when the axis passes through the center of mass. (b) Show that this moment of inertia is I 5 mL2, where m 5 mM/(m 1 M).

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33. A model airplane with mass 0.750 kg is tethered to the M ground by a wire so that it flies in a horizontal circle 30.0 m in radius. The airplane engine provides a net thrust of 0.800 N perpendicular to the tethering wire. (a) Find the torque the net thrust produces about the center of the circle. (b) Find the angular acceleration of the airplane. (c) Find the translational acceleration of the airplane tangent to its flight path. 34. A disk having moment of inertia 100 kg ? m2 is free to

L

Q/C rotate without friction, starting from rest, about a fixed

w

.a

sw

axis through its center. A tangential force whose magnitude can range from F 5 0 to F 5 50.0 N can be applied at any distance ranging from R 5 0 to R 5 3.00 m from the axis of rotation. (a) Find a pair of values of F and R that cause the disk to complete 2.00 rev in 10.0 s. (b) Is your answer for part (a) a unique answer? How many answers exist?

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w

35. The combination of an applied force and a friction force produces a constant total torque of 36.0 N ? m on a wheel rotating about a fixed axis. The applied force acts for 6.00 s. During this time, the angular speed of the wheel increases from 0 to 10.0 rad/s. The applied force is then removed, and the wheel M R comes to rest in 60.0 s. Find (a) the moment of inertia of the wheel, (b) the magnitude of the torque m1 due to friction, and (c) the total number of revolutions of the wheel during the entire interval of 66.0 s. 36. Review. Consider the system shown Q/C in Figure P10.36 with m1 5 20.0 kg, m 2 5 12.5 kg, R 5 0.200 m, and the mass of the pulley M 5 5.00 kg.

m2

Figure P10.36

m

M x

Lx

Figure P10.40 41. Figure P10.41 shows a side view of a car tire before it is mounted on a wheel. Model it as having two sidewalls of uniform thickness 0.635 cm and a tread wall of uniform thickness 2.50 cm and width 20.0 cm. Assume the rubber has uniform density 1.10 3 103 kg/m3. Find its moment of inertia about an axis perpendicular to the page through its center. 33.0 cm

Sidewall

16.5 cm

30.5 cm

Tread

Figure P10.41 42. Following the procedure used in Example 10.7, prove S that the moment of inertia about the y axis of the rigid rod in Figure 10.15 is 13ML2.

Rotation of a Rigid Object About a Fixed Axis

43. Three identical thin rods, each S of length L and mass m, are welded perpendicular to one another as shown in Figure P10.43. The assembly is rotated about an axis that passes through the end of one rod and is parallel to another. Determine the moment of inertia of this structure about this axis.

hole does not pass through the center of the disk. The cam with the hole cut out has mass M. The cam is mounted on a uniform, solid, cylindrical shaft of diameter R and also of mass M. What is the kinetic energy of the cam–shaft combination when it is rotating with angular speed v about the shaft’s axis?

z

y

47. A war-wolf or trebuchet is a device used during the Mid-

m1

m2 3.00 m

sw

2.00 kg

6.00 m

w

2.00 kg

x

.a

O

4.00 m

4.00 kg

w

Figure P10.45

w

46. Many machines employ cams for various purposes, S such as opening and closing valves. In Figure P10.46, the cam is a circular disk of radius R with a hole of diameter R cut through it. As shown in the figure, the

Figure P10.47 Section 10.8 Energy Considerations in Rotational Motion 48. A horizontal 800-N merry-go-round is a solid disk of radius 1.50 m and is started from rest by a constant horizontal force of 50.0 N applied tangentially to the edge of the disk. Find the kinetic energy of the disk after 3.00 s. 49. Big Ben, the nickname for the clock in Elizabeth Tower (named after the Queen in 2012) in London, has an hour hand 2.70 m long with a mass of 60.0 kg and a minute hand 4.50 m long with a mass of 100 kg (Fig. P10.49). Calculate the total rotational kinetic energy of the two hands about the axis of rotation. (You may

Travelpix Ltd/Stone/Getty Images

y 3.00 kg

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45. The four particles in Figure P10.45 are connected by W rigid rods of negligible mass. The origin is at the center of the rectangle. The system rotates in the xy plane about the z axis with an angular speed of 6.00 rad/s. Calculate (a) the moment of inertia of the system about the z axis and (b) the rotational kinetic energy of the system.

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4 4. Rigid rods of negligible mass lying along the y axis conW nect three particles (Fig. P10.44). The system rotates Q/C about the x axis with an y angular speed of 2.00 rad/s. Find (a) the moment of iner- 4.00 kg y 3.00 m tia about the x axis, (b) the total rotational kinetic energy x evaluated from 12Iv 2, (c) the O tangential speed of each 2.00 kg y 2.00 m particle, and (d) the total kinetic energy evaluated from 3.00 kg y 4.00 m 1 2 m v . (e) Compare the a2 i i answers for kinetic energy in Figure P10.44 parts (a) and (b).

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Figure P10.43

Section 10.7 Rotational Kinetic Energy

used to fling large vegetables and pianos as a sport. A simple trebuchet is shown in Figure P10.47. Model it as a stiff rod of negligible mass, 3.00 m long, joining particles of mass m1 5 0.120 kg and m 2 5 60.0 kg at its ends. It can turn on a frictionless, horizontal axle perpendicular to the rod and 14.0 cm from the large-mass particle. The operator releases the trebuchet from rest in a horizontal orientation. (a) Find the maximum speed that the small-mass object attains. (b) While the small-mass object is gaining speed, does it move with constant acceleration? (c) Does it move with constant tangential acceleration? (d) Does the trebuchet move with constant angular acceleration? (e) Does it have constant momentum? (f) Does the trebuchet–Earth system have constant mechanical energy?

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Axis of rotation

Q/C dle Ages to throw rocks at castles and now sometimes

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328 Chapter 10

R 2R

Figure P10.46

Figure P10.49 Problems 49 and 72.

329

Problems model the hands as long, thin rods rotated about one end. Assume the hour and minute hands are rotating at a constant rate of one revolution per 12 hours and 60 minutes, respectively.)

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d

M

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with mass m 5 1.20 kg has a ball of diameter d 5 8.00 cm and mass M 5 2.00 kg attached to one m end. The arrangement is originally vertical and stationary, with the ball at the top as shown in Figure P10.54. The combination is free to pivot Figure P10.54 about the bottom end of the rod after being given a slight nudge. (a) After the combination rotates through 90 degrees, what is its rotational kinetic energy? (b) What is the angular speed of the rod and ball? (c) What is the linear speed of the center of mass of the ball? (d) How does it compare with the speed had the ball fallen freely through the same distance of 28 cm?

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51. The top in Figure P10.51 has a moment of inertia of 4.00 3 1024 kg ? m2 and is initially at rest. It is free to rotate about the stationary axis AA9. A string, wrapped around a peg along the axis A of the top, is pulled in such a manner as to maintain a S F constant tension of 5.57 N. If the string does not slip while it is unwound from the peg, what is the angular speed of the top after 80.0 cm A of string has been pulled off the peg?

54. Review. A thin, cylindri-

AMT cal rod , 5 24.0 cm long

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50. Consider two objects with m1 . m 2 connected by a light string that passes over a pulley having R a moment of inertia of I about I its axis of rotation as shown in Figure P10.50. The string does not slip on the pulley or stretch. m1 The pulley turns without friction. The two objects are 2h released from rest separated by a vertical distance 2h. (a) Use m2 the principle of conservation of energy to find the translational Figure P10.50 speeds of the objects as they pass each other. (b) Find the angular speed of the pulley at this time.

and the pulley is a hollow cylinder with a mass of M 5 0.350 kg, an inner radius of R 1 5 0.020 0 m, and an outer radius of R 2 5 0.030 0 m. Assume the mass of the spokes is negligible. The coefficient of kinetic friction between the block and the horizontal surface is mk 5 0.250. The pulley turns without friction on its axle. The light cord does not stretch and does not slip on the pulley. The block has a velocity of vi 5 0.820 m/s toward the pulley when it passes a reference point on the table. (a) Use energy methods to predict its speed after it has moved to a second point, 0.700 m away. (b) Find the angular speed of the pulley at the same moment.

Figure P10.51

M

w

w

w

.a

sw

52. Why is the following situation impossible? In a large city with an air-pollution problem, a bus has no combustion engine. It runs over its citywide route on energy drawn from a large, rapidly rotating flywheel under the floor of the bus. The flywheel is spun up to its maximum rotation rate of 3 000 rev/min by an electric motor at the bus terminal. Every time the bus speeds up, the flywheel slows down slightly. The bus is equipped with regenerative braking so that the flywheel can speed up when the bus slows down. The flywheel is a uniform solid cylinder with mass 1 200 kg and radius 0.500 m. The bus body does work against air resistance and rolling resistance at the average rate of 25.0 hp as it travels its route with an average speed of 35.0 km/h.

55. Review. An object with a mass of m 5 5.10 kg is M attached to the free end of a light string wrapped around a reel of radius R 5 0.250 m and mass M 5 3.00 kg. The reel is a solid disk, free to rotate in a vertical plane about the horizontal axis passing through its center as shown in Figure P10.55. The suspended object is released from rest 6.00 m above the floor. Determine (a) the tension in the string, (b) the acceleration of the object, and (c) the speed with which the object hits the floor. (d) Verify your answer to part (c) by using the isolated system (energy) model.

53. In Figure P10.53, the hanging object has a mass of m1 5 0.420 kg; the sliding block has a mass of m 2 5 0.850 kg; R2 m2

R

R1

m1 m

Figure P10.53

Figure P10.55

Rotation of a Rigid Object About a Fixed Axis

59. A cylinder of mass 10.0 kg rolls without slipping on a M horizontal surface. At a certain instant, its center of mass has a speed of 10.0 m/s. Determine (a) the translational kinetic energy of its center of mass, (b) the rotational kinetic energy about its center of mass, and (c) its total energy. 60. A solid sphere is released from height h from the top

Q/C of an incline making an angle u with the horizontal. S Calculate the speed of the sphere when it reaches the

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bottom of the incline (a) in the case that it rolls without slipping and (b) in the case that it slides frictionlessly without rolling. (c) Compare the time intervals required to reach the bottom in cases (a) and (b).

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61. (a) Determine the acceleration of the center of mass S of a uniform solid disk rolling down an incline making angle u with the horizontal. (b) Compare the acceleration found in part (a) with that of a uniform hoop. (c) What is the minimum coefficient of friction required to maintain pure rolling motion for the disk? 62. A smooth cube of mass m and edge length r slides with

Q/C speed v on a horizontal surface with negligible friction. S The cube then moves up a smooth incline that makes

an angle u with the horizontal. A cylinder of mass m and radius r rolls without slipping with its center of mass moving with speed v and encounters an incline of the same angle of inclination but with sufficient friction that the cylinder continues to roll without slipping. (a) Which object will go the greater distance up the incline? (b) Find the difference between the maximum distances the objects travel up the incline. (c) Explain what accounts for this difference in distances traveled.

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57. A uniform solid disk of S radius R and mass M is free Pivot R to rotate on a frictionless pivot through a point on its rim (Fig. P10.57). If the disk S g is released from rest in the position shown by the coppercolored circle, (a) what is the speed of its center of mass Figure P10.57 when the disk reaches the position indicated by the dashed circle? (b) What is the speed of the lowest point on the disk in the dashed position? (c) What If? Repeat part (a) using a uniform hoop.

Section 10.9 Rolling Motion of a Rigid Object

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56. This problem describes one S experimental method for determining the moment of inertia of an irregularly shaped object such as the payload for a satellite. Figure P10.56 shows a counterweight of mass m susm pended by a cord wound around a spool of radius r, forming part of a turntable supFigure P10.56 porting the object. The turntable can rotate without friction. When the counterweight is released from rest, it descends through a distance h, acquiring a speed v. Show that the moment of inertia I of the rotating apparatus (including the turntable) is mr 2(2gh/v 2 2 1).

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330 Chapter 10

w

w

w

.a

sw

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5 8. The head of a grass string trimmer has 100 g of cord wound in a light, cylindrical spool with inside diameter 3.00 cm and outside diameter 18.0 cm as shown in Figure P10.58. The cord has a linear density of 10.0 g/m. A single strand of the cord extends 16.0 cm from the outer edge of the spool. (a) When switched on, the trimmer speeds up from 0 to 2 500 rev/min in 0.215 s. What average power is delivered to the head by the trimmer motor while it is accelerating? (b) When the trimmer is cutting grass, it spins at 2 000 rev/min and the grass exerts an average tangential force of 7.65 N on the outer end of the cord, which is still at a radial distance of 16.0 cm from the outer edge of the spool. What is the power delivered to the head under load?

16.

0 cm

63. A uniform solid disk and a uniform hoop are placed S side by side at the top of an incline of height h. (a) If they are released from rest and roll without slipping, which object reaches the bottom first? (b) Verify your answer by calculating their speeds when they reach the bottom in terms of h. 64. A tennis ball is a hollow sphere with a thin wall. It is set Q/C rolling without slipping at 4.03 m/s on a horizontal section of a track as shown in Figure P10.64. It rolls around the inside of a vertical circular loop of radius r 5 45.0 cm. As the ball nears the bottom of the loop, the shape of the track deviates from a perfect circle so that the ball leaves the track at a point h 5 20.0 cm below the horizontal section. (a) Find the ball’s speed at the top of the loop. (b) Demonstrate that the ball will not fall from the track at the top of the loop. (c) Find the ball’s speed as it leaves the track at the bottom. (d) What If? Suppose that static friction between ball and track were

r

3.00 cm

h

18.0 cm

Figure P10.58

Figure P10.64

331

Problems

70. A shaft is turning at angular speed v at time t 5 0. S Thereafter, its angular acceleration is given by a 5 A 1 Bt

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(a) Find the angular speed of the shaft at time t. (b) Through what angle does it turn between t 5 0 and t ? 71. Review. A mixing beater consists of three thin rods, each 10.0 cm long. The rods diverge from a central hub, separated from each other by 120°, and all turn in the same plane. A ball is attached to the end of each rod. Each ball has cross-sectional area 4.00 cm2 and is so shaped that it has a drag coefficient of 0.600. Calculate the power input required to spin the beater at 1 000 rev/min (a) in air and (b) in water. 72. The hour hand and the minute hand of Big Ben, the Elizabeth Tower clock in London, are 2.70 m and 4.50 m long and have masses of 60.0 kg and 100 kg, respectively (see Fig. P10.49). (a) Determine the total torque due to the weight of these hands about the axis of rotation when the time reads (i) 3:00, (ii) 5:15, (iii) 6:00, (iv) 8:20, and (v) 9:45. (You may model the hands as long, thin, uniform rods.) (b) Determine all times when the total torque about the axis of rotation is zero. Determine the times to the nearest second, solving a transcendental equation numerically. 73. A long, uniform rod of length L and mass M is pivoted S about a frictionless, horizontal pin through one end. The rod is nudged from rest in a vertical position as shown in Figure P10.73. At the instant the rod is horizontal, find (a) its angular speed, (b) the magnitude of its angular acceleration, (c) the x and y components of the acceleration of its center of mass, and (d) the components of the reaction force at the pivot.

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66. As shown in Figure 10.13 on page 306, toppling chimS neys often break apart in midfall because the mortar between the bricks cannot withstand much shear stress. As the chimney begins to fall, shear forces must act on the topmost sections to accelerate them tangentially so that they can keep up with the rotation of the lower part of the stack. For simplicity, let us model the chimney as a uniform rod of length , pivoted at the lower end. The rod starts at rest in a vertical position (with the frictionless pivot at the bottom) and falls over under the influence of gravity. What fraction of the length of the rod has a tangential acceleration greater than g sin u, where u is the angle the chimney makes with the vertical axis?

where a is in rad/s2 and t is in seconds. (a) Find the angular speed of the shaft at t 5 3.00 s. (b) Through what angle does it turn between t 5 0 and t 5 3.00 s?

.c

Additional Problems

a 5 210.0 2 5.00t

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65. A metal can containing condensed mushroom soup Q/C has mass 215 g, height 10.8 cm, and diameter 6.38 cm. It is placed at rest on its side at the top of a 3.00-m-long incline that is at 25.08 to the horizontal and is then released to roll straight down. It reaches the bottom of the incline after 1.50 s. (a) Assuming mechanical energy conservation, calculate the moment of inertia of the can. (b) Which pieces of data, if any, are unnecessary for calculating the solution? (c) Why can’t the moment of inertia be calculated from I 5 12mr 2 for the cylindrical can?

69. A shaft is turning at 65.0 rad/s at time t 5 0. Thereafter, its angular acceleration is given by

ee b

negligible so that the ball slid instead of rolling. Would its speed then be higher, lower, or the same at the top of the loop? (e) Explain your answer to part (d).

67. Review. A 4.00-m length of light nylon cord is wound

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AMT around a uniform cylindrical spool of radius 0.500 m M and mass 1.00 kg. The spool is mounted on a friction-

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sw

less axle and is initially at rest. The cord is pulled from the spool with a constant acceleration of magnitude 2.50 m/s2. (a) How much work has been done on the spool when it reaches an angular speed of 8.00 rad/s? (b) How long does it take the spool to reach this angular speed? (c) How much cord is left on the spool when it reaches this angular speed?

68. An elevator system in a tall building consists of a

y

w

Q/C 800-kg car and a 950-kg counterweight joined by a light

w

w

cable of constant length that passes over a pulley of mass 280 kg. The pulley, called a sheave, is a solid cylinder of radius 0.700 m turning on a horizontal axle. The cable does not slip on the sheave. A number n of people, each of mass 80.0 kg, are riding in the elevator car, moving upward at 3.00 m/s and approaching the floor where the car should stop. As an energy-conservation measure, a computer disconnects the elevator motor at just the right moment so that the sheave–car– counterweight system then coasts freely without friction and comes to rest at the floor desired. There it is caught by a simple latch rather than by a massive brake. (a) Determine the distance d the car coasts upward as a function of n. Evaluate the distance for (b) n 5 2, (c) n 5 12, and (d) n 5 0. (e) For what integer values of n does the expression in part (a) apply? (f) Explain your answer to part (e). (g) If an infinite number of people could fit on the elevator, what is the value of d ?

L

x

Pin

Figure P10.73 74. A bicycle is turned upside down while its owner repairs a flat tire on the rear wheel. A friend spins the front wheel, of radius 0.381 m, and observes that drops of water fly off tangentially in an upward direction when the drops are at the same level as the center of the wheel. She measures the height reached by drops moving vertically (Fig. P10.74 on page 332). A drop

332 Chapter 10

Rotation of a Rigid Object About a Fixed Axis

that breaks loose from the tire on one turn rises h 5 54.0 cm above the tangent point. A drop that breaks loose on the next turn rises 51.0 cm above the tangent point. The height to which the drops rise decreases because the angular speed of the wheel decreases. From this information, determine the magnitude of the average angular acceleration of the wheel.

for the pulley? (c) From the analysis model in part (a), find the tension T 1. (d) Similarly, find the tension T 2. (e) From the analysis model in part (b), find a symbolic expression for the moment of inertia of the pulley in terms of the tensions T1 and T2, the pulley radius r, and the acceleration a. (f) Find the numerical value of the moment of inertia of the pulley. 78. Review. A string is wound around a S uniform disk of radius R and mass M. The disk is released from rest h with the string vertical and its top end tied to a fixed bar (Fig. P10.78). R M Show that (a) the tension in the string is one third of the weight of the disk, (b) the magnitude of the Figure P10.78 acceleration of the center of mass is 2g/3, and (c) the speed of the center of mass is (4gh/3)1/2 after the disk has descended through distance h. (d) Verify your answer to part (c) using the energy approach.

v 0

79. The reel shown in Figure P10.79 has radius R and S moment of inertia I. One end of the block of mass m is connected to a spring of force constant k, and the other end is fastened to a cord wrapped around the reel. The reel axle and the incline are frictionless. The reel is wound counterclockwise so that the spring stretches a distance d from its unstretched position and the reel is then released from rest. Find the angular speed of the reel when the spring is again unstretched. I R

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75. A bicycle is turned upside down while its owner repairs S a flat tire on the rear wheel. A friend spins the front wheel, of radius R, and observes that drops of water fly off tangentially in an upward direction when the drops are at the same level as the center of the wheel. She measures the height reached by drops moving vertically (Fig. P10.74). A drop that breaks loose from the tire on one turn rises a distance h1 above the tangent point. A drop that breaks loose on the next turn rises a distance h2 , h1 above the tangent point. The height to which the drops rise decreases because the angular speed of the wheel decreases. From this information, determine the magnitude of the average angular acceleration of the wheel.

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Figure P10.74 Problems 74 and 75.

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76. (a) What is the rotational kinetic energy of the Earth about its spin axis? Model the Earth as a uniform sphere and use data from the endpapers of this book. (b) The rotational kinetic energy of the Earth is decreasing steadily because of tidal friction. Assuming the rotational period decreases by 10.0 ms each year, find the change in one day.

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w

77. Review. As shown in Figure P10.77, two blocks are conGP nected by a string of negligible mass passing over a pulley of radius r = 0.250 m and moment of inertia I. The block on the frictionless incline is moving with a constant acceleration of magnitude a = 2.00 m/s2. From this information, we wish to find the moment of inertia of the pulley. (a) What analysis model is appropriate for the blocks? (b) What analysis model is appropriate

m

u

Figure P10.79 80. A common demonstration, illustrated in Figure P10.80, consists of a ball resting at one end of a uniform board of length , that is hinged at the other end and elevated at an angle u. A light cup is attached to the board at rc so that it will catch the ball when the support stick is removed suddenly. (a) Show that the ball will lag behind the falling board when u is less than 35.38. Cup

S

a

rc

T1 15.0 kg m1

k

T2 u m 2 20.0 kg

37.0

Figure P10.77

Hinged end

Figure P10.80

Support stick

Problems (b) Assuming the board is 1.00 m long and is supported at this limiting angle, show that the cup must be 18.4 cm from the moving end.

top end. Suddenly, a horizontal impulsive force 14.7 ^i N is applied to it. (a) Suppose the force acts at the bottom end of the rod. Find the acceleration of its center of mass and (b) the horizontal force the hinge exerts. (c) Suppose the force acts at the midpoint of the rod. Find the acceleration of this point and (d) the horizontal hinge reaction force. (e) Where can the impulse be applied so that the hinge will exert no horizontal force? This point is called the center of percussion.

81. A uniform solid sphere of radius r is placed on the S inside surface of a hemispherical bowl with radius R. The sphere is released from rest at an angle u to the vertical and rolls without slipping (Fig. P10.81). Determine the angular speed of the sphere when it reaches the bottom of the bowl.

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r

85. A thin rod of length h and mass M is held vertically S with its lower end resting on a frictionless, horizontal surface. The rod is then released to fall freely. (a) Determine the speed of its center of mass just before it hits the horizontal surface. (b) What If? Now suppose the rod has a fixed pivot at its lower end. Determine the speed of the rod’s center of mass just before it hits the surface.

R

u

333

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8 6. Review. A clown balances a small spherical grape at the top of his bald head, which also has the shape of a sphere. After drawing sufficient applause, the grape starts from rest and rolls down without slipping. It will leave contact with the clown’s scalp when the radial line joining it to the center of curvature makes what angle with the vertical?

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82. Review. A spool of wire of mass M and radius R is S S unwound under a constant force F (Fig. P10.82). Assuming the spool is a uniform, solid cylinder that doesn’t slip,Sshow that (a) the acceleration of the center of mass is 4 F /3M and (b) the force of friction is to the right and equal in magnitude to F/3. (c) If the cylinder starts from rest and rolls without slipping, what is the speed of its center of mass after it has rolled through a distance d ?

ee b

Figure P10.81

87. A plank with a mass M 5 6.00 kg rests on top of two identical, solid, cylindrical rollers that have R 5 5.00 cm and m 5 2.00 kg (Fig. P10.87). The plank is pulled by a S constant horizontal force F of magnitude 6.00 N applied to the end of the plank and perpendicular to the axes of the cylinders (which are parallel). The cylinders roll without slipping on a flat surface. There is also no slipping between the cylinders and the plank. (a) Find the initial acceleration of the plank at the moment the rollers are equidistant from the ends of the plank. (b) Find the acceleration of the rollers at this moment. (c) What friction forces are acting at this moment?

ic

S

ys

F

ar ph

M R

Challenge Problems

sw

Figure P10.82

w

w

w

.a

83. A solid sphere of mass m and radius r rolls without slipS ping along the track shown in Figure P10.83. It starts from rest with the lowest point of the sphere at height h above the bottom of the loop of radius R, much larger than r. (a) What is the minimum value of h (in terms of R) such that the sphere completes the loop? (b) What are the force components on the sphere at the point P if h 5 3R ? Solid sphere of mass m and radius r R.

h

R P

Figure P10.83 8 4. A thin rod of mass 0.630 kg and length 1.24 m is at rest, hanging vertically from a strong, fixed hinge at its

S

M m

R

F

m

R

Figure P10.87 8 8. As a gasoline engine operates, a flywheel turning with the crankshaft stores energy after each fuel explosion, providing the energy required to compress the next charge of fuel and air. For the engine of a certain lawn tractor, suppose a flywheel must be no more than 18.0 cm in diameter. Its thickness, measured along its axis of rotation, must be no larger than 8.00 cm. The flywheel must release energy 60.0 J when its angular speed drops from 800 rev/min to 600 rev/min. Design a sturdy steel (density 7.85 3 103 kg/m3) flywheel to meet these requirements with the smallest mass you can reasonably attain. Specify the shape and mass of the flywheel.

Rotation of a Rigid Object About a Fixed Axis

r 5 ri 1

hu 2p

w

w

w

.a

sw

ar ph

where v is the constant speed with which the disc surface passes the laser. (c) From the result in part (b), use integration to find an expression for the angle u as a function of time. (d) From the result in part (c), use differentiation to find the angular acceleration of the disc as a function of time. 91. A spool of thread consists of a cylinder of radius R 1 with S end caps of radius R 2 as depicted in the end view shown in Figure P10.91. The mass of the spool, including the thread, is m, and its moment of inertia about an axis through its center is I. The spool is placed on a rough, horizontal surface so that it rolls without slipping when S a force T acting to the right is applied to the free end of the thread. (a) Show that the magnitude of the friction force exerted by the surface on the spool is given by I 1 mR 1R 2 I 1 mR 22

bT

(b) Determine the direction of the force of friction. R2 R1 S

T

Figure P10.91

(b) Find the magnitude of the acceleration of the block in terms of m, m, M, g, and u. m r

M

u

Figure P10.92

93. A merry-go-round is stationary. A dog is running around the merry-go-round on the ground just outside its circumference, moving with a constant angular speed of 0.750 rad/s. The dog does not change his pace when he sees what he has been looking for: a bone resting on the edge of the merry-go-round onethird of a revolution in front of him. At the instant the dog sees the bone (t 5 0), the merry-go-round begins to move in the direction the dog is running, with a constant angular acceleration of 0.015 0 rad/s2. (a) At what time will the dog first reach the bone? (b) The confused dog keeps running and passes the bone. How long after the merry-go-round starts to turn do the dog and the bone draw even with each other for the second time?

ys

du v 5 dt ri 1 1 h u/2p 2

f5a

m 1 2M

ic

where ri is the radius of the innermost portion of the track and u is the angle through which the disc turns to arrive at the location of the track of radius r. (b) Show that the rate of change of the angle u is given by

Å

4Mgd 1 sin u 2 m cos u 2

om

9 0. To find the total angular displacement during the S playing time of the compact disc in part (B) of Example 10.2, the disc was modeled as a rigid object under constant angular acceleration. In reality, the angular acceleration of a disc is not constant. In this problem, let us explore the actual time dependence of the angular acceleration. (a) Assume the track on the disc is a spiral such that adjacent loops of the track are separated by a small distance h. Show that the radius r of a given portion of the track is given by

v5

.c

where v 0 and s are constants. The angular speed changes from 3.50 rad/s at t 5 0 to 2.00 rad/s at t 5 9.30 s. (a) Use this information to determine s and v 0. Then determine (b) the magnitude of the angular acceleration at t 5 3.00 s, (c) the number of revolutions the wheel makes in the first 2.50 s, and (d) the number of revolutions it makes before coming to rest.

ly

du 5 v 0e2st dt

92. A cord is wrapped around a pulley that is shaped like S a disk of mass m and radius r. The cord’s free end is connected to a block of mass M. The block starts from rest and then slides down an incline that makes an angle u with the horizontal as shown in Figure P10.92. The coefficient of kinetic friction between block and incline is m. (a) Use energy methods to show that the block’s speed as a function of position d down the incline is

ee b

89. As a result of friction, the angular speed of a wheel changes with time according to

s. w

334 Chapter 10

94. A uniform, hollow, cylinM S drical spool has inside radius R/2, outside radius R, and mass M (Fig. P10.94). It is mounted so that it rotates on a fixed, horizontal axle. A counR/2 m terweight of mass m is R/2 y connected to the end of a string wound around the spool. The counterweight Figure P10.94 falls from rest at t 5 0 to a position y at time t. Show that the torque due to the friction forces between spool and axle is tf 5 R c m ag 2

2y t

2b

2M

5y 4t 2

d

c h a p t e r

11

Angular Momentum

11.1 The Vector Product and Torque

om

11.2 Analysis Model: Nonisolated System (Angular Momentum)

.c

11.3 Angular Momentum of a Rotating Rigid Object

11.5 The Motion of Gyroscopes and Tops

ar ph

ys

ic

s. w

ee b

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11.4 Analysis Model: Isolated System (Angular Momentum)

Two motorcycle racers lean precariously into a turn around a racetrack. The analysis of such a leaning turn is based on principles associated with angular momentum. (Stuart Westmorland/The Image Bank/ Getty Images)

w

.a

sw

The central topic of this chapter is angular momentum, a quantity that plays a key role in rotational dynamics. In analogy to the principle of conservation of linear momentum, there is also a principle of conservation of angular momentum. The angular momentum of an isolated system is constant. For angular momentum, an isolated system is one for which no external torques act on the system. If a net external torque acts on a system, it is nonisolated. Like the law of conservation of linear momentum, the law of conservation of angular momentum is a fundamental law of physics, equally valid for relativistic and quantum systems.

w

11.1 The Vector Product and Torque

w

An important consideration in defining angular momentum is the process of multiplying two vectors by means of the operation called the vector product. We will introduce the vector product by considering the vector nature of torque. S Consider a force F acting on a particle located at point P and described by the S vector position r (Fig. 11.1 on page 336). As we saw in Section 10.6, the magnitude of the torque due to this force about an axis throughSthe origin is rF sin f, where f S S is the angle between r and F. The axis aboutSwhich F tends to produce rotation is S perpendicular to the plane formed by r and F. S S S The torque vector t is related to the two vectors r and F. We can establish a S S S mathematical relationship between t , r , and F using a mathematical operation called the vector product:

S

S

t;S r 3 F

(11.1)

335

336 Chapter 11

Angular Momentum

S We now give a formal definition of the vector product. Given any two vectors S S S S A and B , the vector product A 3 B is defined as a third vector C , which has a S S S magnitude of AB sin u, where u is the angle between A and B . That is, if C is given by S S S C 5 A 3 B (11.2)

z

S

t r F

its magnitude is

y

The quantity AB sin u is equal to the area S of the parallelogram formed by A and S B as shown in Figure 11.2. The direction of C is perpendicular to the plane formed S S by A and B , and the best way to determine this direction is to use the right-hand rule illustrated in Figure 11.2. The four fingers of the right hand areSpointed along S S A and then “wrapped” in the direction that would rotate A into BS through the S S angle u. The direction of the upright thumb is the direction of A 3 B 5 C. S S S S Because of the notation, A 3 B is often read “ A cross B ,” so the vector product is also called the cross product. Some properties of the vector product that follow from its definition are as follows:

r

om

P f S

F

.c

Figure 11.1 The torque vector S

1. Unlike the scalar product, the vector product is not commutative. Instead, the order in which the two vectors are multiplied in a vector product is important: S

S

The direction of C is perpendicular S S to the plane formed by A and B, and its direction is determined by the right-hand rule. S

S

S

S

A 3 B 5 2B 3 A (11.4) Therefore, if you change the order of the vectors in a vector product, you must change the sign. You can easily verify this relationship with the righthand rule. S S S S 2. If A is parallel to B (u 5 0 or 1808), then A 3 B 5 0; therefore, it follows S S that A 3 A 5 0. S S S S 3. If A is perpendicular to B , then 0 A 3 B 0 5 AB. 4. The vector product obeys the distributive law:

s. w

Properties of the vector product

S

ee b

ly

t lies in a direction perpendicular to the plane formed by the position vector S r and the applied force S vector F . In the situation shown, S S r and F lie in the xy plane, so the torque is along the z axis.

S

ic

x

(11.3)

C 5 AB sin u

S

S

ys

O

C A B

ar ph

S

S

S

S

S

S

S

S

S

A 3 1 B 1 C 2 5 A 3 B 1 A 3 C

(11.5)

5. The derivative of the vector product with respect to some variable such as t is

S

A

S

S

sw

S S S d S dA dB 1A 3 B2 5 3 B 1 A 3 (11.6) dt dt dt where it is important to preserve the multiplicative order of the terms on the right side in view of Equation 11.4.

u S

S

S

.a

B S

w

C B A

w

Figure 11.2 The vectorS product S S

w

A 3 B is a third vector C having a magnitude AB sin u equal to the area of the parallelogram shown.

Cross products of unit vectors

It is left as an exercise (Problem 4) to show from Equations 11.3 and 11.4 and from the definition of unit vectors that the cross products of the unit vectors ^i, ^j, and k^ obey the following rules:

i^ 3 i^ 5 j^ 3 j^ 5 k^ 3 k^ 5 0

(11.7a)

i^ 3 j^ 5 2 j^ 3 i^ 5 k^

(11.7b)

j^ 3 k^ 5 2k^ 3 j^ 5 ^i

(11.7c)

k^ 3 i^ 5 2 i^ 3 k^ 5 ^j

(11.7d) S

Pitfall Prevention 11.1 The Vector Product Is a Vector Remember that the result of taking a vector product between two vectors is a third vector. Equation 11.3 gives only the magnitude of this vector.

S

S

S

Signs are interchangeable in cross products. For example, A 3 12B 2 5 2 A 3 B and i^ 3 12 j^ 2 5 2 i^ 3 ^j. S S The cross product of any two vectors A and B can be expressed in the following determinant form:

i^ j^ k^ A A A A A A A 3 B 5 † Ax Ay Az † 5 ` y z ` i^ 1 ` z x ` j^ 1 ` x y ` k^ By Bz Bz Bx Bx By Bx By Bz

S

S

11.1 The Vector Product and Torque 337

Expanding these determinants gives the result S

S A 3 B 5 1 Ay Bz 2 Az By 2 i^ 1 1 Az Bx 2 Ax Bz 2 j^ 1 1 Ax By 2 Ay Bx 2 k^

(11.8)

Given the definition of the cross product, we can now assign a direction to the S torque vector. If the force lies in the xy plane as in Figure 11.1, the torque t is represented by a vector parallel to the z axis. The force in Figure 11.1 creates a torque that tends to rotate the particle counterclockwise about the z axis; the direction of S t is toward increasing z, and S t is therefore in the positive z direction. If we reversed S S the direction of F in Figure 11.1, t would be in the negative z direction.

ly

.c

om

Q uick Quiz 11.1 Which of the following statements about the relationship between the magnitude of the cross productSof two vectors and the product S of theSmagS 0 A 3 B 0 is larger than AB. (b) 0 A 3 B 0 is nitudes of the vectors S is true? (a) S smaller than AB. (c) 0 A 3 B 0 could be larger or smaller than AB, depending on S S the angle between the vectors. (d) 0 A 3 B 0 could be equal to AB.

ee b

Example 11.1 The Vector Product

S Two vectors lying in the xy plane are given by the equations A 5 2 i^ 1 3 ^j and S S S S S S S B 5 2 ^i 1 2 ^j. Find A 3 B and verify that A 3 B 5 2 B 3 A .

s. w

S o l u ti o n

ic

Conceptualize Given the unit-vector notations of the vectors, think about the directions the vectors point in space. Draw them on graph paper and imagine the parallelogram shown in Figure 11.2 for these vectors.

ys

Categorize Because we use the definition of the cross product discussed in this section, we categorize this example as a substitution problem.

Perform the multiplication:

ar ph

Write the cross product of the two vectors:

sw

Use Equations 11.7a through 11.7d to evaluate the various terms: S

S

S

S

.a

To verify that A 3 B 5 2 B 3 A , evaluate S S B 3 A:

w

w

Use Equations 11.7a through 11.7d to evaluate the various terms:

w

S

S

S A 3 B 5 1 2 i^ 1 3 j^ 2 3 1 2 i^ 1 2 j^ 2

S

S A 3 B 5 2 i^ 3 1 2 i^ 2 1 2 i^ 3 2 j^ 1 3 j^ 3 1 2 i^ 2 1 3 j^ 3 2 ^j

S

S

A 3 B 5 0 1 4 k^ 1 3 k^ 1 0 5 7 k^

S

S B 3 A 5 1 2 i^ 1 2 j^ 2 3 1 2 i^ 1 3 j^ 2 S

Perform the multiplication:

S

S

S

S B 3 A 5 1 2 i^ 2 3 2 i^ 1 1 2 i^ 2 3 3 j^ 1 2 j^ 3 2 i^ 1 2 j^ 3 3 ^j

S

S

B 3 A 5 0 2 3 k^ 2 4 k^ 1 0 5 27 k^ S

S

Therefore, A 3 B 5 2 B 3 A . As an alternative method for finding A 3 B , you could use Equation 11.8. Try it!

Example 11.2 The Torque Vector S

A force of F 5 1 2.00 i^ 1 3.00 j^ 2 N is applied to an object that is pivoted about a fixed axis aligned along the z coordinate axis. The force is applied at a point located at S r 5 1 4.00 i^ 1 5.00 j^ 2 m. Find the torque S t applied to the object. S o l u ti o n

Conceptualize Given the unit-vector notations, think about the directions of the force and position vectors. If this force were applied at this position, in what direction would an object pivoted at the origin turn? continued

338 Chapter 11

Angular Momentum

▸ 11.2 c o n t i n u e d Categorize Because we use the definition of the cross product discussed in this section, we categorize this example as a substitution problem. Set up the torque vector using Equation 11.1:

S

Perform the multiplication:

S

S

t5S r 3 F 5 3 1 4.00 i^ 1 5.00 j^ 2 m 4 3 3 1 2.00 i^ 1 3.00 j^ 2 N 4 t 5 3 1 4.00 2 1 2.00 2 i^ 3 i^ 1 1 4.00 2 1 3.00 2 i^ 3 j^

S

t 5 3 0 1 12.0 k^ 2 10.0 k^ 1 0 4 N # m 5 2.0 k^ N # m

S

om

Use Equations 11.7a through 11.7d to evaluate the various terms:

1 1 5.00 2 1 2.00 2 j^ 3 i^ 1 1 5.00 2 1 3.00 2 j^ 3 j^ 4 N # m

ly

.c

Notice that both S r and F are in the xy plane. As expected, the torque vector is perpendicular to this plane, having only a z component. We have followed the rules for significant figures discussed in Section 1.6, which lead to an answer with two significant figures. We have lost some precision because we ended up subtracting two numbers that are close.

ee b

s. w

11.2 A nalysis Model: Nonisolated System (Angular Momentum)

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.a

Figure 11.3 As the skater passes the pole, she grabs hold of it, which causes her to swing around the pole rapidly in a circular path.

ar ph

ys

ic

Imagine a rigid pole sticking up through the ice on a frozen pond (Fig. 11.3). A skater glides rapidly toward the pole, aiming a little to the side so that she does not hit it. As she passes the pole, she reaches out to her side and grabs it, an action that causes her to move in a circular path around the pole. Just as the idea of linear momentum helps us analyze translational motion, a rotational analog—angular momentum—helps us analyze the motion of this skater and other objects undergoing rotational motion. In Chapter 9, we developed the mathematical form of linear momentum and then proceeded to show how this new quantity was valuable in problem solving. We will follow a similar procedure for angular momentum. Consider a particle of mass m located at the vector position S r and moving with linear momentum S p as in Figure 11.4. In describing translational motion, we found that the net force on the particle equals the time rate of change of its linear S momentum, g F 5 d S p /dt (see Eq. 9.3). Let us take the cross product of each side of Equation 9.3 with S r , which gives the net torque on the particle on the left side of the equation: dS p S r 3 a F 5 aS t5S r 3 dt

S

Now let’s add to the right side the term 1 d S p , which is zero because r /dt 2 3 S S dS r /dt 5 S v and S v and p are parallel. Therefore, S at 5 r 3 S

dS p dt

1

dS r 3S p dt

We recognize the right side of this equation as the derivative of S r 3S p (see Eq. 11.6). Therefore, r 3S p2 d1S S t 5 (11.9) a dt

which looks very similar in form to Equation 9.3, g F 5 d S p /dt. Because torque plays the same role in rotational motion that force plays in translational motion, S this result suggests that the combination r 3S p should play the same role in rotaS

339

11.2 Analysis Model: Nonisolated System (Angular Momentum) S tional motion that p plays in translational motion. We call this combination the angular momentum of the particle: S

WW Angular momentum of a particle

The instantaneous angular momentum L of a particle relative to an axis through the origin O is defined by the cross product of the particle’s instantaS neous position vector S : r and its instantaneous linear momentum p S

L ;S r 3S p

(11.10)

We can now write Equation 11.9 as at 5 S

S

dL dt

(11.11)

S

The angular momentum L of a particle about an axis is a vector perpendicular to both the S particle’s position r relative to S the axis and its momentum p.

om

which is the rotational analogSof Newton’s second law, g F 5 d S p /dt. Torque causes the angular momentum L to change just as force causes linear momentum S p to change. S Notice that Equation 11.11 is valid only if g S t and L are measured about the same axis. Furthermore, the expression is valid for any axis fixed in an inertial frame. The SI unit of angular momentum is kg ? m2/s. Notice also that both the magS nitude and the direction of L depend S on the choice of axis. Following the rightto the plane formed by hand rule, we see that the direction of L is perpendicular S S S S r and p . In Figure 11.4, S r and p areSin the xy plane, so L points in the z direction. S Because p 5 mS v , the magnitude of L is

.c

S

ly

z S

ee b

L 5 mvr sin f

s. w

(11.12)

ar ph

ys

ic

S where f is the angle between S r and p . It follows that L is zero when S r is parallel to S p (f 5 0 or 1808). In other words, when the translational velocity of the particle is along a line that passes through the axis, the particle has zero angular momentum S S with respect to the axis. On the other hand, if r is perpendicular to p (f 5 908), then L 5 mvr. At that instant, the particle moves exactly as if it were on the rim of a S wheel rotating about the axis in a plane defined by S r and p .

O

m

S

r

y

S

p

f

x

Figure 11.4 S The angular

momentum L of a particle is a S vector given by L 5 S r 3S p.

Pitfall Prevention 11.2 Is Rotation Necessary for Angular Momentum? We can define angular momentum even if the particle is not moving in a circular path. A particle moving in a straight line has angular momentum about any axis displaced from the path of the particle.

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sw

Q uick Quiz 11.2 Recall the skater described at the beginning of this section. Let her mass be m. (i) What would be her angular momentum relative to the pole at the instant she is a distance d from the pole if she were skating directly toward it at speed v? (a) zero (b) mvd (c) impossible to determine (ii) What would be her angular momentum relative to the pole at the instant she is a distance d from the pole if she were skating at speed v along a straight path that is a perpendicular distance a from the pole? (a) zero (b) mvd (c) mva (d) impossible to determine

S

S

L r p

w

w

Example 11.3 Angular Momentum of a Particle in Circular Motion y

A particle moves in the xy plane in a circular path of radius r as shown in Figure 11.5. Find the magnitude and direction of its angular momentum relative to an axis through O when its velocity is S v.

S

v

S o l u ti o n

Conceptualize The linear momentum of the particle is always changing in direction (but not in magnitude). You might therefore be tempted to conclude that the angular momentum of the particle is always changing. In this situation, however, that is not the case. Let’s see why.

S

Figure 11.5 (Example 11.3) A particle moving in a circle of radius r has an angular momentum about an axis through O that has magnitude S mvr. The vector L 5 S r 3S p points out of the page.

r

O

m x

continued

340 Chapter 11

Angular Momentum

▸ 11.3 c o n t i n u e d Categorize We use the definition of the angular momentum of a particle discussed in this section, so we categorize this example as a substitution problem. S

Use Equation 11.12 to evaluate the magnitude of L :

L 5 mvr sin 908 5 mvr S

.c

Angular Momentum of a System of Particles

om

This value of L is constant because all three factors on the right are constant. The direction of L also is constant, S even though the direction of p 5 mS v keeps changing. To verify this statement, apply the right-hand rule to find the S S S S S direction of L 5 r 3 p 5 m r 3 S v in Figure 11.5. Your thumb points out of the page, so that is the direction of L . S S Hence, we can write the vector expression L 5 1 mvr 2 k^ . If the particle were to move clockwise, L would point downS ward and into the page and L 5 2 1 mvr 2 k^ . A particle in uniform circular motion has a constant angular momentum about an axis through the center of its path.

ee b

ly

Using the techniques of Section 9.7, we can show that Newton’s second law for a system of particles is dS p tot S F 5 a ext dt

s. w

This equation states that the net external force on a system of particles is equal to the time rate of change of the total linear momentum of the system. Let’s see if a similar statement can be made for rotational motion. The total angular momentum of a system of particles about some axis is defined as the vector sum of the angular momenta of the individual particles:

ic

L tot 5 L 1 1 L 2 1 c1 L n 5 a L i i where the vector sum is over all n particles in the system. Differentiating this equation with respect to time gives S

S

S

S

ar ph

ys

S

d L tot d Li 5 a 5 aS ti dt dt i i S

S

w

w

w

.a

sw

where we have used Equation 11.11 to replace the time rate of change of the angular momentum of each particle with the net torque on the particle. The torques acting on the particles of the system are those associated with internal forces between particles and those associated with external forces. The net torque associated with all internal forces, however, is zero. Recall that Newton’s third law tells us that internal forces between particles of the system are equal in magnitude and opposite in direction. If we assume these forces lie along the line of separation of each pair of particles, the total torque around some axis passing through an origin O due to each action–reaction force pair is zero (that is, the moment arm d from O to the line of action of the forces is equal for both particles, and the forces are in opposite directions). In the summation, therefore, the net internal torque is zero. We conclude that the total angular momentum of a system can vary with time only if a net external torque is acting on the system:

The net external torque on a system equals the time rate of change of angular momentum of the system

a text S

S

d L tot 5 dt

(11.13)

This equation is indeed the rotational analog of g F ext 5 d S p tot /dt for a system of particles. Equation 11.13 is the mathematical representation of the angular momentum version of the nonisolated system model. If a system is nonisolated in the sense that there is a net torque on it, the torque is equal to the time rate of change of angular momentum. Although we do not prove it here, this statement is true regardless of the motion of the center of mass. It applies even if the center of mass is accelerating, provided S

341

11.2 Analysis Model: Nonisolated System (Angular Momentum)

the torque and angular momentum are evaluated relative to an axis through the center of mass. Equation 11.13 can be rearranged and integrated to give text 2 dt D L tot 5 3 1 a S S

This equation represents the angular impulse–angular momentum theorem. Compare this equation to the translational version, Equation 9.40.

om

Analysis Model Nonisolated System (Angular Momentum) Imagine a system that rotates about an axis. If there is a net external torque acting on the system, the time rate of change of the angular momentum of the system is equal to the net external torque:

.c

S

d L tot dt

(11.13)

ly

a text 5 S

System boundary

Angular momentum

ee b

Examples:

External torque

ar ph

ys

ic

s. w

• a flywheel in an automobile engine increases its angular momentum when the The rate of change in the engine applies torque to it angular momentum of the • the tub of a washing machine decreases in angular momentum due to frictional nonisolated system is equal to the net external torque torque after the machine is turned off on the system. • the axis of the Earth undergoes a precessional motion due to the torque exerted on the Earth by the gravitational force from the Sun • the armature of a motor increases its angular momentum due to the torque exerted by a surrounding magnetic field (Chapter 31)

Example 11.4 A System of Objects AM A sphere of mass m1 and a block of mass m 2 are connected by a light cord that passes over a pulley as shown in Figure 11.6. The radius of the pulley is R, and the mass of the thin rim is M. The spokes of the pulley have negligible mass. The block slides on a frictionless, horizontal surface. Find an expression for the linear acceleration of the two objects, using the concepts of angular momentum and torque.

S

w

S o l u ti o n

.a

sw

v

w

Conceptualize W hen the system is released, the block slides to the left, the sphere

w

drops downward, and the pulley rotates counterclockwise. This situation is similar to problems we have solved earlier except that now we want to use an angular momentum approach.

R

S

v

m2

m1

Figure 11.6 (Example 11.4) When the system is released, the sphere moves downward and the block moves to the left.

Categorize We identify the block, pulley, and sphere as a nonisolated system for angular momentum, subject to the external torque due to the gravitational force on the sphere. We shall calculate the angular momentum about an axis that coincides with the axle of the pulley. The angular momentum of the system includes that of two objects moving translationally (the sphere and the block) and one object undergoing pure rotation (the pulley). Analyze At any instant of time, the sphere and the block have a common speed v, so the angular momentum of the sphere about the pulley axle is m1vR and that of the block is m 2vR. At the same instant, all points on the rim of the pulley also move with speed v, so the angular momentum of the pulley is MvR. Now let’s address the total external torque acting on the system about the pulley axle. Because it has a moment arm of zero, the force exerted by the axle on the pulley does not contribute to the torque. Furthermore, the normal force

continued

342 Chapter 11

Angular Momentum

▸ 11.4 c o n t i n u e d g , so these forces do not contribute to the torque. The acting on the block is balanced by the gravitational force m 2S gravitational force m 1S g acting on the sphere produces a torque about the axle equal in magnitude to m1gR, where R is the moment arm of the force about the axle. This result is the total external torque about the pulley axle; that is, g text 5 m 1gR.

dL a text 5 dt d 3 1 m 1 1 m 2 1 M 2 vR 4 m 1gR 5 dt

Substitute this expression and the total external torque into Equation 11.13, the mathematical representation of the nonisolated system model for angular momentum:

m 1g

m1 1 m2 1 M

ee b

(3) a 5

ly

Recognizing that dv/dt 5 a, solve Equation (2) for a :

dv dt

.c

(2) m 1gR 5 1 m 1 1 m 2 1 M 2 R

om

(1) L 5 m1vR 1 m 2vR 1 MvR 5 (m1 1 m 2 1 M)vR

Write an expression for the total angular momentum of the system:

s. w

Finalize W hen we evaluated the net torque about the axle, we did not include the forces that the cord exerts on the objects because these forces are internal to the system under consideration. Instead, we analyzed the system as a whole. Only external torques contribute to the change in the system’s angular momentum. Let M S 0 in Equation (3) and call the result Equation A. Now go back to Equation (5) in Example 5.10, let u S 0, and call the result Equation B. Do Equations A and B match? Looking at Figures 5.15 and 11.6 in these limits, should the two equations match?

ys

ic

z

ar ph

11.3 Angular Momentum of a Rotating Rigid Object S

v

sw

S

vi

w

x

y

S

mi

w

r

.a

L

S

In Example 11.4, we considered the angular momentum of a deformable system of particles. Let us now restrict our attention to a nondeformable system, a rigid object. Consider a rigid object rotating about a fixed axis that coincides with the z axis of a coordinate system as shown in Figure 11.7. Let’s determine the angular momentum of this object. Each particle of the object rotates in the xy plane about the z axis with an angular speed v. The magnitude of the angular momentum of a particle of mass mi about the z axis is miviri . Because vi 5 ri v (Eq. 10.10), we can express the magnitude of the angular momentum of this particle as

w

Figure 11.7 W hen a rigid object rotates about an axis, the anguS lar momentum L is in the same direction as the angular velocity S v according to the expression S S L 5 I v.

Li 5 miri2v

S

S The vector L i for this particle is directed along the z axis, as is the vector v . We can now find the angular momentum (which in this situation has only a z component) of the whole object by taking the sum of Li over all particles:

L z 5 a L i 5 a m i ri 2v 5 a a m i ri 2 b v i

i

i

Lz 5 Iv

(11.14)

where we have recognized oi mi ri2 as the moment of inertia I of the object about the z axis (Eq. 10.19). Notice that Equation 11.14 is mathematically similar in form to Equation 9.2 for linear momentum: S p 5 mS v. Now let’s differentiate Equation 11.14 with respect to time, noting that I is constant for a rigid object: dL z dv 5I 5 Ia (11.15) dt dt

11.3 Angular Momentum of a Rotating Rigid Object

343

where a is the angular acceleration relative to the axis of rotation. Because dL z /dt is equal to the net external torque (see Eq. 11.13), we can express Equation 11.15 as

o text 5 Ia

(11.16)

WW Rotational form of Newton’s second law

ly

.c

om

That is, the net external torque acting on a rigid object rotating about a fixed axis equals the moment of inertia about the rotation axis multiplied by the object’s angular acceleration relative to that axis. This result is the same as Equation 10.18, which was derived using a force approach, but we derived Equation 11.16 using the concept of angular momentum. As we saw in Section 10.7, Equation 11.16 is the mathematical representation of the rigid object under a net torque analysis model. This equation is also valid for a rigid object rotating about a moving axis, provided the moving axis (1) passes through the center of mass and (2) is a symmetry axis. If a symmetrical object rotates about a fixed axis passing through itsS center S S of mass, you can write Equation 11.14 in vector form as L 5 I v , where L is the total angular momentum of the object measured with respect to the axis of rotation. Furthermore, the expression is valid for any object, regardless of its symmetry, S if L stands for the component of angular momentum along the axis of rotation.1

s. w

ee b

Q uick Quiz 11.3 A solid sphere and a hollow sphere have the same mass and radius. They are rotating with the same angular speed. Which one has the higher angular momentum? (a) the solid sphere (b) the hollow sphere (c) both have the same angular momentum (d) impossible to determine

ic

Example 11.5 Bowling Ball

z

Estimate the magnitude of the angular momentum of a bowling ball spinning at 10 rev/s as shown in Figure 11.8.

ar ph

ys

S

S o l u ti o n

Conceptualize Imagine spinning a bowling ball on

sw

the smooth floor of a bowling alley. Because a bowling ball is relatively heavy, the angular momentum should be relatively large.

L

Figure 11.8 (Example 11.5) A bowling ball that rotates about the z axis in the direction shown has an angular S momentum L in the positive z direction. If the direction of rotation is reversed, then S L points in the negative z direction.

y

w

w

.a

Categorize We evaluate the angular momentum x using Equation 11.14, so we categorize this example as a substitution problem. We start by making some estimates of the relevant physical parameters and model the ball as a uniform solid sphere. A typical bowling ball might have a mass of 7.0 kg and a radius of 12 cm. I 5 25MR 2 5 25 1 7.0 kg 2 1 0.12 m 2 2 5 0.040 kg # m2

Evaluate the magnitude of the angular momentum from Equation 11.14:

Lz 5 Iv 5 1 0.040 kg # m2 2 1 10 rev/s 2 1 2p rad/rev 2 5 2.53 kg # m2/s

w

Evaluate the moment of inertia of the ball about an axis through its center from Table 10.2:

Because of the roughness of our estimates, we should keep only one significant figure, so L z 5 3 kg ? m2/s. S

S

S

S general, the expression L 5 I v is not always valid. If a rigid object rotates about an arbitrary axis, then L and v may point in different directions. In this case, the moment of inertia cannot be treated as a scalar. Strictly speaking, S S L 5 I v applies only to rigid objects of any shape that rotate about one of three mutually perpendicular axes (called principal axes) through the center of mass. This concept is discussed in more advanced texts on mechanics. 1In

344 Chapter 11

Angular Momentum

Example 11.6 The Seesaw AM

(A) Find an expression for the magnitude of the system’s angular momentum. S o l u ti o n

y O

u

x S

md g

Figure 11.9 (Example 11.6) A father and daughter demonstrate angular momentum on a seesaw.

S

mf g

om

A father of mass mf and his daughter of mass md sit on opposite ends of a seesaw at equal distances from the pivot at the center (Fig. 11.9). The seesaw is modeled as a rigid rod of mass M and length , and is pivoted without friction. At a given moment, the combination rotates in a vertical plane with an angular speed v.

.c

Conceptualize Identify the z axis through O as the axis of rotation in Figure 11.9. The rotating system has angular momentum about that axis.

,2 M a 1 m f 1 m db v 4 3

L 5 Iv 5

ic

Find the magnitude of the angular momentum of the system:

s. w

ee b

ly

Categorize Ignore any movement of arms or legs of the father and daughter and model them both as particles. The system is therefore modeled as a rigid object. This first part of the example is categorized as a substitution problem. The moment of inertia of the system equals the sum of the moments of inertia of the three components: the seesaw and the two individuals. We can refer to Table 10.2 to obtain the expression for the moment of inertia of the rod and use the particle expression I 5 mr 2 for each person. , 2 , 2 ,2 M 1 Find the total moment of inertia of the system about the M, 2 1 m f a b 1 m d a b 5 I 5 12 a 1 m f 1 m db 2 2 4 3 z axis through O :

ar ph

ys

(B) Find an expression for the magnitude of the angular acceleration of the system when the seesaw makes an angle u with the horizontal. S o l u ti o n

Conceptualize G enerally, fathers are more massive than daughters, so the system is not in equilibrium and has an angular acceleration. We expect the angular acceleration to be positive in Figure 11.9.

.a

sw

Categorize The combination of the board, father, and daughter is a rigid object under a net torque because of the external torque associated with the gravitational forces on the father and daughter. We again identify the axis of rotation as the z axis in Figure 11.9.

w

Analyze To find the angular acceleration of the system at any angle u, we first calculate the net torque on the system and then use o text 5 Ia from the rigid object under a net torque model to obtain an expression for a. , cos u 2

tf 5 m f g

Evaluate the torque due to the gravitational force on the daughter:

td 5 2m d g

w

w

Evaluate the torque due to the gravitational force on the father:

Evaluate the net external torque exerted on the system: Use Equation 11.16 and I from part (A) to find a:

1S t f out of page 2

, cos u 1 S t d into page 2 2

1 a text 5 tf 1 td 5 2 1 m f 2 m d 2 g , cos u

a5

2 1 m f 2 m d 2 g cos u a text 5 I , 3 1 M/3 2 1 m f 1 m d 4

Finalize For a father more massive than his daughter, the angular acceleration is positive as expected. If the seesaw begins in a horizontal orientation (u 5 0) and is released, the rotation is counterclockwise in Figure 11.9 and the father’s end of the seesaw drops, which is consistent with everyday experience. W h at I f ? Imagine the father moves inward on the seesaw to a distance d from the pivot to try to balance the two sides. What is the angular acceleration of the system in this case when it is released from an arbitrary angle u?

11.4 Analysis Model: Isolated System (Angular Momentum)

345

▸ 11.6 c o n t i n u e d Answer The angular acceleration of the system should decrease if the system is more balanced. Find the total moment of inertia about the z axis through O for the modified system:

I5

1 2 12 M,

, 2 ,2 M 1 mf d 2 1 md a b 5 a 1 m db 1 m f d 2 2 4 3

1 a text 5 tf 1 td 5 m f gd cos u 2 2m d g , cos u

Find the net torque exerted on the system about an axis through O: Find the new angular acceleration of the system:

1 m f d 2 12md , 2 g cos u a text 5 2 1 , /4 2 3 1 M/3 2 1 m d 4 1 m f d 2 I

om

a5

1 m f d 2 12md , 2 g cos u

ly

50 1 ,2/4 2 3 1 M/3 2 1 m d 4 1 m f d 2 md , m f d 2 12m d , 5 0 S d 5 a b mf 2

a5

ee b

Find the required position of the father by setting a 5 0:

.c

The seesaw is balanced when the angular acceleration is zero. In this situation, both father and daughter can push off the ground and rise to the highest possible point.

In the rare case that the father and daughter have the same mass, the father is located at the end of the seesaw, d 5 ,/2.

s. w

ys

ic

11.4 Analysis Model: Isolated System (Angular Momentum)

ar ph

In Chapter 9, we found that the total linear momentum of a system of particles remains constant if the system is isolated, that is, if the net external force acting on the system is zero. We have an analogous conservation law in rotational motion:

sw

The total angular momentum of a system is constant in both magnitude and direction if the net external torque acting on the system is zero, that is, if the system is isolated.

w

.a

This statement is often called2 the principle of conservation of angular momentum and is the basis of the angular momentum version of the isolated system model. This principle follows directly from Equation 11.13, which indicates that if a text

S

d L tot 5 5 0 dt

(11.17)

S

(11.18)

D L tot 5 0

w

then

w

S

Equation 11.18 can be written as

S

L tot 5 constant or

S

S

L i 5 Lf

For an isolated system consisting of a small number of particles, we write this conserS S vation law as L tot 5 g L n 5 constant, where the index n denotes the nth particle in the system. If an isolated rotating system is deformable so that its mass undergoes redistribution in some way, the system’s moment of inertia changes. Because the magnitude of the angular momentum of the system is L 5 Iv (Eq. 11.14), conservation 2 The most general conservation of angular momentum equation is Equation 11.13, which describes how the system interacts with its environment.

WW Conservation of angular momentum

346 Chapter 11

Angular Momentum

of angular momentum requires that the product of I and v must remain constant. Therefore, a change in I for an isolated system requires a change in v. In this case, we can express the principle of conservation of angular momentum as

When his arms and legs are close to his body, the skater’s moment of inertia is small and his angular speed is large.

(11.19)

Ii vi 5 If vf 5 constant

ee b

ly

.c

Clive Rose/Getty Images

om

This expression is valid both for rotation about a fixed axis and for rotation about an axis through the center of mass of a moving system as long as that axis remains fixed in direction. We require only that the net external torque be zero. Many examples demonstrate conservation of angular momentum for a deformable system. You may have observed a figure skater spinning in the finale of a program (Fig. 11.10). The angular speed of the skater is large when his hands and feet are close to the trunk of his body. (Notice the skater’s hair!) Ignoring friction between skater and ice, there are no external torques on the skater. The moment of inertia of his body increases as his hands and feet are moved away from his body at the finish of the spin. According to the isolated system model for angular momentum, his angular speed must decrease. In a similar way, when divers or acrobats wish to make several somersaults, they pull their hands and feet close to their bodies to rotate at a higher rate. In these cases, the external force due to gravity acts through the center of mass and hence exerts no torque about an axis through this point. Therefore, the angular momentum about the center of mass must be conserved; that is, Ii vi 5 If vf . For example, when divers wish to double their angular speed, they must reduce their moment of inertia to half its initial value. In Equation 11.18, we have a third version of the isolated system model. We can now state that the energy, linear momentum, and angular momentum of an isolated system are all constant: DE system 5 0 (if there are no energy transfers across the system boundary)

S Dp tot 5 0

(if the net external torque on the system is zero)

ar ph

D L tot 5 0

(if the net external force on the system is zero)

A system may be isolated in terms of one of these quantities but not in terms of another. If a system is nonisolated in terms of momentum or angular momentum, it will often be nonisolated also in terms of energy because the system has a net force or torque on it and the net force or torque will do work on the system. We can, however, identify systems that are nonisolated in terms of energy but isolated in terms of momentum. For example, imagine pushing inward on a balloon (the system) between your hands. Work is done in compressing the balloon, so the system is nonisolated in terms of energy, but there is zero net force on the system, so the system is isolated in terms of momentum. A similar statement could be made about twisting the ends of a long, springy piece of metal with both hands. Work is done on the metal (the system), so energy is stored in the nonisolated system as elastic potential energy, but the net torque on the system is zero. Therefore, the system is isolated in terms of angular momentum. Other examples are collisions of macroscopic objects, which represent isolated systems in terms of momentum but nonisolated systems in terms of energy because of the output of energy from the system by mechanical waves (sound).

w

w

w

.a

sw

Al Bello/Getty Images

tum is conserved as Russian gold medalist Evgeni Plushenko performs during the Turin 2006 Winter Olympic Games.

S

ys

Figure 11.10 A ngular momen-

ic

s. w

To slow down for the finish of his spin, the skater moves his arms and legs outward, increasing his moment of inertia.

Q uick Quiz 11.4 A competitive diver leaves the diving board and falls toward the water with her body straight and rotating slowly. She pulls her arms and legs into a tight tuck position. What happens to her rotational kinetic energy? (a) It increases. (b) It decreases. (c) It stays the same. (d) It is impossible to determine.

11.4 Analysis Model: Isolated System (Angular Momentum)

347

Analysis Model Isolated System (Angular Momentum) Imagine a system rotates about an axis. If there is no net external torque on the system, there is no change in the angular momentum of the system: S

Angular momentum

(11.18) The angular momentum of the isolated system is constant.

om

Applying this law of conservation of angular momentum to a system whose moment of inertia changes gives

(11.19)

Ii vi 5 If vf 5 constant

.c

D L tot 5 0

• after a supernova explosion, the core of a star collapses to a small radius and spins at a much higher rate • the square of the orbital period of a planet is proportional to the cube of its semimajor axis; Kepler’s third law (Chapter 13) • in atomic transitions, selection rules on the quantum numbers must be obeyed in order to conserve angular momentum (Chapter 42) • in beta decay of a radioactive nucleus, a neutrino must be emitted in order to conserve angular momentum (Chapter 44)

ly

Examples:

System boundary

ee b

Example 11.7 Formation of a Neutron Star AM

s. w

A star rotates with a period of 30 days about an axis through its center. The period is the time interval required for a point on the star’s equator to make one complete revolution around the axis of rotation. After the star undergoes a supernova explosion, the stellar core, which had a radius of 1.0 3 104 km, collapses into a neutron star of radius 3.0 km. Determine the period of rotation of the neutron star.

ic

S o l u ti o n

ys

Conceptualize The change in the neutron star’s motion is similar to that of the skater described earlier, but in the reverse direction. As the mass of the star moves closer to the rotation axis, we expect the star to spin faster.

sw

ar ph

Categorize Let us assume that during the collapse of the stellar core, (1) no external torque acts on it, (2) it remains spherical with the same relative mass distribution, and (3) its mass remains constant. We categorize the star as an isolated system in terms of angular momentum. We do not know the mass distribution of the star, but we have assumed the distribution is symmetric, so the moment of inertia can be expressed as kMR 2, where k is some numerical constant. (From Table 10.2, for example, we see that k 5 25 for a solid sphere and k 5 23 for a spherical shell.)

.a

Analyze Let’s use the symbol T for the period, with Ti being the initial period of the star and Tf being the period of the neutron star. The star’s angular speed is given by v 5 2p/T. Ii vi 5 If vf

Use v 5 2p/T to rewrite this equation in terms of the initial and final periods:

Ii a

w

w

w

From the isolated system model for angular momentum, write Equation 11.19 for the star:

2p 2p b 5 If a b Ti Tf

Substitute the moments of inertia in the preceding equation:

kMR i 2 a

Solve for the final period of the star:

Tf 5 a

Substitute numerical values:

Tf 5 a

2p 2p b 5 kMR f 2 a b Ti Tf

Rf

Ri

2

b Ti

2 3.0 km b 1 30 days 2 5 2.7 3 1026 days 5 0.23 s 4 1.0 3 10 km

Finalize The neutron star does indeed rotate faster after it collapses, as predicted. It moves very fast, in fact, rotating about four times each second!

348 Chapter 11

Angular Momentum

Example 11.8 The Merry-Go-Round AM A horizontal platform in the shape of a circular disk rotates freely in a horizontal plane about a frictionless, vertical axle (Fig. 11.11). The platform has a mass M 5 100 kg and a radius R 5 2.0 m. A student whose mass is m 5 60 kg walks slowly from the rim of the disk toward its center. If the angular speed of the system is 2.0 rad/s when the student is at the rim, what is the angular speed when she reaches a point r 5 0.50 m from the center?

M m

R

.c

ee b

Categorize Because the platform rotates on a frictionless axle, we identify the system of the student and the platform as an isolated system in terms of angular momentum.

Figure 11.11 (Example 11.8) As the student walks toward the center of the rotating platform, the angular speed of the system increases because the angular momentum of the system remains constant.

ly

Conceptualize The speed change here is similar to those of the spinning skater and the neutron star in preceding discussions. This problem is different because part of the moment of inertia of the system changes (that of the student) while part remains fixed (that of the platform).

om

S o l u ti o n

Analyze Let us denote the moment of inertia of the platform as Ip and that of the student as Is . We model the student as a particle.

Ii 5 Ipi 1 Isi 5 12 MR 2 1 mR 2

Find the moment of inertia of the system when the student walks to the position r , R:

If 5 Ipf 1 Isf 5 12 MR 2 1 mr 2

Write Equation 11.19 for the system:

Ii vi 5 If vf

Substitute the moments of inertia:

1 12 MR 2 1 mR 2 2 v i 5 1 12 MR 2 1 mr 2 2 v f

Substitute numerical values:

ic

ys

ar ph

sw

Solve for the final angular speed:

s. w

Find the initial moment of inertia Ii of the system (student plus platform) about the axis of rotation:

1 2 2 MR

1 mR 2

2

1 mr 2

vf 5 a 1

2 MR

1 2 1 100

vf 5 c 1

2 1 100

bv i

kg 2 1 2.0 m 2 2 1 1 60 kg 2 1 2.0 m 2 2

kg 2 1 2.0 m 2 2 1 1 60 kg 2 1 0.50 m 2 2

d 1 2.0 rad/s 2 5 4.1 rad/s

.a

Finalize A s expected, the angular speed increases. The fastest that this system could spin would be when the stu-

w

w

dent moves to the center of the platform. Do this calculation to show that this maximum angular speed is 4.4 rad/s. Notice that the activity described in this problem is dangerous as discussed with regard to the Coriolis force in Section 6.3. What if you measured the kinetic energy of the system before and after the student walks inward? Are the initial kinetic energy and the final kinetic energy the same?

w

W h at I f ?

Answer You may be tempted to say yes because the system is isolated. Remember, however, that energy can be transformed among several forms, so we have to handle an energy question carefully. Find the initial kinetic energy: Find the final kinetic energy:

K i 5 12 Ii v i 2 5 12 1 440 kg # m2 2 1 2.0 rad/s 2 2 5 880 J

K f 5 12 If v f2 5 12 1 215 kg # m2 2 1 4.1 rad/s 2 2 5 1.80 3 103 J

Therefore, the kinetic energy of the system increases. The student must perform muscular activity to move herself closer to the center of rotation, so this extra kinetic energy comes from potential energy stored in the student’s body from previous meals. The system is isolated in terms of energy, but a transformation process within the system changes potential energy to kinetic energy.

349

11.4 Analysis Model: Isolated System (Angular Momentum)

Example 11.9 Disk and Stick Collision AM

Before S

vdi

A 2.0-kg disk traveling at 3.0 m/s strikes a 1.0-kg stick of length 4.0 m that is lying flat on nearly frictionless ice as shown in the overhead view of Figure 11.12a. The disk strikes at the endpoint of the stick, at a distance r 5 2.0 m from the stick’s center. Assume the collision is elastic and the disk does not deviate from its original line of motion. Find the translational speed of the disk, the translational speed of the stick, and the angular speed of the stick after the collision. The moment of inertia of the stick about its center of mass is 1.33 kg ? m2. S o l u ti o n

r

a

om

After S

Figure 11.12 (Example

Conceptualize Examine Figure 11.12a and imagine what

11.9) Overhead view of a disk striking a stick in an elastic collision. (a) Before the collision, the disk moves toward the stick. (b) The collision causes the stick to rotate and move to the right.

ee b

ly

.c

happens after the disk hits the stick. Figure 11.12b shows what you might expect: the disk continues to move at a slower speed, and the stick is in both translational and rotational motion. We assume the disk does not deviate from its original line of motion because the force exerted by the stick on the disk is parallel to the original path of the disk.

vdf

v S

vs

b

s. w

Categorize Because the ice is frictionless, the disk and stick form an isolated system in terms of momentum and angular momentum. Ignoring the sound made in the collision, we also model the system as an isolated system in terms of energy. In addition, because the collision is assumed to be elastic, the kinetic energy of the system is constant. Analyze First notice that we have three unknowns, so we need three equations to solve simultaneously. S Dp tot 5 0 S 1 m d v df 1 m sv s 2 2 m d v di 5 0

ic

ys

Apply the isolated system model for momentum to the system and then rearrange the result:

ar ph

Apply the isolated system model for angular momentum to the system and rearrange the result. Use an axis passing through the center of the stick as the rotation axis so that the path of the disk is a distance r 5 2.0 m from the rotation axis:

.a

sw

Apply the isolated system model for energy to the system, rearrange the equation, and factor the combination of terms related to the disk:

w

w

Multiply Equation (1) by r and add to Equation (2):

w

Solve for v:

Divide Equation (3) by Equation (1):

(1) md(vdi 2 vdf) 5 msvs S

D L tot 5 0 S 1 2rm d v df 1 Iv 2 2 1 2rm d v di 2 5 0

(2) 2rmd(vdi 2 vdf) 5 Iv

DK 5 0 S 1 12m dv df2 1 12m sv s2 1 12I v 2 2 2 12m d v di2 5 0 (3) md(vdi 2 vdf)(vdi 1 vdf) 5 msvs2 1 Iv2 rmd(vdi 2 vdf) 5 rmsvs 2rmd(vdi 2 vdf) 5 Iv 0 5 rmsvs 1 Iv (4) v 5 2

rm sv s I

m d 1 v di 2 v df 2 1 v di 1 v df 2 m d 1 v di 2 v df 2

(5) v di 1 v df 5 v s 1

5

m sv s 2 1 I v 2 m sv s

I v2 m sv s

Substitute Equation (4) into Equation (5):

(6) v di 1 v df 5 v s a1 1

Substitute vdf from Equation (1) into Equation (6):

v di 1 av di 2

r 2m s b I

ms r 2m s v s b 5 v s a1 1 b md I

continued

350 Chapter 11

Angular Momentum

▸ 11.9 c o n t i n u e d Solve for vs and substitute numerical values:

vs 5 5

Substitute numerical values into Equation (4):

2v di 1 1 1 m s /m d 2 1 1 r 2m s /I 2

2 1 3.0 m/s 2 5 1.3 m/s 1 1 1 1.0 kg/2.0 kg 2 1 3 1 2.0 m 2 2 1 1.0 kg 2 /1.33 kg # m2 4

v52

v df 5 v di 2

1.33 kg # m2

5 22.0 rad/s

1.0 kg ms 1 1.3 m/s 2 5 2.3 m/s v s 5 3.0 m/s 2 md 2.0 kg

om

Solve Equation (1) for vdf and substitute numerical values:

1 2.0 m 2 1 1.0 kg 2 1 1.3 m/s 2

Finalize These values seem reasonable. The disk is moving more slowly after the collision than it was before the col-

Comparison of Values in Example 11.9 Before and After the Collision

v (m/s)

v (rad/s)

L (kg ? m 2/s)

p (kg ? m/s)

K trans ( J)

K rot ( J)

212 9.0 — 0 0 0 212 9.0 0

s. w

Before Disk 3.0 — 6.0 0 0 0 Stick Total for system — — 6.0 After Disk 2.3 — 4.7 Stick 1.3 22.0 1.3 Total for system — — 6.0

ee b

Table 11.1

ly

.c

lision, and the stick has a small translational speed. Table 11.1 summarizes the initial and final values of variables for the disk and the stick, and it verifies the conservation of linear momentum, angular momentum, and kinetic energy for the isolated system.

ys

ic

29.3 5.4 — 22.7 0.9 2.7 212 6.3 2.7

ar ph

Note: Linear momentum, angular momentum, and total kinetic energy of the system are all conserved.

sw

11.5 The Motion of Gyroscopes and Tops

w

w

w

.a

An unusual and fascinating type of motion you have probably observed is that of a top spinning about its axis of symmetry as shown in Figure 11.13a. If the top spins rapidly, the symmetry axis rotates about the z axis, sweeping out a cone (see Fig. 11.13b). The motion of the symmetry axis about the vertical—known as precessional motion—is usually slow relative to the spinning motion of the top. It is quite natural to wonder why the top does not fall over. Because the center of mass is not directly above the pivot point O, a net torque is acting on the top about an axis passing through O, a torque resulting from the gravitational force MS g . The top would certainly fall over if it were not spinning. Because it is spinS ning, however, it has an angular momentum L directed along its symmetry axis. We shall show that this symmetry axis moves about the z axis (precessional motion occurs) because the torque produces a change in the direction of the symmetry axis. This illustration is an excellent example of the importance of the vector nature of angular momentum. The essential features of precessional motion can be illustrated by considering the simple gyroscope shown in Figure 11.14a. The two forces acting on the gyroscope are shown in Figure 11.14b: the downward gravitational force M S g and the normal force S n acting upward at the pivot point O. The normal force produces no torque about an axis passing through the pivot because its moment arm through r 3 MS g that point is zero. The gravitational force, however, produces a torque S t5S S about an axis passing through O, where the direction of t is perpendicular to the t lies in a horizontal xy plane plane formed by S r and MS g. By necessity, the vector S

11.5 The Motion of Gyroscopes and Tops 351

perpendicular to the angular momentum vector. The net torque and angular momentum of the gyroscope are related through Equation 11.13: a t ext S

The right-hand rule indicates S S S S S that r F r M g is in the xy plane.

S

dL 5 dt

z

S

L

CM S

Mg x

n

r

y

O

S

ly

.c

a

S

S

om

This expression shows that in the infinitesimal time interval dt, the nonzero torque S produces a change in angular momentum d LS , a change that is in the same direcS tion as S t . Therefore, like the torque vector, d L must also be perpendicular to L . Figure 11.14c illustrates the resulting precessional motion of the symmetry axis of S the gyroscope. In a time interval dt, the change in angular momentum is d L 5 S S S S S t dt. Because L f 2 L i 5SS d L is perpendicular to L , the magnitude of L does not S S change 1 0 L i 0 5 0 L f 0 2 . Rather, what is changing is the direction of L . Because the S S change in angular momentum d L is in the direction of t , which lies in the xy plane, the gyroscope undergoes precessional motion. To simplify the description of the system, we assume the total angular momenS tum of the precessing wheel is the sum of the angular momentum I v due to the spinning and the angular momentum due to the motion of the center of mass about the pivot. In our treatment, we shall neglect the contribution from the centerS of-mass motion and take the total angular momentum to be simply I v . In practice, S this approximation is good if v is made very large. The vector diagram in Figure 11.14c shows that in the time interval dt, the angular momentum vector rotates through an angle df, which is also the angle through which the gyroscope axle rotates. From the vector triangle formed by the vectors S S S L i , L f , and d L , we see that

S

ee b

L

S

s. w

Li

x

1 Mg rCM 2 dt dL a text dt 5 5 df 5 L L L

S

ic

ys

ar ph w

w

w

.a

sw

z r CM S

n

S

S

S

Li

x

(11.20)

O t

S

Lf

Li df

y

S

dL

b

Figure 11.13 Precessional motion of a top spinning about its symmetry axis. (a) The only external forces acting on the top S are the normal force n and the gravitational force M S g . The direction of the angular momentum S L is along the axis of symmetry. S S S (b) Because L f 5 D L 1 L i , the top precesses about the z axis.

O S

t

Lf

Mg

y

S

a

b

S

S

The gravitational force M g in the negative z direction produces a torque on the gyroscope in the positive y direction about the pivot.

y

O

The direction of L is parallel S to that of in a .

Dividing through by dt and using the relationship L 5 Iv, we find that the rate at which the axle rotates about the vertical axis is MgrCM df 5 vp 5 dt Iv

Lf

S

The torque results in a change in angular S momentum d L in a direction parallel to the torque vector. The gyroscope axle sweeps out an angle d f in a time interval dt. c

Figure 11.14 (a) A spinning gyroscope is placed on a pivot at the right end. (b) Diagram for the spinning gyroscope showing forces, torque, and angular momentum. (c) Overhead view (looking down the z axis) of the gyroscope’s initial and final angular momentum vectors for an infinitesimal time interval dt.

Angular Momentum

The angular speed vp is called the precessional frequency. This result is valid only when vp ,, v. Otherwise, a much more complicated motion is involved. As you can see from Equation 11.20, the condition vp ,, v is met when v is large, that is, when the wheel spins rapidly. Furthermore, notice that the precessional frequency decreases as v increases, that is, as the wheel spins faster about its axis of symmetry. As an example of the usefulness of gyroscopes, suppose you are in a spacecraft in deep space and you need to alter your trajectory. To fire the engines in the correct direction, you need to turn the spacecraft. How, though, do you turn a spacecraft in empty space? One way is to have small rocket engines that fire perpendicularly out the side of the spacecraft, providing a torque around its center of mass. Such a setup is desirable, and many spacecraft have such rockets. Let us consider another method, however, that does not require the consumption of rocket fuel. Suppose the spacecraft carries a gyroscope that is not rotating as in Figure 11.15a. In this case, the angular momentum of the spacecraft about its center of mass is zero. Suppose the gyroscope is set into rotation, giving the gyroscope a nonzero angular momentum. There is no external torque on the isolated system (spacecraft and gyroscope), so the angular momentum of this system must remain zero according to the isolated system (angular momentum) model. The zero value can be satisfied if the spacecraft rotates in the direction opposite that of the gyroscope so that the angular momentum vectors of the gyroscope and the spacecraft cancel, resulting in no angular momentum of the system. The result of rotating the gyroscope, as in Figure 11.15b, is that the spacecraft turns around! By including three gyroscopes with mutually perpendicular axles, any desired rotation in space can be achieved. This effect created an undesirable situation with the Voyager 2 spacecraft during its flight. The spacecraft carried a tape recorder whose reels rotated at high speeds. Each time the tape recorder was turned on, the reels acted as gyroscopes and the spacecraft started an undesirable rotation in the opposite direction. This rotation had to be counteracted by Mission Control by using the sideward-firing jets to stop the rotation!

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352 Chapter 11

.c

a

ys

ic

s. w

ee b

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When the gyroscope turns counterclockwise, the spacecraft turns clockwise.

ar ph

b

sw

Figure 11.15 (a) A spacecraft carries a gyroscope that is not spinning. (b) The gyroscope is set into rotation.

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Definitions

.a

Summary

S

S

Given two vectors A and B, the vecS S S tor product A 3 B is a vector C having a magnitude

S

The torque S t on a particle due to a force F about an axis through the origin in an inertial frame is defined to be t;S r 3 F

(11.3)

C 5 AB sin u S

S

where u is the angle between A and B . The S S S direction of the vector C 5 A 3 B is perS S pendicular to the plane formed by A and B , and this direction is determined by the righthand rule.

S

S

(11.1)

S

The angular momentum L about an axis through the origin of a particle having linear momentum S p 5 mS v is S

L ;S r 3S p

S

(11.10)

where r is the vector position of the particle relative to the origin.

Objective Questions 353

Concepts and Principles

continued

The z component of angular momentum of a rigid object rotating about a fixed z axis is

(11.14)

Lz 5 Iv

where I is the moment of inertia of the object about the axis of rotation and v is its angular speed.

System boundary

.c

External torque

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Angular momentum

ee b

Angular momentum

s. w

The rate of change in the angular momentum of the nonisolated system is equal to the net external torque on the system.

Isolated System (Angular Momentum). If a system experiences no external torque from the environment, the total angular momentum of the system is conserved:

S a text 5

S

d L tot dt

(11.13)

sw

ar ph

ys

ic

Nonisolated System (Angular Momentum). If a system interacts with its environment in the sense that there is an external torque on the system, the net external torque acting on a system is equal to the time rate of change of its angular momentum:

The angular momentum of the isolated system is constant.

Objective Questions

S

(11.18)

D L tot 5 0

Applying this law of conservation of angular momentum to a system whose moment of inertia changes gives

(11.19)

Ii vi 5 If vf 5 constant

1. denotes answer available in Student Solutions Manual/Study Guide

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.a

1. An ice skater starts a spin with her arms stretched out to the sides. She balances on the tip of one skate to turn without friction. She then pulls her arms in so that her moment of inertia decreases by a factor of 2. In the process of her doing so, what happens to her kinetic energy? (a) It increases by a factor of 4. (b) It increases by a factor of 2. (c) It remains constant. (d) It decreases by a factor of 2. (e) It decreases by a factor of 4. 2. A pet mouse sleeps near the eastern edge of a stationary, horizontal turntable that is supported by a frictionless, vertical axle through its center. The mouse wakes up and starts to walk north on the turntable. (i) As it takes its first steps, what is the direction of the mouse’s displacement relative to the stationary ground below? (a) north (b) south (c) no displacement. (ii) In this process, the spot on the turntable where the mouse had been snoozing undergoes a displacement in what direction relative to the ground below? (a) north (b) south (c) no displacement. Answer yes or no for the

following questions. (iii) In this process, is the mechanical energy of the mouse–turntable system constant? (iv) Is the momentum of the system constant? (v) Is the angular momentum of the system constant? 3. Let us name three perpendicular directions as right, up, and toward you as you might name them when you are facing a television screen that lies in a vertical plane. Unit vectors for these directions are r^ , u^ , and t^, respectively. Consider the quantity (23u^ 3 2t^). (i) Is the magnitude of this vector (a) 6, (b) 3, (c) 2, or (d) 0? (ii) Is the direction of this vector (a) down, (b) toward you, (c) up, (d) away from you, or (e) left? 4. Let the four compass directions north, east, south, and west be represented by unit vectors n^ , e^ , s^ , and w^ , respectively. Vertically up and down are represented as u^ and d^ . Let us also identify unit vectors that are halfway between these directions such as ne for northeast. Rank the magnitudes of the following cross products from largest to smallest. If any are equal in magnitude l

System boundary

om

Analysis Models for Problem Solving

Angular Momentum they walk, what happens to the angular speed of the turntable? (a) It increases. (b) It decreases. (c) It stays constant. Consider the ponies–turntable system in this process and answer yes or no for the following questions. (ii) Is the mechanical energy of the system conserved? (iii) Is the momentum of the system conserved? (iv) Is the angular momentum of the system conserved?

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8. Consider an isolated system moving through empty space. The system consists of objects that interact with each other and can change location with respect to one another. Which of the following quantities can change in time? (a) The angular momentum of the system. (b) The linear momentum of the system. (c) Both the angular momentum and linear momentum of the system. (d) Neither the angular momentum nor linear momentum of the system.

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.c

l

l

Conceptual Questions

1. denotes answer available in Student Solutions Manual/Study Guide

1. Stars originate as large bodies of slowly rotating gas. Because of gravity, these clumps of gas slowly decrease in size. What happens to the angular speed of a star as it shrinks? Explain.

9. If global warming continues over the next one hundred years, it is likely that some polar ice will melt and the water will be distributed closer to the equator. (a) How would that change the moment of inertia of the Earth? (b) Would the duration of the day (one revolution) increase or decrease? 10. A cat usually lands on its feet regardless of the position from which it is dropped. A slow-motion film of a cat falling shows that the upper half of its body twists in one direction while the lower half twists in the opposite direction. (See Fig. CQ11.10.) Why does this type of rotation occur?

ys

ic

2. A scientist arriving at a hotel asks a bellhop to carry a heavy suitcase. When the bellhop rounds a corner, the suitcase suddenly swings away from him for some unknown reason. The alarmed bellhop drops the suitcase and runs away. What might be in the suitcase?

ee b

l

or are equal to zero, show that in your ranking. (a) n^ 3 n^ (b) w^ 3 ne (c) u^ 3 ne (d) n^ 3 nw (e) n^ 3 e^ 5. Answer yes or no to the following questions. (a) Is it possible to calculate the torque acting on a rigid object without specifying an axis of rotation? (b) Is the torque independent of the location of the axis of rotation? S S is in 6. Vector A is in the negative y direction, and vector B S A 3 the negative x direction. (i) What is the direction of S B ? (a) no direction because it is a scalar (b) xS (c) S 2y (d) z (e) 2z (ii) What is the direction of B 3 A ? Choose from the same possibilities (a) through (e). 7. Two ponies of equal mass are initially at diametrically opposite points on the rim of a large horizontal turntable that is turning freely on a frictionless, vertical axle through its center. The ponies simultaneously start walking toward each other across the turntable. (i) As

s. w

354 Chapter 11

ar ph

3. Why does a long pole help a tightrope walker stay balanced?

w

5. Both torque and work are products of force and displacement. How are they different? Do they have the same units? 6. In some motorcycle races, the riders drive over small hills and the motorcycle becomes airborne for a short time interval. If the motorcycle racer keeps the throttle open while leaving the hill and going into the air, the motorcycle tends to nose upward. Why? 7. If the torque acting on a particle about an axis through a certain origin is zero, what can you say about its angular momentum about that axis? 8. A ball is thrown in such a way that it does not spin about its own axis. Does this statement imply that the angular momentum is zero about an arbitrary axis? Explain.

Agence Nature/Photo Researchers, Inc.

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.a

sw

4. Two children are playing with a roll of paper towels. One child holds the roll between the index fingers of her hands so that it is free to rotate, and the second child pulls at constant speed on the free end of the paper towels. As the child pulls the paper towels, the radius of the roll of remaining towels decreases. (a) How does the torque on the roll change with time? (b) How does the angular speed of the roll change in time? (c) If the child suddenly jerks the end paper towel with a large force, is the towel more likely to break from the others when it is being pulled from a nearly full roll or from a nearly empty roll?

Figure CQ11.10 11. In Chapters 7 and 8, we made use of energy bar charts to analyze physical situations. Why have we not used bar charts for angular momentum in this chapter?

355

Problems

Problems AMT Analysis Model tutorial available in

The problems found in this chapter may be assigned online in Enhanced WebAssign

Enhanced WebAssign

GP Guided Problem M Master It tutorial available in Enhanced WebAssign

1. straightforward; 2. intermediate; 3. challenging

BIO

W Watch It video solution available in Enhanced WebAssign

1. full solution available in the Student Solutions Manual/Study Guide

Q/C

S

N 5 4 ^i 1 5 ^j 2 2k^ , calcu 1. Given M 5 2 ^i 2 3 ^j 1 k^ Sand S W late the vector product M 3 N .

2. The displacement vectors 42.0 cm at 15.08 and 23.0 cm at 65.08 both start from the origin and form two sides of a parallelogram. Both angles are measured counterclockwise from the x axis. (a) Find the area of the parallelogram. (b) Find the length of its longer diagonal. S

S

s. w

3. Two vectors areSgiven by A 5 ^i 1 2 ^j and B 5S22 ^i 1S3 ^j. S A 3 B and (b) the angle between A and B. Find (a) M

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ic

4. Use the definition of the vector product and the definiS tions of the unit vectors ^i, ^j, and k^ to prove Equations 11.7. You may assume the x axis points to the right, the y axis up, and the z axis horizontally toward you (not away from you). This choice is said to make the coordinate system a right-handed system.

ar ph

5. Calculate the net torque (magnitude and direction) on the beam in Figure P11.5 about (a) an axis through O perpendicular to the page and (b) an axis through C perpendicular to the page. 25 N

sw O

.a

2.0 m C

20°

10 N

4.0 m

w

w

30 N

A

B

S

F3 D

O

C S

S

F2

F1

Figure P11.9 S

10. A student claims that S he has found a vector A such Q/C that 1 2 ^i 2 3 ^j 1 4k^ 2 3 A 5 1 4 ^i 1 3 ^j 2 k^ 2 . (a) Do you believe this claim? (b) Explain why or why not. Section 11.2 Analysis Model: Nonisolated System (Angular Momentum) 11. A light, rigid rod of length , 5 1.00 m joins two parM ticles, with masses m1 5 4.00 kg and m 2 5 3.00 kg, at its ends. The combination rotates in the xy plane about a pivot through the center of the rod (Fig. P11.11). Determine the angular momentum of the system about the origin when the speed of each particle is 5.00 m/s.

30°

45°

S

9. Two forces F1 and F 2 act along the two sides of an equilateral triangle as shown in Figure P11.9. Point O is the intersection ofS the altitudes of the triangle. (a) Find a third force F 3 to be applied at B and along BC that will make the total torque zero about the point O. S (b) What If? Will the total torque change if F 3 is applied not at B but at any other point along BC?

.c

S

ly

S

ee b

Section 11.1 The Vector Product and Torque

om

S

y

Figure P11.5

S

v

S

m2

w

6. Two vectors areSgiven by these expressions: A 5 23 i^ 1 7 j^ 2 4 k^ and BS 5 S6 ^i 2 10 ^j 1 9k^ . EvaluateS the Squantities (a) cos21[ A ? B /AB] and (b) sin21[ 0 A 3 B 0 /AB]. (c) Which give(s) the angle between the vectors? S

S

S

S

S

S

7. If 0 A 3 B 0 5 A ? B , what is the angle between A and B?

S 8. A particle is located at the vector position r 5

Q/C 1 4.00 ^i 1 6.00 ^j 2 m, and a force exerted on it is given by S

F 5 1 3.00 i^ 1 2.00 j^ 2 N. (a) What is the torque acting on the particle about the origin? (b) Can there be another point about which the torque caused by this force on this particle will be in the opposite direction and half as large in magnitude? (c) Can there be more than one such point? (d) Can such a point lie on the y axis? (e) Can more than one such point lie on the y axis? (f) Determine the position vector of one such point.

x m1

,

S

v

Figure P11.11 12. A 1.50-kg particle moves in the xy plane with a velocS W ity of v 5 1 4.20 i^ 2 3.60 j^ 2 m/s. Determine the angular momentum of the particle about the origin when its r 5 1 1.50 ^i 1 2.20 ^j 2 m. position vector is S

13. A particle of mass m moves in the xy plane with a velocity S of S v 5 v x i^ 1 v y ^j. Determine the angular momentum

356 Chapter 11

Angular Momentum

m

v1 vxi i

u

S

v2

ys

Figure P11.15

20. A 5.00-kg particle starts from the origin at time zero.

Q/C Its velocity as a function of time is given by

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16. Review. A conical pendulum consists S of a bob of mass m in motion in a circular path in a horizontal plane as shown in Figure P11.16. During the motion, the supporting wire of length , maintains a constant angle u with the vertical. Show that the magnitude of the angular momentum of the bob about the vertical dashed line is

sw

u

m

Figure P11.16

.a m 2g ,3 sin4 u cos u

w

L5a

1/2

b

w

w

17. A particle of mass m moves in a circle of radius R at a S constant speed v as shown in Figure P11.17. The motion begins at point Q at time t 5 0. Determine the angular momentum of the particle about the axis perpendicular to the page through point P as a function of time. y S

v

R P

m Q

om

19. The position vector of a particle of mass 2.00 kg as S M a function of time is given by r 5 1 6.00 ^i 1 5.00t ^j 2 , r is in meters and t is in seconds. Determine the where S angular momentum of the particle about the origin as a function of time.

x

R

Figure P11.18

ic

O

m

S

S

vi

R

s. w

y

M

.c

15. Review. A projectile of mass m is launched with an iniS Q/C tial velocity v i making an angle u with the horizontal as S shown in Figure P11.15. The projectile moves in the gravitational field of the Earth. Find the angular momentum of the projectile about the origin (a) when the projectile is at the origin, (b) when it is at the highest point of its trajectory, and (c) just before it hits the ground. (d) What torque causes its angular momentum to change?

8.00 cm and mass M 5 2.00 kg. The spokes have negligible mass. (a) What is the magnitude of the net torque on the system about the axle of the pulley? (b) When the counterweight has a speed v, the pulley has an angular speed v 5 v/R. Determine the magnitude of the total angular momentum of the system about the axle of the pulley. (c) Using your result from S t 5 d L /dt, calculate the acceleration of part (b) and S the counterweight.

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14. Heading straight toward the summit of Pike’s Peak, an airplane of mass 12 000 kg flies over the plains of Kansas at nearly constant altitude 4.30 km with constant velocity 175 m/s west. (a) What is the airplane’s vector angular momentum relative to a wheat farmer on the ground directly below the airplane? (b) Does this value change as the airplane continues its motion along a straight line? (c) What If? What is its angular momentum relative to the summit of Pike’s Peak?

18. A counterweight of mass m 5 4.00 kg is attached to

AMT a light cord that is wound around a pulley as in FigW ure P11.18. The pulley is a thin hoop of radius R 5

ee b

of the particle about the origin when its position vector r 5 x ^i 1 y ^j. is S

x

Figure P11.17 Problems 17 and 32.

S

v 5 6t 2 ^i 1 2t ^j

where S v is in meters per second and t is in seconds. (a) Find its position as a function of time. (b) Describe its motion qualitatively. Find (c) its acceleration as a function of time, (d) the net force exerted on the particle as a function of time, (e) the net torque about the origin exerted on the particle as a function of time, (f) the angular momentum of the particle as a function of time, (g) the kinetic energy of the particle as a function of time, and (h) the power injected into the system of the particle as a function of time. 21. A ball having mass m is fas-

Q/C tened at the end of a flagpole S that is connected to the side

m

of a tall building at point P as shown in Figure P11.21. The length of the flagpole is ,, and u it makes an angle u with the x axis. The ball becomes loose P and starts to fall with acceleraFigure P11.21 tion 2g ^j. (a) Determine the angular momentum of the ball about point P as a function of time. (b) For what physical reason does the angular momentum change? (c) What is the rate of change of the angular momentum of the ball about point P ?

Problems

Section 11.3 Angular Momentum of a Rotating Rigid Object

the ring. (a) What angular momentum does the space station acquire? (b) For what time interval must the rockets be fired if each exerts a thrust of 125 N?

22. A uniform solid sphere of radius r 5 0.500 m and mass m 5 15.0 kg turns counterclockwise about a vertical axis through its center. Find its vector angular momentum about this axis when its angular speed is 3.00 rad/s.

r

.c

Figure P11.29 Problems 29 and 40.

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Section 11.4 Analysis Model: Isolated System (Angular Momentum)

ee b

30. A disk with moment of inertia I1 rotates about a fricW tionless, vertical axle with angular speed vi . A second S disk, this one having moment of inertia I2 and initially not rotating, drops onto the first disk (Fig. P11.30). Because of friction between the surfaces, the two eventually reach the same angular speed vf . (a) Calculate vf . (b) Calculate the ratio of the final to the initial rotational energy.

s. w

25. A uniform solid disk of mass m 5 3.00 kg and radius W r 5 0.200 m rotates about a fixed axis perpendicular to its face with angular frequency 6.00 rad/s. Calculate the magnitude of the angular momentum of the disk when the axis of rotation (a) passes through its center of mass and (b) passes through a point midway between the center and the rim.

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23. Big Ben (Fig. P10.49, page 328), the Parliament tower clock in London, has hour and minute hands with lengths of 2.70 m and 4.50 m and masses of 60.0 kg and 100 kg, respectively. Calculate the total angular momentum of these hands about the center point. (You may model the hands as long, thin rods rotating about one end. Assume the hour and minute hands are rotating at a constant rate of one revolution per 12 hours and 60 minutes, respectively.) 24. Show that the kinetic energy of an object rotating S about a fixed axis with angular momentum L 5 Iv can be written as K 5 L2/2I.

26. Model the Earth as a uniform sphere. (a) Calculate

Q/C the angular momentum of the Earth due to its spin-

S

vi

ic

ning motion about its axis. (b) Calculate the angular momentum of the Earth due to its orbital motion about the Sun. (c) Explain why the answer in part (b) is larger than that in part (a) even though it takes significantly longer for the Earth to go once around the Sun than to rotate once about its axis.

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28. The distance between the centers of the wheels of a motorcycle is 155 cm. The center of mass of the motorcycle, including the rider, is 88.0 cm above the ground and halfway between the wheels. Assume the mass of each wheel is small compared with the body of the motorcycle. The engine drives the rear wheel only. What horizontal acceleration of the motorcycle will make the front wheel rise off the ground? 29. A space station is constructed in the shape of a hollow

AMT ring of mass 5.00 3 104 kg. Members of the crew walk

on a deck formed by the inner surface of the outer cylindrical wall of the ring, with radius r 5 100 m. At rest when constructed, the ring is set rotating about its axis so that the people inside experience an effective free-fall acceleration equal to g. (See Fig. P11.29.) The rotation is achieved by firing two small rockets attached tangentially to opposite points on the rim of

S

vf

ys

ar ph

.a

sw

27. A particle of mass 0.400 kg is attached to the 100-cm M mark of a meterstick of mass 0.100 kg. The meterstick rotates on the surface of a frictionless, horizontal table with an angular speed of 4.00 rad/s. Calculate the angular momentum of the system when the stick is pivoted about an axis (a) perpendicular to the table through the 50.0-cm mark and (b) perpendicular to the table through the 0-cm mark.

357

I2

I1 Before

After

Figure P11.30 31. A playground merry-go-round of radius R 5 2.00 m

2 AMT has a moment of inertia I 5 250 kg ? m and is rotating W at 10.0 rev/min about a frictionless, vertical axle. Fac-

ing the axle, a 25.0-kg child hops onto the merry-goround and manages to sit down on the edge. What is the new angular speed of the merry-go-round?

32. Figure P11.17 represents a small, flat puck with mass

Q/C m 5 2.40 kg sliding on a frictionless, horizontal sur-

face. It is held in a circular orbit about a fixed axis by a rod with negligible mass and length R 5 1.50 m, pivoted at one end. Initially, the puck has a speed of v 5 5.00 m/s. A 1.30-kg ball of putty is dropped vertically onto the puck from a small distance above it and immediately sticks to the puck. (a) What is the new period of rotation? (b) Is the angular momentum of the puck–putty system about the axis of rotation constant in this process? (c) Is the momentum of the system constant in the process of the putty sticking to the puck? (d) Is the mechanical energy of the system constant in the process?

Angular Momentum

sw

a

v

m1 m2 a

b

.a

Figure P11.34

35. A uniform cylindrical turntable of radius 1.90 m and

w

Q/C mass 30.0 kg rotates counterclockwise in a horizontal

w

w

plane with an initial angular speed of 4p rad/s. The fixed turntable bearing is frictionless. A lump of clay of mass 2.25 kg and negligible size is dropped onto the turntable from a small distance above it and immediately sticks to the turntable at a point 1.80 m to the east of the axis. (a) Find the final angular speed of the clay and turntable. (b) Is the mechanical energy of the turntable–clay system constant in this process? Explain and use numerical results to verify your answer. (c) Is the momentum of the system constant in this process? Explain your answer.

36. A puck of mass m1 5 80.0 g and radius r 1 5 4.00 cm v 5 1.50 m/s as glides across an air table at a speed of S shown in Figure P11.36a. It makes a glancing collision with a second puck of radius r 2 5 6.00 cm and mass m 2 5 120 g (initially at rest) such that their rims just touch. Because their rims are coated with instant-acting glue,

b

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Figure P11.36

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.c

37. A wooden block of mass M resting on a frictionless, S horizontal surface is attached to a rigid rod of length , and of negligible mass (Fig. P11.37). The rod is pivoted at the other end. A bullet of mass m traveling parallel to the horizontal surface and perpendicular to the rod with speed v hits the block and becomes embedded in it. (a) What is the angular momentum of the bullet– block system about a vertical axis through the pivot? (b) What fraction of the original kinetic energy of the bullet is converted into internal energy in the system during the collision?

ys

vf

ar ph

vi

S

ic

34. A student sits on a freely rotating stool holding two W dumbbells, each of mass 3.00 kg (Fig. P11.34). When his arms are extended horizontally (Fig. P11.34a), the dumbbells are 1.00 m from the axis of rotation and the student rotates with an angular speed of 0.750 rad/s. The moment of inertia of the student plus stool is 3.00 kg · m2 and is assumed to be constant. The student pulls the dumbbells inward horizontally to a position 0.300 m from the rotation axis (Fig. P11.34b). (a) Find the new angular speed of the student. (b) Find the kinetic energy of the rotating system before and after he pulls the dumbbells inward.

the pucks stick together and rotate after the collision (Fig. P11.36b). (a) What is the angular momentum of the system relative to the center of mass? (b) What is the angular speed about the center of mass?

ee b

33. A 60.0-kg woman stands at the western rim of a M horizontal turntable having a moment of inertia of Q/C 500 kg ? m2 and a radius of 2.00 m. The turntable is initially at rest and is free to rotate about a frictionless, vertical axle through its center. The woman then starts walking around the rim clockwise (as viewed from above the system) at a constant speed of 1.50 m/s relative to the Earth. Consider the woman–turntable system as motion begins. (a) Is the mechanical energy of the system constant? (b) Is the momentum of the system constant? (c) Is the angular momentum of the system constant? (d) In what direction and with what angular speed does the turntable rotate? (e) How much chemical energy does the woman’s body convert into mechanical energy of the woman–turntable system as the woman sets herself and the turntable into motion?

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358 Chapter 11

M

S

v

m

Figure P11.37 38. Review. A thin, uniform, rectangular signboard hangs vertically above the door of a shop. The sign is hinged to a stationary horizontal rod along its top edge. The mass of the sign is 2.40 kg, and its vertical dimension is 50.0 cm. The sign is swinging without friction, so it is a tempting target for children armed with snowballs. The maximum angular displacement of the sign is 25.08 on both sides of the vertical. At a moment when the sign is vertical and moving to the left, a snowball of mass 400 g, traveling horizontally with a velocity of 160 cm/s to the right, strikes perpendicularly at the lower edge of the sign and sticks there. (a) Calculate the angular speed of the sign immediately before the impact. (b) Calculate its angular speed immediately after the impact. (c) The spattered sign will swing up through what maximum angle? v i is fired 39. A wad of sticky clay with mass m and velocity S at a solid cylinder of mass M and radius R (Fig. P11.39). Q/C S The cylinder is initially at rest and is mounted on a fixed horizontal axle that runs through its center of mass. The line of motion of the projectile is perpendicular to the axle and at a distance d , R from the center. (a) Find the angular speed of the system just after the clay strikes and sticks to the surface of the cylin-

Problems der. (b) Is the mechanical energy of the clay–cylinder system constant in this process? Explain your answer. (c) Is the momentum of the clay–cylinder system constant in this process? Explain your answer.

of the spacecraft around the same axis is Is 5 5.00 3 105 kg ? m2. Neither the spacecraft nor the gyroscope is originally rotating. The gyroscope can be powered up in a negligible period of time to an angular speed of 100 rad/s. If the orientation of the spacecraft is to be changed by 30.08, for what time interval should the gyroscope be operated?

S

m vi

M

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40. Why is the following situation impossible? A space station shaped like a giant wheel has a radius of r 5 100 m and a moment of inertia of 5.00 3 108 kg ? m2. A crew of 150 people of average mass 65.0 kg is living on the rim, and the station’s rotation causes the crew to experience an apparent free-fall acceleration of g (Fig. P11.29). A research technician is assigned to perform an experiment in which a ball is dropped at the rim of the station every 15 minutes and the time interval for the ball to drop a given distance is measured as a test to make sure the apparent value of g is correctly maintained. One evening, 100 average people move to the center of the station for a union meeting. The research technician, who has already been performing his experiment for an hour before the meeting, is disappointed that he cannot attend the meeting, and his mood sours even further by his boring experiment in which every time interval for the dropped ball is identical for the entire evening. 41. A 0.005 00-kg bullet traveling horizontally with speed Q/C 1.00 3 103 m/s strikes an 18.0-kg door, embedding itself 10.0 cm from the side opposite the hinges as shown in Figure P11.41. The 1.00-m wide door is free to swing on its frictionless hinges. (a) Before it hits the door, does the bullet have angular momentum relative to the door’s axis of rotation? (b) If so, evaluate this angular momentum. If not, explain why there is no angular momentum. (c) Is the mechanical energy of the bullet– door system constant during this collision? Answer without doing a calculation. (d) At what angular speed does the door swing open immediately after the collision? (e) Calculate the total energy of the bullet–door system and determine whether it is less than or equal to the kinetic energy of the bullet before the collision.

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Figure P11.39

43. The angular momentum vector of a precessing gyroscope sweeps out a cone as shown in Figure P11.43. The angular speed of the tip of the angular momentum vector, called its precessional frequency, is given by vp 5 t/L, where t is the magnitude of the torque on the gyroscope and L is the magnitude of its angular momentum. In the motion called precession of the equinoxes, the Earth’s axis of rotation precesses about the perpendicular to its orbital plane with a period of 2.58 3 104 yr. Model the Earth as a uniform sphere and calculate the torque on the Earth that is causing this precession.

ee b

R

d

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Hinge

18.0 kg

0.005 00 kg

Figure P11.41 An overhead view of a bullet striking a door. Section 11.5 The Motion of Gyroscopes and Tops 42. A spacecraft is in empty space. It carries on board a gyroscope with a moment of inertia of Ig 5 20.0 kg ? m2 about the axis of the gyroscope. The moment of inertia

359

vp

t L

S

L

Figure P11.43 A precessing angular momentum vector sweeps out a cone in space.

Additional Problems 4 4. A light rope passes over a light,

Q/C frictionless pulley. One end is fasS tened to a bunch of bananas of

mass M, and a monkey of mass M clings to the other end (Fig. P11.44). M The monkey climbs the rope in an attempt to reach the bananas. (a) Treating the system as consistM ing of the monkey, bananas, rope, and pulley, find the net torque on the system about the pulley axis. (b) Using the result of part (a), Figure P11.44 determine the total angular momentum about the pulley axis and describe the motion of the system. (c) Will the monkey reach the bananas?

45. Comet Halley moves about the Sun in an elliptical orbit, with its closest approach to the Sun being about 0.590 AU and its greatest distance 35.0 AU (1 AU 5 the Earth–Sun distance). The angular momentum of the comet about the Sun is constant, and the gravitational force exerted by the Sun has zero moment arm. The comet’s speed at closest approach is 54.0 km/s. What is its speed when it is farthest from the Sun? 46. Review. Two boys are sliding toward each other on a

Q/C frictionless, ice-covered parking lot. Jacob, mass 45.0 kg,

is gliding to the right at 8.00 m/s, and Ethan, mass 31.0 kg, is gliding to the left at 11.0 m/s along the same

Angular Momentum

48. A skateboarder with his board can be modeled as a

Q/C particle of mass 76.0 kg, located at his center of mass,

2

m

3

m

d

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d

om

m

Figure P11.49

50. Two children are playing on stools at a restaurant coun-

Q/C ter. Their feet do not reach the footrests, and the tops

of the stools are free to rotate without friction on pedestals fixed to the floor. One of the children catches a tossed ball, in a process described by the equation 1 0.730 kg # m2 2 1 2.40 j^ rad/s 2

1 1 0.120 kg 2 1 0.350 ^i m 2 3 1 4.30 k^ m/s 2

S 5 3 0.730 kg # m2 1 1 0.120 kg 2 1 0.350 m 2 2 4 v

S (a) Solve the equation for the unknown v . (b) Complete the statement of the problem to which this equation applies. Your statement must include the given numerical information and specification of the unknown to be determined. (c) Could the equation equally well describe the other child throwing the ball? Explain your answer.

w

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0.500 m above the ground. As shown in Figure P11.48, the skateboarder starts from rest in a crouching position at one lip of a half-pipe (point A). The half-pipe forms one half of a cylinder of radius 6.80 m with its axis horizontal. On his descent, the skateboarder moves without friction and maintains his crouch so that his center of mass moves through one quarter of a circle. (a) Find his speed at the bottom of the half-pipe (point B). (b) Find his angular momentum about the center of curvature at this point. (c) Immediately after passing point B, he stands up and raises his arms, lifting his center of gravity to 0.950 m above the concrete (point C). Explain why his angular momentum is constant in this maneuver, whereas the kinetic energy of his body is not constant. (d) Find his speed immediately after he stands up. (e) How much chemical energy in the skateboarder’s legs was converted into mechanical energy in the skateboarder–Earth system when he stood up?

1

2d 3

P

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47. We have all complained that there aren’t enough hours in a day. In an attempt to fix that, suppose all the people in the world line up at the equator and all start running east at 2.50 m/s relative to the surface of the Earth. By how much does the length of a day increase? Assume the world population to be 7.00 3 109 people with an average mass of 55.0 kg each and the Earth to be a solid homogeneous sphere. In addition, depending on the details of your solution, you may need to use the approximation 1/(1 2 x) < 1 1 x for small x.

Assuming m and d are known, find (a) the moment of inertia of the system of three particles about the pivot, (b) the torque acting on the system at t 5 0, (c) the angular acceleration of the system at t 5 0, (d) the linear acceleration of the particle labeled 3 at t 5 0, (e) the maximum kinetic energy of the system, (f) the maximum angular speed reached by the rod, (g) the maximum angular momentum of the system, and (h) the maximum speed reached by the particle labeled 2.

ee b

line. When they meet, they grab each other and hang on. (a) What is their velocity immediately thereafter? (b) What fraction of their original kinetic energy is still mechanical energy after their collision? That was so much fun that the boys repeat the collision with the same original velocities, this time moving along parallel lines 1.20 m apart. At closest approach, they lock arms and start rotating about their common center of mass. Model the boys as particles and their arms as a cord that does not stretch. (c) Find the velocity of their center of mass. (d) Find their angular speed. (e) What fraction of their original kinetic energy is still mechanical energy after they link arms? (f) Why are the answers to parts (b) and (e) so different?

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360 Chapter 11

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A

51. A projectile of mass m moves to the right with a speed vi GP (Fig. P11.51a). The projectile strikes and sticks to the end S of a stationary rod of mass M, length d, pivoted about a frictionless axle perpendicular to the page through O (Fig. P11.51b). We wish to find the fractional change of kinetic energy in the system due to the collision. (a) What is the appropriate analysis model to describe the projectile and the rod? (b) What is the angular momentum of the system before the collision about an axis through O? (c) What is the moment of inertia of the system about an axis through O after the projectile sticks to the rod? (d) If the angular speed of the system after the collision is v, what is the angular momentum of the system after the collision? (e) Find the angular speed v after the collision in terms of the given quantiS

B C

vi

m

Figure P11.48 49. A rigid, massless rod has three particles with equal S masses attached to it as shown in Figure P11.49. The rod is free to rotate in a vertical plane about a frictionless axle perpendicular to the rod through the point P and is released from rest in the horizontal position at t 5 0.

v O

O

d

M a

b

Figure P11.51

361

Problems ties. (f) What is the kinetic energy of the system before the collision? (g) What is the kinetic energy of the system after the collision? (h) Determine the fractional change of kinetic energy due to the collision.

of mass at speeds of 5.00 m/s. Treating the astronauts as particles, calculate (a) the magnitude of the angular momentum of the two-astronaut system and (b) the rotational energy of the system. By pulling on the rope, one astronaut shortens the distance between them to 5.00 m. (c) What is the new angular momentum of the system? (d) What are the astronauts’ new speeds? (e) What is the new rotational energy of the system? (f) How much chemical potential energy in the body of the astronaut was converted to mechanical energy in the system when he shortened the rope?

52. A puck of mass m 5 50.0 g is attached to a taut cord pass-

AMT ing through a small hole in a frictionless, horizontal M surface (Fig. P11.52). The puck is initially orbiting with

vi

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m

S

ly

ri

56. Two astronauts (Fig. P11.55), each having a mass M, S are connected by a rope of length d having negligible mass. They are isolated in space, orbiting their center of mass at speeds v. Treating the astronauts as particles, calculate (a) the magnitude of the angular momentum of the two-astronaut system and (b) the rotational energy of the system. By pulling on the rope, one of the astronauts shortens the distance between them to d/2. (c) What is the new angular momentum of the system? (d) What are the astronauts’ new speeds? (e) What is the new rotational energy of the system? (f) How much chemical potential energy in the body of the astronaut was converted to mechanical energy in the system when he shortened the rope?

.c

speed vi 5 1.50 m/s in a circle of radius ri 5 0.300 m. The cord is then slowly pulled from below, decreasing the radius of the circle to r 5 0.100 m. (a) What is the puck’s speed at the smaller radius? (b) Find the tension in the cord at the smaller radius. (c) How much work is done by the hand in pulling the cord so that the radius of the puck’s motion changes from 0.300 m to 0.100 m?

Figure P11.52 Problems 52 and 53.

57. Native people throughout North and South America

Q/C used a bola to hunt for birds and animals. A bola can S consist of three stones, each with mass m, at the ends

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ys

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53. A puck of mass m is attached to a taut cord passing S through a small hole in a frictionless, horizontal surface (Fig. P11.52). The puck is initially orbiting with speed vi in a circle of radius ri . The cord is then slowly pulled from below, decreasing the radius of the circle to r. (a) What is the puck’s speed when the radius is r? (b) Find the tension in the cord as a function of r. (c) How much work is done by the hand in pulling the cord so that the radius of the puck’s motion changes from ri to r?

w

w

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sw

54. Why is the following situation impossible? A meteoroid strikes the Earth directly on the equator. At the time it lands, it is traveling exactly vertical and downward. Due to the impact, the time for the Earth to rotate once increases by 0.5 s, so the day is 0.5 s longer, undetectable to laypersons. After the impact, people on the Earth ignore the extra half-second each day and life goes on as normal. (Assume the density of the Earth is uniform.)

w

55. Two astronauts (Fig. P11.55), each having a mass of M 75.0 kg, are connected by a 10.0-m rope of negligible mass. They are isolated in space, orbiting their center

of three light cords, each with length ,. The other ends of the cords are tied together to form a Y. The hunter holds one stone and swings the other two above his head (Figure P11.57a). Both these stones move together in a horizontal circle of radius 2, with speed v 0. At a moment when the horizontal component of their velocity is directed toward the quarry, the hunter releases the stone in his hand. As the bola flies through the air, the cords quickly take a stable arrangement with constant 120-degree angles between them (Fig. P11.57b). In the vertical direction, the bola is in free fall. Gravitational forces exerted by the Earth make the junction of the cords move with the downward g . You may ignore the vertical motion as acceleration S you proceed to describe the horizontal motion of the bola. In terms of m, ,, and v 0, calculate (a) the magnitude of the momentum of the bola at the moment of release and, after release, (b) the horizontal speed of the center of mass of the bola and (c) the angular momentum of the bola about its center of mass. (d) Find the angular speed of the bola about its center of mass after it has settled into its Y shape. Calculate m

CM d

,

, , m

a

Figure P11.55 Problems 55 and 56.

b

Figure P11.57

m

362 Chapter 11

Angular Momentum

the kinetic energy of the bola (e) at the instant of release and (f) in its stable Y shape. (g) Explain how the conservation laws apply to the bola as its configuration changes. Robert Beichner suggested the idea for this problem.

rolling occurs. (c) Assume the coefficient of friction between disk and surface is m. What is the time interval after setting the disk down before pure rolling motion begins? (d) How far does the disk travel before pure rolling begins?

58. A uniform rod of mass 300 g and length 50.0 cm

Q/C rotates in a horizontal plane about a fixed, frictionless,

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Figure P11.61

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62. In Example 11.9, we investigated an elastic collision between a disk and a stick lying on a frictionless surface. Suppose everything is the same as in the example except that the collision is perfectly inelastic so that the disk adheres to the stick at the endpoint at which it strikes. Find (a) the speed of the center of mass of the system and (b) the angular speed of the system after the collision. 63. A solid cube of side 2a and mass M is sliding on a fricS tionless surface with uniform velocity S v as shown in Figure P11.63a. It hits a small obstacle at the end of the table that causes the cube to tilt as shown in Figure P11.63b. Find the minimum value of the magniv such that the cube tips over and falls off the tude of S table. Note: The cube undergoes an inelastic collision at the edge.

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59. Global warming is a cause for concern because even small changes in the Earth’s temperature can have significant consequences. For example, if the Earth’s polar ice caps were to melt entirely, the resulting additional water in the oceans would flood many coastal areas. Model the polar ice as having mass 2.30 3 1019 kg and forming two flat disks of radius 6.00 3 105 m. Assume the water spreads into an unbroken thin, spherical shell after it melts. Calculate the resulting change in the duration of one day both in seconds and as a percentage.

M

s. w

vertical pin through its center. Two small, dense beads, each of mass m, are mounted on the rod so that they can slide without friction along its length. Initially, the beads are held by catches at positions 10.0 cm on each side of the center and the system is rotating at an angular speed of 36.0 rad/s. The catches are released simultaneously, and the beads slide outward along the rod. (a) Find an expression for the angular speed vf of the system at the instant the beads slide off the ends of the rod as it depends on m. (b) What are the maximum and the minimum possible values for vf and the values of m to which they correspond?

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60. The puck in Figure P11.60 has a mass of 0.120 kg. The distance of the puck from the center of rotation is originally 40.0 cm, and the puck is sliding with a speed of 80.0 cm/s. The string is pulled downward 15.0 cm through the hole in the frictionless table. Determine the work done on the puck. (Suggestion: Consider the change of kinetic energy.)

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R

M

S

a

b

Figure P11.63

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m

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F

Figure P11.60

6 4. A solid cube of wood of side 2a and mass M is resting S on a horizontal surface. The cube is constrained to rotate about a fixed axis AB (Fig. P11.64). A bullet of mass m and speed v is shot at the face opposite ABCD at a height of 4a/3. The bullet becomes embedded in the cube. Find the minimum value of v required to tip the cube so that it falls on face ABCD. Assume m ,, M.

Challenge Problems 61. A uniform solid disk of radius R is set into rotation S with an angular speed vi about an axis through its center. While still rotating at this speed, the disk is placed into contact with a horizontal surface and immediately released as shown in Figure P11.61. (a) What is the angular speed of the disk once pure rolling takes place? (b) Find the fractional change in kinetic energy from the moment the disk is set down until pure

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Figure P11.64

c h a p t e r

12

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Static Equilibrium and Elasticity

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12.1 Analysis Model: Rigid Object in Equilibrium

12.3 Examples of Rigid Objects in Static Equilibrium 12.4 Elastic Properties of Solids

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12.2 More on the Center of Gravity

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In Chapters 10 and 11, we studied the dynamics of rigid objects. Part of this chapter addresses the conditions under which a rigid object is in equilibrium. The term equilibrium implies that the object moves with both constant velocity and constant angular velocity relative to an observer in an inertial reference frame. We deal here only with the special case in which both of these velocities are equal to zero. In this case, the object is in what is called static equilibrium. Static equilibrium represents a common situation in engineering practice, and the principles it involves are of special interest to civil engineers, architects, and mechanical engineers. If you are an engineering student, you will undoubtedly take an advanced course in statics in the near future. The last section of this chapter deals with how objects deform under load conditions. An elastic object returns to its original shape when the deforming forces are removed. Several elastic constants are defined, each corresponding to a different type of deformation.

Balanced Rock in Arches National Park, Utah, is a 3 000 000-kg boulder that has been in stable equilibrium for several millennia. It had a smaller companion nearby, called “Chip Off the Old Block,” that fell during the winter of 1975. Balanced Rock appeared in an early scene of the movie Indiana Jones and the Last Crusade. We will study the conditions under which an object is in equilibrium in this chapter. (John W. Jewett, Jr.)

12.1 Analysis Model: Rigid Object in Equilibrium In Chapter 5, we discussed the particle in equilibrium model, in which a particle moves with constant velocity because the net force acting on it is zero. The situation with real (extended) objects is more complex because these objects often cannot be modeled as particles. For an extended object to be in equilibrium, a second condition must be satisfied. This second condition involves the rotational motion of the extended object.

363

364 Chapter 12

Static Equilibrium and Elasticity S

S

F

u

Consider a single force F acting on a rigid object as shown in Figure 12.1. Recall S that the torque associated with the force F about an axis through O is given by Equation 11.1:

P

S

d

r

S

S

t5S r 3 F

t is Fd (see Equation 10.14), where d is the moment arm shown The magnitude of S in Figure 12.1. According to Equation 10.18, the net torque on a rigid object causes it to undergo an angular acceleration. In this discussion, we investigate those rotational situations in which the angular acceleration of a rigid object is zero. Such an object is in rotational equilibrium. Because o text 5 Ia for rotation about a fixed axis, the necessary condition for rotational equilibrium is that the net torque about any axis must be zero. We now have two necessary conditions for equilibrium of a rigid object:

O S

Figure 12.1 A single force F acts

om

on a rigid object at the point P.

a F ext 5 0 S

not mean an absence of rotational motion. An object that is rotating at a constant angular speed can be under the influence of a net torque of zero. This possibility is analogous to the translational situation: zero net force does not mean an absence of translational motion.

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Zero Torque Zero net torque does

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1. The net external force on the object must equal zero:

Pitfall Prevention 12.1

(12.1)

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2. The net external torque on the object about any axis must be zero: S a text 5 0

(12.2)

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These conditions describe the rigid object in equilibrium analysis model. The first condition is a statement of translational equilibrium; it states that the translational acceleration of the object’s center of mass must be zero when viewed from an inertial reference frame. The second condition is a statement of rotational equilibrium; it states that the angular acceleration about any axis must be zero. In the special case of static equilibrium, which is the main subject of this chapter, the object in equilibrium is at rest relative to the observer and so has no translational or angular speed (that is, v CM 5 0 and v 5 0).

S

F

d CM

S

F

Q uick Quiz 12.2 Consider the object subject to the three forces in Figure 12.3. Choose the correct statement with regard to this situation. (a) The object is in force equilibrium but not torque equilibrium. (b) The object is in torque equilibrium but not force equilibrium. (c) The object is in both force equilibrium and torque equilibrium. (d) The object is in neither force equilibrium nor torque equilibrium.

.a

Figure 12.2 (Quick Quiz 12.1)

sw

d

Q uick Quiz 12.1 Consider the object subject to the two forces of equal magnitude in Figure 12.2. Choose the correct statement with regard to this situation. (a) The object is in force equilibrium but not torque equilibrium. (b) The object is in torque equilibrium but not force equilibrium. (c) The object is in both force equilibrium and torque equilibrium. (d) The object is in neither force equilibrium nor torque equilibrium.

w

Two forces of equal magnitude are applied at equal distances from the center of mass of a rigid object.

w

S

S

F1

w

F2

S

F3

Figure 12.3 (Quick Quiz 12.2) Three forces act on an object. Notice that the lines of action of all three forces pass through a common point.

The two vector expressions given by Equations 12.1 and 12.2 are equivalent, in general, to six scalar equations: three from the first condition for equilibrium and three from the second (corresponding to x, y, and z components). Hence, in a complex system involving several forces acting in various directions, you could be faced with solving a set of equations with many unknowns. Here, we restrict our discussion to situations in which all the forces lie in the xy plane. (Forces whose vector representations are in the same plane are said to be coplanar.) With this restriction, we must deal with only three scalar equations. Two come from balancing the forces in the x and y directions. The third comes from the torque equation, namely that the net torque about a perpendicular axis through any point in the xy plane must be zero. This perpendicular axis will necessarily be parallel to

12.2 More on the Center of Gravity 365

the z axis, so the two conditions of the rigid object in equilibrium model provide the equations

o Fx 5 0 o Fy 5 0 o tz 5 0

(12.3)

where the location of the axis of the torque equation is arbitrary.

Analysis Model Rigid Object in Equilibrium

S

(12.1)

S a t ext 5 0

(12.2)

a0 tz 0

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12.2 More on the Center of Gravity

Each particle of the object has a specific mass and specific coordinates. y (x1, y1)

CM

m2 (x3, y3) m3

x

O

Figure 12.4 A n object can be divided into many small particles. These particles can be used to locate the center of mass. y

(x1, y1)

i

We use a similar equation to define the y coordinate of the center of mass, replacing each x with its y counterpart. Let us now examine the situation from another point of view by considering the gravitational force exerted on each particle as shown in Figure 12.5. Each particle contributes a torque about an axis through the origin equal in magnitude to the particle’s weight mg multiplied by its moment arm. For example, the magnitude of the torque due to the force m 1S g 1 is m1g 1x 1, where g 1 is the value of the gravitational acceleration at the position of the particle of mass m1. We wish to locate the center of gravity, the point at which application of the single gravitational force M S g CG (where M 5 m1 1 m 2 1 m 3 1 ??? is the total mass of the object and S g CG is the acceleration due to gravity at the location of the center of gravity) has the same effect on

(x2, y2)

m1

w

w

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sw

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Whenever we deal with a rigid object, one of the forces we must consider is the gravitational force acting on it, and we must know the point of application of this force. As we learned in Section 9.5, associated with every object is a special point called its center of gravity. The combination of the various gravitational forces acting on all the various mass elements of the object is equivalent to a single gravitational force acting through this point. Therefore, to compute the torque due to the gravitational force on an object of mass M, we need only consider the force M S g acting at the object’s center of gravity. S How do we find this special point? As mentioned in Section 9.5, if we assume g is uniform over the object, the center of gravity of the object coincides with its center of mass. To see why, consider an object of arbitrary shape lying in the xy plane as illustrated in Figure 12.4. Suppose the object is divided into a large number of particles of masses m1, m 2, m 3, . . . having coordinates (x 1, y1), (x 2, y 2), (x 3, y 3), . . . . In Equation 9.29, we defined the x coordinate of the center of mass of such an object to be m ix i m 1x 1 1 m 2x 2 1 m 3x 3 1 c a i x CM 5 5 m1 1 m2 1 m3 1 c m i a

w

a0 Fx 0 Fy 0

• a balcony juts out from a building and must support the weight of several humans without collapsing • a gymnast performs the difficult iron cross maneuver in an Olympic event • a ship moves at constant speed through x calm water and maintains a perfectly O level orientation (Chapter 14) • polarized molecules in a dielectric material in a constant electric field take on an average equilibrium orientation that remains fixed in time (Chapter 26)

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y

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a F ext 5 0

Examples:

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Imagine an object that can rotate, but is exhibiting no translational acceleration a and no rotational acceleration a. Such an object is in both translational and rotational equilibrium, so the net force and the net torque about any axis are both equal to zero:

(x2, y2) S

m2 g 2

S

m1 g 1 CG

(x3, y3) S

m3 g 3 O S

x

S

Fg M g CG

Figure 12.5 By dividing an object into many particles, we can find its center of gravity.

366 Chapter 12

Static Equilibrium and Elasticity

rotation as does the combined effect of all the individual gravitational forces m iS gi . g CG acting at the center of gravity to the sum Equating the torque resulting from M S of the torques acting on the individual particles gives 1 m 1 1 m 2 1 m 3 1 c2 g CG x CG 5 m 1g1x 1 1 m 2g2x 2 1 m 3g3x 3 1 c

This expression accounts for the possibility that the value of g can in general vary over the object. If we assume uniform g over the object (as is usually the case), the g factors cancel and we obtain m 1x 1 1 m 2x 2 1 m 3x 3 1 c x CG 5 (12.4) m1 1 m2 1 m3 1 c

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Comparing this result with Equation 9.29 shows that the center of gravity is located S is uniform over the entire object. Several examat the center of mass as long as g ples in the next section deal with homogeneous, symmetric objects. The center of gravity for any such object coincides with its geometric center.

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ee b

ly

Q uick Quiz 12.3 A meterstick of uniform density is hung from a string tied at the 25-cm mark. A 0.50-kg object is hung from the zero end of the meterstick, and the meterstick is balanced horizontally. What is the mass of the meterstick? (a) 0.25 kg (b) 0.50 kg (c) 0.75 kg (d) 1.0 kg (e) 2.0 kg (f) impossible to determine

12.3 Examples of Rigid Objects in Static Equilibrium

ic

ys

ar ph

Problem-Solving Strategy Rigid Object in Equilibrium

When analyzing a rigid object in equilibrium under the action of several external forces, use the following procedure.

w

w

w

.a

Figure 12.6 This one-bottle wine holder is a surprising display of static equilibrium.

The photograph of the one-bottle wine holder in Figure 12.6 shows one example of a balanced mechanical system that seems to defy gravity. For the system (wine holder plus bottle) to be in equilibrium, the net external force must be zero (see Eq. 12.1) and the net external torque must be zero (see Eq. 12.2). The second condition can be satisfied only when the center of gravity of the system is directly over the support point.

sw

© Cengage Learning/Charles D. Winters

The center of gravity of the system (bottle plus holder) is directly over the support point.

1. Conceptualize. Think about the object that is in equilibrium and identify all the forces on it. Imagine what effect each force would have on the rotation of the object if it were the only force acting.

2. Categorize. Confirm that the object under consideration is indeed a rigid object in equilibrium. The object must have zero translational acceleration and zero angular acceleration. 3. Analyze. Draw a diagram and label all external forces acting on the object. Try to guess the correct direction for any forces that are not specified. When using the particle under a net force model, the object on which forces act can be represented in a free-body diagram with a dot because it does not matter where on the object the forces are applied. When using the rigid object in equilibrium model, however, we cannot use a dot to represent the object because the location where forces act is important in the calculation. Therefore, in a diagram showing the forces on an object, we must show the actual object or a simplified version of it. Resolve all forces into rectangular components, choosing a convenient coordinate system. Then apply the first condition for equilibrium, Equation 12.1. Remember to keep track of the signs of the various force components.

367

12.3 Examples of Rigid Objects in Static Equilibrium

Choose a convenient axis for calculating the net torque on the rigid object. Remember that the choice of the axis for the torque equation is arbitrary; therefore, choose an axis that simplifies your calculation as much as possible. Usually, the most convenient axis for calculating torques is one through a point through which the lines of action of several forces pass, so their torques around this axis are zero. If you don’t know a force or don’t need to know a force, it is often beneficial to choose an axis through the point at which this force acts. Apply the second condition for equilibrium, Equation 12.2. Solve the simultaneous equations for the unknowns in terms of the known quantities.

om

▸ Problem-Solving Strategy c o n t i n u e d

4. Finalize. Make sure your results are consistent with your diagram. If you selected a

ee b

ly

.c

direction that leads to a negative sign in your solution for a force, do not be alarmed; it merely means that the direction of the force is the opposite of what you guessed. Add up the vertical and horizontal forces on the object and confirm that each set of components adds to zero. Add up the torques on the object and confirm that the sum equals zero.

s. w

Example 12.1 The Seesaw Revisited AM

n

2

d

ys

ic

A seesaw consisting of a uniform board of mass M and length , supports at rest a father and daughter with masses mf and md , respectively, as shown in Figure 12.7. The support (called the fulcrum) is under the center of gravity of the board, the father is a distance d from the center, and the daughter is a distance ,/2 from the center.

S

ar ph

(A) D etermine the magnitude of the upward force S n exerted by the support on the board. Solution

S

Mg S

mf g

S

md g

Figure 12.7 (Example 12.1) A balanced system.

sw

Conceptualize Let us focus our attention on the board and consider the gravitational forces on the father and daughter as forces applied directly to the board. The daughter would cause a clockwise rotation of the board around the support, whereas the father would cause a counterclockwise rotation.

w

.a

Categorize Because the text of the problem states that the system is at rest, we model the board as a rigid object in equilibrium. Because we will only need the first condition of equilibrium to solve this part of the problem, however, we could also simply model the board as a particle in equilibrium.

w

Analyze Define upward as the positive y direction and substitute the forces on the board into Equation 12.1:

w

n: Solve for the magnitude of the force S

n 2 mf g 2 md g 2 Mg 5 0 (1) n 5 mf g 1 md g 1 Mg 5 (mf 1 md 1 M)g

(B) Determine where the father should sit to balance the system at rest. Solution

Categorize This part of the problem requires the introduction of torque to find the position of the father, so we model the board as a rigid object in equilibrium. Analyze The board’s center of gravity is at its geometric center because we are told that the board is uniform. If we choose a rotation axis perpendicular to the page through the center of gravity of the board, the torques produced by n and the gravitational force on the board about this axis are zero.

S

continued

368 Chapter 12

Static Equilibrium and Elasticity

▸ 12.1 c o n t i n u e d Substitute expressions for the torques on the board due to the father and daughter into Equation 12.2:

1 mf g2 1 d2 2 1 md g2

Solve for d:

d5 a

, 50 2

md , b mf 2

Finalize This result is the same one we obtained in Example 11.6 by evaluating the angular acceleration of the system and setting the angular acceleration equal to zero.

om

W h at I f ? Suppose we had chosen another point through which the rotation axis were to pass. For example, suppose the axis is perpendicular to the page and passes through the location of the father. Does that change the results to parts (A) and (B)?

ee b

ly

.c

Answer Part (A) is unaffected because the calculation of the net force does not involve a rotation axis. In part (B), we would conceptually expect there to be no change if a different rotation axis is chosen because the second condition of equilibrium claims that the torque is zero about any rotation axis. Let’s verify this answer mathematically. Recall that the sign of the torque associated with a force is positive if that force tends to rotate the system counterclockwise, whereas the sign of the torque is negative if the force tends to rotate the system clockwise. Let’s choose a rotation axis perpendicular to the page and passing through the location of the father. , n 1 d 2 2 1 Mg 2 1 d 2 2 1 m d g 2 ad 1 b 5 0 2

s. w

Substitute expressions for the torques on the board around this axis into Equation 12.2:

, 1 m f 1 m d 1 M 2 g 1 d 2 2 1 Mg 2 1 d 2 2 1 m d g 2 ad 1 b 5 0 2 , 1 mf g2 1 d2 2 1 md g2 a b 5 0 2

S

ys

ic

Substitute from Equation (1) in part (A) and solve for d:

This result is in agreement with the one obtained in part (B).

ar ph

d5a

md , b mf 2

Example 12.2 Standing on a Horizontal Beam AM A uniform horizontal beam with a length of , 5 8.00 m and a weight of W b 5 200 N is attached to a wall by a pin connection. Its far end is supported by a cable that makes an angle of f 5 53.08 with the beam (Fig. 12.8a). A person of weight Wp 5 600 N stands a distance d 5 2.00 m from the wall. Find the tension in the cable as well as the magnitude and direction of the force exerted by the wall on the beam.

sw

S

R

.a

Wb

w

Conceptualize Imagine the person in Figure 12.8a moving outward on the beam. It seems reasonable that the farther he moves outward, the larger the torque he applies about the pivot and the larger the tension in the cable must be to balance this torque. Categorize Because the system is at rest, we categorize the beam as a rigid object in equilibrium. Analyze We identify all the external forces acting on the beam: the 200-N gravitational force, the

f

u

f

w

w

S o l u ti o n

S

T

Wp

a

b R sin u T sin f R cos u

Figure 12.8 (Example 12.2) (a) A uniform beam supported by a cable. A person walks outward on the beam. (b) The force diagram for the beam. (c) The force diagram for the beam showing the S S components of R and T.

Wb

d Wp 2 c

T cos f

12.3 Examples of Rigid Objects in Static Equilibrium

369

▸ 12.2 c o n t i n u e d S

S

force T exerted by the cable, the force R exerted by the wall at the pivot, and the 600-N force that the person exerts on the beam. These forces are all indicated in the force diagram for the beam shown in Figure 12.8b. When we assign directions for forces, it is sometimes helpful to imagine what would happen if a force were suddenly removed. For example, if the wall were to vanish suddenly, the left end of the beam would move to the left as it begins to fall. This scenario tells us that the wall is not only holding the beam up but is also pressing outward against it. Therefore, S we draw the vector R in the direction shown in Figure 12.8b. Figure 12.8c shows the horizontal and vertical compoS S nents of T and R . (1) (2)

o F o F

x

5 R cos u 2 T cos f 5 0

y

5 R sin u 1 T sin f 2 Wp 2 W b 5 0

om

Applying the first condition of equilibrium, substitute expressions for the forces on the beam into component equations from Equation 12.1:

ee b

ly

.c

where we have chosen rightward and upward as our positive directions. Because R, T, and u are all unknown, we cannot obtain a solution from these expressions alone. (To solve for the unknowns, the number of simultaneous equations must generally equal the number of unknowns.) Now let’s invoke the condition for rotational equilibrium. A convenient axis to choose for our torque equation is S the one that passes through the pin connection. The feature that makes this axis so convenient is that the force R S and the horizontal component of T both have a moment arm of zero; hence, these forces produce no torque about this axis.

Wp d 1 Wb 1 ,/2 2 , sin f

sw .a

w

Solve Equation (1) for R and substitute numerical values:

1 600 N 2 1 2.00 m 2 1 1 200 N 2 1 4.00 m 2 5 313 N 1 8.00 m 2 sin 53.08

Wp 1 Wb 2 T sin f R sin u 5 tan u 5 R cos u T cos f

ar ph

Rearrange Equations (1) and (2) and then divide: Solve for u and substitute numerical values:

5

ic

T5

ys

This equation contains only T as an unknown because of our choice of rotation axis. Solve for T and substitute numerical values:

s. w

, a tz 5 1 T sin f 2 1 , 2 2 Wp d 2 W b a 2 b 5 0

Substitute expressions for the torques on the beam into Equation 12.2:

u 5 tan21 a 5 tan21 c R5

Wp 1 Wb 2 T sin f T cos f

b

600 N 1 200 N 2 1 313 N 2 sin 53.08 d 5 71.18 1 313 N 2 cos 53.08

1 313 N 2 cos 53.08 T cos f 5 5 581 N cos u cos 71.18

S

w

Finalize The positive value for the angle u indicates that our estimate of the direction of R was accurate.

w

Had we selected some other axis for the torque equation, the solution might differ in the details but the answers would be the same. For example, had we chosen an axis through the center of gravity of the beam, the torque equation would involve both T and R. This equation, coupled with Equations (1) and (2), however, could still be solved for the unknowns. Try it! W h at I f ?

W hat if the person walks farther out on the beam? Does T change? Does R change? Does u change?

Answer T must increase because the gravitational force on the person exerts a larger torque about the pin connection, which must be countered by a larger torque in the opposite direction due to an increased value of T. If T increases, the S vertical component of R decreases to maintain force equilibrium in the vertical direction. Force equilibrium in the S horizontal direction, however, requires an increased horizontal component of R to balance the horizontal component S of the increased T. This fact suggests that u becomes smaller, but it is hard to predict what happens to R. Problem 66 asks you to explore the behavior of R.

370 Chapter 12

Static Equilibrium and Elasticity u

Example 12.3 The Leaning Ladder AM

a S

A uniform ladder of length , rests against a smooth, vertical wall (Fig. 12.9a). The mass of the ladder is m, and the coefficient of static friction between the ladder and the ground is ms 5 0.40. Find the minimum angle umin at which the ladder does not slip.

P

S

n

S o l u ti o n

Conceptualize Think about any ladders you have climbed. Do you want

u

u

S

S

mg

fs

om

a large friction force between the bottom of the ladder and the surface or a small one? If the friction force is zero, will the ladder stay up? Simulate a ladder with a ruler leaning against a vertical surface. Does the ruler slip at some angles and stay up at others?

O b

a

Categorize We do not wish the ladder to slip, so we model it as a rigid object in equilibrium.

ly

.c

Figure 12.9 (S PExample 12.3) (a) A uniform ladder at rest, leaning against a smooth wall. The ground is rough. (b) The forces on the ladder. S

n

ee b

Analyze A diagram showing all the external forces acting on the ladder is illustrated in Figure 12.9b. The force exerted S

s. w

by the ground on the ladder is the vector sum of a normal force S n and the force of static friction fs . The wall exerts a S normal force P on the top of the ladder, but there is no friction force here becauseuthe wall S is smooth. So the net force mg O S on the top of the ladder is perpendicular to the wall and of magnitude P. f (1)

Apply the first condition for equilibrium to the ladder in both the x and the y directions:

x

s

y

(3) P 5 fs

ys

Solve Equation (1) for P :

(4) n 5 mg

Solve Equation (2) for n:

ar ph

When the ladder is on the verge of slipping, the force of static friction must have its maximum value, which is given by fs,max 5 msn. Combine this equation with Equations (3) and (4):

.a

sw

Apply the second condition for equilibrium to the ladder, evaluating torques about an axis perpendicular to the page through O:

(5) P max 5 fs,max 5 msn 5 msmg

, a tO 5 P , sin u 2 mg 2 cos u 5 0

w

mg mg sin u 5 tan u 5 S u 5 tan 21 a b cos u 2P 2P

w

Solve for tan u:

o F 5 f 2b P 5 0 o F 5 n 2 mg 5 0

ic

(2)

s

w

Under the conditions that the ladder is just ready to slip, u becomes umin and Pmax is given by Equation (5). Substitute:

u min 5 tan 21 a

mg

2Pmax

b 5 tan21 a

1 1 d 5 518 b 5 tan21 c 2ms 2 1 0.40 2

Finalize Notice that the angle depends only on the coefficient of friction, not on the mass or length of the ladder.

Example 12.4 Negotiating a Curb AM S

(A) Estimate the magnitude of the force F a person must apply to a wheelchair’s main wheel to roll up over a sidewalk curb (Fig. 12.10a). This main wheel that comes in contact with the curb has a radius r, and the height of the curb is h.

12.3 Examples of Rigid Objects in Static Equilibrium

371

▸ 12.4 c o n t i n u e d S o l u ti o n S

F

Conceptualize Think about wheelchair access to buildings. Generally, there are ramps built for individuals in wheelchairs. Steplike structures such as curbs are serious barriers to a wheelchair.

O

Categorize Imagine the person exerts enough force so that the bottom of the main wheel just loses contact with the lower surface and hovers at rest. We model the wheel in this situation as a rigid object in equilibrium.

rh

om

a

ly ee b O

2r h

S

R

D

S

S

mg

R

u

u S

A

F

S

mg

c

d

Figure 12.10 (Example 12.4) (a) A person in a wheelchair attempts to roll up over a curb. (b) Details of the wheel and curb. S The person applies a force F to the top of the wheel. (c) A force diagram for the wheel when it is just about to be raised. Three S forces act on the wheel at this instant: F, which is exerted by the S hand; R , which is exerted by the curb; and the gravitational force mS g . (d) The vector sum of the three external forces acting on the wheel is zero. (1) d 5 "r 2 2 1 r 2 h 2 2 5 "2rh 2 h 2 (2) a tA 5 mgd 2 F 1 2r 2 h 2 5 0

Substitute for d from Equation (1):

mg"2rh 2 h2 2 F 1 2r 2 h 2 5 0

Solve for F :

(3) F 5

Simplify:

F 5 mg

Substitute the known values:

F 5 1 700 N 2

w

d

.c

S

F

s. w

w

Apply the second condition for equilibrium to the wheel, taking torques about an axis through A:

B

b

ic

ys

ar ph

sw

w

.a

Use the triangle OAC in Figure 12.10b to find the moment arm d of the gravitational force m S g acting on the wheel relative to an axis through point A:

A h

Analyze Usually, the person’s hands supply the required force to a slightly smaller wheel that is concentric with the main wheel. For simplicity, let’s assume the radius of this second wheel is the same as the radius of the main wheel. Let’s estimate a combined gravitational force of magnitude mg 5 1 400 N for the person and the wheelchair, acting along a line of action passing through the axle of the main wheel, and choose a wheel radius of r 5 30 cm. We also pick a curb height of h 5 10 cm. Let’s also assume the wheelchair and occupant are symmetric and each wheel supports a weight of 700 N. We then proceed to analyze only one of the main wheels. Figure 12.10b shows the geometry for a single wheel. When the wheel is just about to be raised from the street, the normal force exerted by the ground on the wheel at point B goes to zero. Hence, at this time only three forces act on the wheel as shown in the force diagram in S Figure 12.10c. The force R , which is the force exerted by the curb on the wheel, acts at point A, so if we choose to have our axis of rotation be perpendicular to the page and pass through point A, we do not need to include S S R in our torque equation. The moment arm of F relative to an axis through A is given by 2r 2 h (see Fig. 12.10c).

r

C

mg"2rh 2 h2 2r 2 h

"h "2r 2 h h 5 mg 2r 2 h Å 2r 2 h 0.1 m 1 Å 2 0.3 m 2 2 0.1 m

5 3 3 102 N

continued

372 Chapter 12

Static Equilibrium and Elasticity

▸ 12.4 c o n t i n u e d S

(B) D etermine the magnitude and direction of R .

o F o F

(4)

Apply the first condition for equilibrium to the x and y components of the forces on the wheel:

(5)

x

5 F 2 R cos u 5 0

y

5 R sin u 2 mg 5 0

mg R sin u 5 tan u 5 R cos u F u 5 tan21 a

Solve Equation (5) for R and substitute numerical values:

R5

mg

5

F

b 5 tan21 a

700 N b 5 708 300 N

700 N 5 8 3 102 N sin 708

ee b

sin u

mg

.c

Solve for the angle u:

ly

Divide Equation (5) by Equation (4):

om

S o l u ti o n

Finalize Notice that we have kept only one digit as significant. (We have written the angle as 708 because 7 3 1018 is awkward!) The results indicate that the force that must be applied to each wheel is substantial. You may want to estimate the force required to roll a wheelchair up a typical sidewalk accessibility ramp for comparison.

s. w

W h at I f ? Would it be easier to negotiate the curb if the person grabbed the wheel at point D in Figure 12.10c and pulled upward? S

Answer If the force F in Figure 12.10c is rotated counterclockwise by 908 and applied at D, its moment arm about an

sw

Solve this equation for F 9 and substitute for d:

w

.a

Take the ratio of this force to the original force from Equation (3) and express the result in terms of h/r, the ratio of the curb height to the wheel radius:

w

w

Substitute the ratio h/r 5 0.33 from the given values:

5 mgd 2 F 9(d 1 r) 5 0

ys

o t

A

ar ph

Modify Equation (2) for this situation:

ic

axis through A is d 1 r. Let’s call the magnitude of this new force F9.

Fr 5

mgd d1r

5

mg"2rh 2 h2 "2rh 2 h2 1 r

mg"2rh 2 h2

Fr 2r 2 h "2rh 2 h 1 r 5 5 5 2 F mg"2rh 2 h "2rh 2 h2 1 r 2

2r 2 h

Fr 2 2 0.33 5 0.96 5 F "2 1 0.33 2 2 1 0.33 2 2 1 1

h 22a b r

h h 2 2a b 2 a b 1 1 r Å r

This result tells us that, for these values, it is slightly easier to pull upward at D than horizontally at the top of the wheel. For very high curbs, so that h/r is close to 1, the ratio F 9/F drops to about 0.5 because point A is located near the right edge of the wheel in Figure 12.10b. The force at D is applied at a distance of about 2r from A, whereas the force at the top of the wheel has a moment arm of only about r. For high curbs, then, it is best to pull upward at D, although a large value of the force is required. For small curbs, it is best to apply the force at the top of the wheel. The ratio F 9/F becomes larger than 1 at about h/r 5 0.3 because point A is now close to the bottom of the wheel and the force applied at the top of the wheel has a larger moment arm than when applied at D. Finally, let’s comment on the validity of these mathematical results. Consider Figure 12.10d and imagine that the S vector F is upward instead of to the right. There is no way the three vectors can add to equal zero as required by the first equilibrium condition. Therefore, our results above may be qualitatively valid, but not exact quantitatively. To S cancel the horizontal component of R , the force at D must be applied at an angle to the vertical rather than straight upward. This feature makes the calculation more complicated and requires both conditions of equilibrium.

12.4 Elastic Properties of Solids 373

12.4 Elastic Properties of Solids

ee b

stress strain

Elastic modulus ;

ly

.c

om

Except for our discussion about springs in earlier chapters, we have assumed objects remain rigid when external forces act on them. In Section 9.8, we explored deformable systems. In reality, all objects are deformable to some extent. That is, it is possible to change the shape or the size (or both) of an object by applying external forces. As these changes take place, however, internal forces in the object resist the deformation. We shall discuss the deformation of solids in terms of the concepts of stress and strain. Stress is a quantity that is proportional to the force causing a deformation; more specifically, stress is the external force acting on an object per unit crosssectional area. The result of a stress is strain, which is a measure of the degree of deformation. It is found that, for sufficiently small stresses, stress is proportional to strain; the constant of proportionality depends on the material being deformed and on the nature of the deformation. We call this proportionality constant the elastic modulus. The elastic modulus is therefore defined as the ratio of the stress to the resulting strain: (12.5)

ic

s. w

The elastic modulus in general relates what is done to a solid object (a force is applied) to how that object responds (it deforms to some extent). It is similar to the spring constant k in Hooke’s law (Eq. 7.9) that relates a force applied to a spring and the resultant deformation of the spring, measured by its extension or compression. We consider three types of deformation and define an elastic modulus for each:

ar ph

ys

1. Young’s modulus measures the resistance of a solid to a change in its length. 2. Shear modulus measures the resistance to motion of the planes within a solid parallel to each other. 3. Bulk modulus measures the resistance of solids or liquids to changes in their volume.

The amount by which the length of the bar changes due to the applied force is L. A

sw

Young’s Modulus: Elasticity in Length

S

Figure 12.11 A force F is applied to the free end of a bar clamped at the other end.

w

Y;

tensile stress F/A 5 tensile strain DL/Li

(12.6)

Young’s modulus is typically used to characterize a rod or wire stressed under either tension or compression. Because strain is a dimensionless quantity, Y has units of force per unit area. Typical values are given in Table 12.1 on page 374. For relatively small stresses, the bar returns to its initial length when the force is removed. The elastic limit of a substance is defined as the maximum stress that can be applied to the substance before it becomes permanently deformed and does not return to its initial length. It is possible to exceed the elastic limit of a substance by

F

L

w

w

.a

Consider a long bar of cross-sectional area A and initial length Li that is clamped at one end as in Figure 12.11. When an external force is applied perpendicular to the cross section, internal molecular forces in the bar resist distortion (“stretching”), but the bar reaches an equilibrium situation in which its final length Lf is greater than Li and in which the external force is exactly balanced by the internal forces. In such a situation, the bar is said to be stressed. We define the tensile stress as the ratio of the magnitude of the external force F to the cross-sectional area A, where the cross section is perpendicular to the force vector. The tensile strain in this case is defined as the ratio of the change in length DL to the original length Li . We define Young’s modulus by a combination of these two ratios:

S

Li

WW Young’s modulus

374 Chapter 12

Static Equilibrium and Elasticity

Table 12.1

Typical Values for Elastic Moduli

Young’s Modulus Shear Modulus Bulk Modulus Substance (N/m 2) (N/m 2) (N/m 2)

om

Tungsten 35 3 1010 14 3 1010 20 3 1010 Steel 20 3 1010 8.4 3 1010 6 3 1010 Copper 11 3 1010 4.2 3 1010 14 3 1010 Brass 9.1 3 1010 3.5 3 1010 6.1 3 1010 Aluminum 7.0 3 1010 2.5 3 1010 7.0 3 1010 Glass 6.5–7.8 3 1010 2.6–3.2 3 1010 5.0–5.5 3 1010 Quartz 5.6 3 1010 2.6 3 1010 2.7 3 1010 Water — — 0.21 3 1010 Mercury — — 2.8 3 1010

applying a sufficiently large stress as seen in Figure 12.12. Initially, a stress-versusstrain curve is a straight line. As the stress increases, however, the curve is no longer a straight line. When the stress exceeds the elastic limit, the object is permanently distorted and does not return to its original shape after the stress is removed. As the stress is increased even further, the material ultimately breaks.

100 0

ly

200

Elastic Breaking limit point Elastic behavior Strain 0.002 0.004 0.006 0.008 0.01

ee b

300

Shear Modulus: Elasticity of Shape

Another type of deformation occurs when an object is subjected to a force parallel to one of its faces while the opposite face is held fixed by another force (Fig. 12.13a). The stress in this case is called a shear stress. If the object is originally a rectangular block, a shear stress results in a shape whose cross section is a parallelogram. A book pushed sideways as shown in Figure 12.13b is an example of an object subjected to a shear stress. To a first approximation (for small distortions), no change in volume occurs with this deformation. We define the shear stress as F/A, the ratio of the tangential force to the area A of the face being sheared. The shear strain is defined as the ratio Dx/h, where Dx is the horizontal distance that the sheared face moves and h is the height of the object. In terms of these quantities, the shear modulus is

s. w

400

.c

Stress (MPa)

Shear modulus

ar ph

ys

ic

Figure 12.12 Stress-versus-strain curve for an elastic solid.

sw

S;

shear stress F/A 5 shear strain Dx/h

(12.7)

w

w

w

.a

Values of the shear modulus for some representative materials are given in Table 12.1. Like Young’s modulus, the unit of shear modulus is the ratio of that for force to that for area.

Bulk Modulus: Volume Elasticity Bulk modulus characterizes the response of an object to changes in a force of uniform magnitude applied perpendicularly over the entire surface of the object as shown in Figure 12.14. (We assume here the object is made of a single substance.) x

A

S

F

h

a

Fixed face

s

S

–F

c ysi

mation in which a rectangular block is distorted by two forces of equal magnitude but opposite directions applied to two parallel faces. (b) A book is under shear stress when a hand placed on the cover applies a horizontal force away from the spine.

Ph

Figure 12.13 (a) A shear defor-

The shear stress causes the top face of the block to move to the right relative to the bottom.

S S

fss

b

S

F

The shear stress causes the front cover of the book to move to the right relative to the back cover.

12.4 Elastic Properties of Solids 375

As we shall see in Chapter 14, such a uniform distribution of forces occurs when an object is immersed in a fluid. An object subject to this type of deformation undergoes a change in volume but no change in shape. The volume stress is defined as the ratio of the magnitude of the total force F exerted on a surface to the area A of the surface. The quantity P 5 F/A is called pressure, which we shall study in more detail in Chapter 14. If the pressure on an object changes by an amount DP 5 DF/A, the object experiences a volume change DV. The volume strain is equal to the change in volume DV divided by the initial volume Vi . Therefore, from Equation 12.5, we can characterize a volume (“bulk”) compression in terms of the bulk modulus, which is defined as volume stress DF/A DP 52 52 volume strain DV/Vi DV/Vi

WW Bulk modulus

(12.8)

om

B;

S

S

Fback

s. w

ee b

ly

S

Fleft

S

Fright

S

Ffront

Vi V S

Fbottom

The cube undergoes a change in volume but no change in shape.

Figure 12.14 A cube is under uniform pressure and is therefore compressed on all sides by forces normal to its six faces. The arrowheads of force vectors on the sides of the cube that are not visible are hidden by the cube.

sw

ar ph

ys

ic

Q uick Quiz 12.4 For the three parts of this Quick Quiz, choose from the following choices the correct answer for the elastic modulus that describes the relationship between stress and strain for the system of interest, which is in italics: (a) Young’s modulus (b) shear modulus (c) bulk modulus (d) none of those choices (i) A block of iron is sliding across a horizontal floor. The friction force between the sliding block and the floor causes the block to deform. (ii) A trapeze artist swings through a circular arc. At the bottom of the swing, the wires supporting the trapeze are longer than when the trapeze artist simply hangs from the trapeze due to the increased tension in them. (iii) A spacecraft carries a steel sphere to a planet on which atmospheric pressure is much higher than on the Earth. The higher pressure causes the radius of the sphere to decrease.

Vi

Ftop

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A negative sign is inserted in this defining equation so that B is a positive number. This maneuver is necessary because an increase in pressure (positive DP) causes a decrease in volume (negative DV ) and vice versa. Table 12.1 lists bulk moduli for some materials. If you look up such values in a different source, you may find the reciprocal of the bulk modulus listed. The reciprocal of the bulk modulus is called the compressibility of the material. Notice from Table 12.1 that both solids and liquids have a bulk modulus. No shear modulus and no Young’s modulus are given for liquids, however, because a liquid does not sustain a shearing stress or a tensile stress. If a shearing force or a tensile force is applied to a liquid, the liquid simply flows in response.

Prestressed Concrete

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If the stress on a solid object exceeds a certain value, the object fractures. The maximum stress that can be applied before fracture occurs—called the tensile strength, compressive strength, or shear strength—depends on the nature of the material and on the type of applied stress. For example, concrete has a tensile strength of about 2 3 106 N/m2, a compressive strength of 20 3 106 N/m2, and a shear strength of 2 3 106 N/m2. If the applied stress exceeds these values, the concrete fractures. It is common practice to use large safety factors to prevent failure in concrete structures. Concrete is normally very brittle when it is cast in thin sections. Therefore, concrete slabs tend to sag and crack at unsupported areas as shown in Figure 12.15a. The slab can be strengthened by the use of steel rods to reinforce the concrete as illustrated in Figure 12.15b. Because concrete is much stronger under compression (squeezing) than under tension (stretching) or shear, vertical columns of concrete can support

Concrete

a

Steel rod under tension

Steel reinforcing rod

Load force Cracks

b

c

Figure 12.15 (a) A concrete slab with no reinforcement tends to crack under a heavy load. (b) The strength of the concrete is increased by using steel reinforcement rods. (c) The concrete is further strengthened by prestressing it with steel rods under tension.

376 Chapter 12

Static Equilibrium and Elasticity

very heavy loads, whereas horizontal beams of concrete tend to sag and crack. A significant increase in shear strength is achieved, however, if the reinforced concrete is prestressed as shown in Figure 12.15c. As the concrete is being poured, the steel rods are held under tension by external forces. The external forces are released after the concrete cures; the result is a permanent tension in the steel and hence a compressive stress on the concrete. The concrete slab can now support a much heavier load.

Example 12.5 Stage Design

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In Example 8.2, we analyzed a cable used to support an actor as he swings onto the stage. Now suppose the tension in the cable is 940 N as the actor reaches the lowest point. What diameter should a 10-m-long steel cable have if we do not want it to stretch more than 0.50 cm under these conditions?

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S o l u ti o n

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Conceptualize Look back at Example 8.2 to recall what is happening in this situation. We ignored any stretching of the cable there, but we wish to address this phenomenon in this example.

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Categorize We perform a simple calculation involving Equation 12.6, so we categorize this example as a substitution problem.

Assuming the cross section is circular, find the diameter of the cable from d 5 2r and A 5 pr 2:

d 5 2r 5 2

Substitute numerical values:

d52

FL i A 52 p Å Å pYDL

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A5

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FLi Y DL

Solve Equation 12.6 for the cross-sectional area of the cable:

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1 940 N 2 1 10 m 2 5 3.5 3 1023 m 5 3.5 mm Å p 1 20 3 1010 N/m2 2 1 0.005 0 m 2

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To provide a large margin of safety, you would probably use a flexible cable made up of many smaller wires having a total cross-sectional area substantially greater than our calculated value.

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Example 12.6 Squeezing a Brass Sphere

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S o l u ti o n

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A solid brass sphere is initially surrounded by air, and the air pressure exerted on it is 1.0 3 105 N/m2 (normal atmospheric pressure). The sphere is lowered into the ocean to a depth where the pressure is 2.0 3 107 N/m2. The volume of the sphere in air is 0.50 m3. By how much does this volume change once the sphere is submerged?

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Conceptualize Think about movies or television shows you have seen in which divers go to great depths in the water in submersible vessels. These vessels must be very strong to withstand the large pressure under water. This pressure squeezes the vessel and reduces its volume. Categorize We perform a simple calculation involving Equation 12.8, so we categorize this example as a substitution problem. Solve Equation 12.8 for the volume change of the sphere:

DV 5 2

Vi DP B

Substitute numerical values:

DV 5 2

1 0.50 m3 2 1 2.0 3 107 N/m2 2 1.0 3 105 N/m2 2 6.1 3 1010 N/m2

5 21.6 3 1024 m3 The negative sign indicates that the volume of the sphere decreases.

Objective Questions 377

Summary Definitions We can describe the elastic properties of a substance using the concepts of stress and strain. Stress is a quantity proportional to the force producing a deformation; strain is a measure of the degree of deformation. Stress is proportional to strain, and the constant of proportionality is the elastic modulus: stress Elastic modulus ; (12.5) strain

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The gravitational force exerted on an object can be considered as acting at a single point called the center of gravity. An object’s center of gravity coincides with its center of mass if the object is in a uniform gravitational field.

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Concepts and Principles

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Analysis Model for Problem Solving

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Three common types of deformation are represented by (1) the resistance of a solid to elongation under a load, characterized by Young’s modulus Y; (2) the resistance of a solid to the motion of internal planes sliding past each other, characterized by the shear modulus S; and (3) the resistance of a solid or fluid to a volume change, characterized by the bulk modulus B.

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Rigid Object in Equilibrium A rigid object in equilibrium exhibits no translational or angular acceleration. The net external force acting on it is zero, and the net external torque on it is zero about any axis: a F ext 5 0 S

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S a t ext 5 0

y

a0 Fx 0 Fy 0

a0 tz 0

(12.1) (12.2) O

x

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The first condition is the condition for translational equilibrium, and the second is the condition for rotational equilibrium.

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Objective Questions

1. denotes answer available in Student Solutions Manual/Study Guide

1. The acceleration due to gravity becomes weaker by about three parts in ten million for each meter of increased elevation above the Earth’s surface. Suppose a skyscraper is 100 stories tall, with the same floor plan for each story and with uniform average density. Compare the location of the building’s center of mass and the location of its center of gravity. Choose one: (a) Its center of mass is higher by a distance of several meters. (b) Its center of mass is higher by a distance of several millimeters. (c) Its center of mass and its center of gravity are in the same location. (d) Its center of gravity is higher by a distance of several millimeters. (e) Its center of gravity is higher by a distance of several meters.

2. A rod 7.0 m long is pivoted at a point 2.0 m from the left end. A downward force of 50 N acts at the left end, and a downward force of 200 N acts at the right end. At what distance to the right of the pivot can a third force of 300 N acting upward be placed to produce rotational equilibrium? Note: Neglect the weight of the rod. (a) 1.0 m (b) 2.0 m (c) 3.0 m (d) 4.0 m (e) 3.5 m 3. Consider the object in Figure OQ12.3. A single force is exerted on the object. The line of action of the force does not pass through the object’s center of mass. The acceleration of the object’s center of mass due to this force (a) is the same as if the force were applied at the

Static Equilibrium and Elasticity A through E, where E is the center of mass of the frame. Rank the torques tA , tB , tC , tD , and tE from largest to smallest, noting that zero is greater than a negative quantity. If two torques are equal, note their equality in your ranking.

Figure OQ12.3

C A B

Figure OQ12.7

8. In analyzing the equilibrium of a flat, rigid object, you are about to choose an axis about which you will calculate torques. Which of the following describes the choice you should make? (a) The axis should pass through the object’s center of mass. (b) The axis should pass through one end of the object. (c) The axis should be either the x axis or the y axis. (d) The axis should pass through any point within the object. (e) Any axis within or outside the object can be chosen. 9. A certain wire, 3 m long, stretches by 1.2 mm when under tension 200 N. (i) Does an equally thick wire 6 m long, made of the same material and under the same tension, stretch by (a) 4.8 mm, (b) 2.4 mm, (c) 1.2 mm, (d) 0.6 mm, or (e) 0.3 mm? (ii) A wire with twice the diameter, 3 m long, made of the same material and under the same tension, stretches by what amount? Choose from the same possibilities (a) through (e).

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4. Two forces are acting on an object. Which of the following statements is correct? (a) The object is in equilibrium if the forces are equal in magnitude and opposite in direction. (b) The object is in equilibrium if the net torque on the object is zero. (c) The object is in equilibrium if the forces act at the same point on the object. (d) The object is in equilibrium if the net force and the net torque on the object are both zero. (e) The object cannot be in equilibrium because more than one force acts on it. 5. In the cabin of a ship, a soda can rests in a saucershaped indentation in a built-in counter. The can tilts as the ship slowly rolls. In which case is the can most stable against tipping over? (a) It is most stable when it is full. (b) It is most stable when it is half full. (c) It is most stable when it is empty. (d) It is most stable in two of these cases. (e) It is equally stable in all cases.

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F

300 N

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center of mass, (b) is larger than the acceleration would be if the force were applied at the center of mass, (c) is smaller than the acceleration would be if the force were applied at the center of mass, or (d) is zero because the force causes only angular acceleration about the center of mass.

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378 Chapter 12

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6. A 20.0-kg horizontal plank 4.00 m long rests on two supports, one at the left end and a second 1.00 m from the right end. What is the magnitude of the force exerted on the plank by the support near the right end? (a) 32.0 N (b) 45.2 N (c) 112 N (d) 131 N (e) 98.2 N

1. denotes answer available in Student Solutions Manual/Study Guide

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Conceptual Questions

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7. Assume a single 300-N force is exerted on a bicycle frame as shown in Figure OQ12.7. Consider the torque produced by this force about axes perpendicular to the plane of the paper and through each of the points

10. The center of gravity of an ax is on the centerline of the handle, close to the head. Assume you saw across the handle through the center of gravity and weigh the two parts. What will you discover? (a) The handle side is heavier than the head side. (b) The head side is heavier than the handle side. (c) The two parts are equally heavy. (d) Their comparative weights cannot be predicted.

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1. A ladder stands on the ground, leaning against a wall. Would you feel safer climbing up the ladder if you were told that the ground is frictionless but the wall is rough or if you were told that the wall is frictionless but the ground is rough? Explain your answer. 2. The center of gravity of an object may be located outside the object. Give two examples for which that is the case. 3. (a) Give an example in which the net force acting on an object is zero and yet the net torque is nonzero. (b) Give an example in which the net torque acting on an object is zero and yet the net force is nonzero. 4. Stand with your back against a wall. Why can’t you put your heels firmly against the wall and then bend forward without falling?

5. An arbitrarily shaped piece of plywood can be suspended from a string attached to the ceiling. Explain how you could use a plumb bob to find its center of gravity. 6. A girl has a large, docile dog she wishes to weigh on a small bathroom scale. She reasons that she can determine her dog’s weight with the following method. First she puts the dog’s two front feet on the scale and records the scale reading. Then she places only the dog’s two back feet on the scale and records the reading. She thinks that the sum of the readings will be the dog’s weight. Is she correct? Explain your answer. 7. Can an object be in equilibrium if it is in motion? Explain. 8. What kind of deformation does a cube of Jell-O exhibit when it jiggles?

Problems

379

Problems AMT Analysis Model tutorial available in

The problems found in this chapter may be assigned online in Enhanced WebAssign

Enhanced WebAssign

GP Guided Problem M Master It tutorial available in Enhanced WebAssign

1. straightforward; 2. intermediate; 3. challenging

BIO

W Watch It video solution available in Enhanced WebAssign

1. full solution available in the Student Solutions Manual/Study Guide

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1. What are the necessary condiFy S tions for equilibrium of the object shown in Figure P12.1? Fx Calculate torques about an Ry axis through point O. u 2. Why is the following situation R x O impossible? A uniform beam of mass mb 5 3.00 kg and length S Fg , 5 1.00 m supports blocks with masses m1 5 5.00 kg and Figure P12.1 m2 5 15.0 kg at two positions as shown in Figure P12.2. The beam rests on two triangular blocks, with point P a distance d 5 0.300 m to the right of the center of gravity of the beam. The position of the object of mass m2 is adjusted along the length of the beam until the normal force on the beam at O is zero.

4. Consider the following distribution of objects: a M 5.00-kg object with its center of gravity at (0, 0) m, a 3.00-kg object at (0, 4.00) m, and a 4.00-kg object at (3.00, 0) m. Where should a fourth object of mass 8.00 kg be placed so that the center of gravity of the four-object arrangement will be at (0, 0)? 5. Pat builds a track for his model car out of solid wood as shown in Figure P12.5. The track is 5.00 cm wide, 1.00 m high, and 3.00 m long. The runway is cut so that it forms a parabola with the equation y 5 (x 2 3)2/9. Locate the horizontal coordinate of the center of gravity of this track.

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Section 12.1 Analysis Model: Rigid Object in Equilibrium

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m1

d

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m2

P

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CG

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Figure P12.2

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Section 12.2 More on the Center of Gravity

y

(x 3)2 9

1.00 m 5.00 cm

x

3.00 m

Figure P12.5 6. A circular pizza of radius R has a circular piece of S radius R/2 removed from one side as shown in Figure P12.6. The center of gravity has moved from C to C9 along the x axis. Show that the distance from C to C9 is R/6. Assume the thickness and density of the pizza are uniform throughout.

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Problems 45, 48, 49, and 92 in Chapter 9 can also be assigned with this section.

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3. A carpenter’s square has the shape of an L as shown in W Figure P12.3. Locate its center of gravity.

C

C

4.0 cm

Figure P12.6

18.0 cm 4.0 cm 12.0 cm

Figure P12.3

7. Figure P12.7 on page 380 shows three uniform objects: a rod with m1 5 6.00 kg, a right triangle with m 2 5 3.00 kg, and a square with m 3 5 5.00 kg. Their coordinates in meters are given. Determine the center of gravity for the three-object system.

380 Chapter 12

Static Equilibrium and Elasticity

y (m)

Sam m1

(9, 7)

(2, 7)

(–5, 5)

Joe

(8, 5)

m3

m2 (–2, 2)

1.00 m

(4, 1)

x (m)

2.00 m 7.60 m

Figure P12.11

Section 12.3 Examples of Rigid Objects in Static Equilibrium

12. A vaulter holds a 29.4-N pole in equilibrium by exertS U with her leading hand and a ing an upward force S downward force D with her trailing hand as shown in Figure P12.12. Point C is the center of gravity of the S S pole. What are the magnitudes of (a) U and (b) D ? 1.50 m

the front axle. Find the force exerted by the ground on each wheel.

3r

S

D

2.25 m

C

B

S

Fg

Figure P12.12

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U

A

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9. Find the mass m of the counterweight needed to balW ance a truck with mass M 5 1 500 kg on an incline of u 5 458 (Fig. P12.9). Assume both pulleys are frictionless and massless.

S

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0.750 m

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8. A 1 500-kg automobile has a wheel base (the distance

AMT between the axles) of 3.00 m. The automobile’s center M of mass is on the centerline at a point 1.20 m behind

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Problems 14, 26, 27, 28, 31, 33, 34, 60, 66, 85, 89, 97, and 100 in Chapter 5 can also be assigned with this section.

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Figure P12.7

13. A 15.0-m uniform ladder weighing 500 N rests against

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AMT a frictionless wall. The ladder makes a 60.08 angle M with the horizontal. (a) Find the horizontal and verti-

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m

M u

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Figure P12.9

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10. A mobile is constructed of light rods, light strings, and W beach souvenirs as shown in Figure P12.10. If m 4 5 12.0 g, find values for (a) m1, (b) m 2, and (c) m 3.

w

4.00 cm

6.00 cm

w

2.00 cm 5.00 cm 3.00 cm 4.00 cm

m3

m2

cal forces the ground exerts on the base of the ladder when an 800-N firefighter has climbed 4.00 m along the ladder from the bottom. (b) If the ladder is just on the verge of slipping when the firefighter is 9.00 m from the bottom, what is the coefficient of static friction between ladder and ground?

14. A uniform ladder of length L and mass m1 rests against S a frictionless wall. The ladder makes an angle u with the horizontal. (a) Find the horizontal and vertical forces the ground exerts on the base of the ladder when a firefighter of mass m 2 has climbed a distance x along the ladder from the bottom. (b) If the ladder is just on the verge of slipping when the firefighter is a distance d along the ladder from the bottom, what is the coefficient of static friction between ladder and ground? 15. A flexible chain weighing 40.0 N hangs between two hooks located at the same height (Fig. P12.15). At each hook, the tangent to the chain makes an angle u 5 42.08 with the horizontal. Find (a) the magnitude of the force each hook exerts on the chain and (b) the

m4 m1

Figure P12.10 11. A uniform beam of length 7.60 m and weight 4.50 3 102 N is carried by two workers, Sam and Joe, as shown in Figure P12.11. Determine the force that each person exerts on the beam.

u

Figure P12.15

Problems

17.

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W

vertical component of this force. Now solve the same problem from the force diagram from part (a) by computing torques around the junction between the cable and the beam at the right-hand end of the beam. Find (e) the vertical component of the force exerted by the pole on the beam, (f) the tension in the cable, and (g) the horizontal component of the force exerted by the pole on the beam. (h) Compare the solution to parts (b) through (d) with the solution to parts (e) through (g). Is either solution more accurate? 9. Sir Lost-a-Lot dons his armor and sets out from the 1 castle on his trusty steed (Fig. P12.19). Usually, the drawbridge is lowered to a horizontal position so that the end of the bridge rests on the stone ledge. Unfortunately, Lost-a-Lot’s squire didn’t lower the drawbridge far enough and stopped it at u 5 20.08 above the horizontal. The knight and his horse stop when their combined center of mass is d 5 1.00 m from the end of the bridge. The uniform bridge is , 5 8.00 m long and has mass 2 000 kg. The lift cable is attached to the bridge 5.00 m from the hinge at the castle end and to a point on the castle wall h 5 12.0 m above the bridge. Lost-a-Lot’s mass combined with his armor and steed is 1 000 kg. Determine (a) the tension in the cable and (b) the horizontal and (c) the vertical force components acting on the bridge at the hinge.

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tension in the chain at its midpoint. Suggestion: For part of the chain. (b), make a force diagram for half A uniform beam of length L and mass m shown in Figure P12.16 is inclined at an angle u to the horizontal. Its upper L end is connected to a wall by a rope, and its lower end rests u on a rough, horizontal surface. The coefficient of static Figure P12.16 friction between the beam and surface is ms . Assume the angle u is such that the static friction force is at its maximum value. (a) Draw a force diagram for the beam. (b) Using the condition of rotational equilibrium, find an expression for the tension T in the rope in terms of m, g, and u. (c) Using the condition of translational equilibrium, find a second expression for T in terms of ms , m, and g. (d) Using the results from parts (a) through (c), obtain an expression for ms involving only the angle u. (e) What happens if the ladder is lifted upward and its base is placed back on the ground slightly to the left of its position in Figure P12.16? Explain. Figure P12.17 shows a claw hammer being used to pull a nail out of a horizontal board. The mass of the hammer is 1.00 kg. A force of 150 N is exerted horizontally as shown, and the nail does not yet move relative to the board. Find (a) the force exerted by the hammer claws on the nail and (b) the force exerted by the surface on the point of contact with the hammer head. Assume the force the hammer exerts on the nail is parallel to the nail.

381

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S

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F

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30.0 cm

30.0

Single point of contact

5.00 cm

Figure P12.17

18. A 20.0-kg floodlight in a park is W supported at the end of a horizonQ/C tal beam of negligible mass that is u hinged to a pole as shown in Figure P12.18. A cable at an angle of u 5 30.08 with the beam helps support the light. (a) Draw a force diagram for the beam. By computing torques Figure P12.18 about an axis at the hinge at the lefthand end of the beam, find (b) the tension in the cable, (c) the horizontal component of the force exerted by the pole on the beam, and (d) the

d

u

Figure P12.19 Problems 19 and 20. 2 0. Review. While Lost-a-Lot ponders his next move in the situation described in Problem 19 and illustrated in Figure P12.19, the enemy attacks! An incoming projectile breaks off the stone ledge so that the end of the drawbridge can be lowered past the wall where it usually rests. In addition, a fragment of the projectile bounces up and cuts the drawbridge cable! The hinge between the castle wall and the bridge is frictionless, and the bridge swings down freely until it is vertical and smacks into the vertical castle wall below the castle entrance. (a) How long does Lost-a-Lot stay in contact with the bridge while it swings downward? (b) Find the angular acceleration of the bridge just as it starts to move. (c) Find the angular speed of the bridge when it strikes the wall below the hinge. Find the force exerted by the hinge on the bridge (d) immediately after the cable breaks and (e) immediately before it strikes the castle wall.

Static Equilibrium and Elasticity and makes an angle of u 5 60.08 with the ground. The upper and lower ends of the ladder rest on frictionless surfaces. The lower end is connected to the wall by a L horizontal rope that is frayed and can support a maximum tension Rope u of only 80.0 N. (a) Draw a force diagram for the ladder. (b) Find the normal force exerted on the Figure P12.24 bottom of the ladder. (c) Find the tension in the rope when the monkey is two-thirds of the way up the ladder. (d) Find the maximum distance d that the monkey can climb up the ladder before the rope breaks. (e) If the horizontal surface were rough and the rope were removed, how would your analysis of the problem change? What other information would you need to answer parts (c) and (d)?

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21. John is pushing his daughter Rachel in a wheelbarrow when it is stopped by a brick 8.00 cm high (Fig. P12.21). The handles make an angle of u 5 15.08 with the ground. Due to the weight of Rachel and the wheelbarrow, a downward force of 400 N is exerted at the center of the wheel, which has a radius of 20.0 cm. (a) What force must John apply along the handles to just start the wheel over the brick? (b) What is the force (magnitude and direction) that the brick exerts on the wheel just as the wheel begins to lift over the brick? In both parts, assume the brick remains fixed and does not slide along the ground. Also assume the force applied by John is directed exactly toward the center of the wheel.

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382 Chapter 12

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22. John is pushing his daughter Rachel in a wheelbarrow S when it is stopped by a brick of height h (Fig. P12.21). The handles make an angle of u with the ground. Due to the weight of Rachel and the wheelbarrow, a downward force mg is exerted at the center of the wheel, which has a radius R. (a) What force F must John apply along the handles to just start the wheel over the brick? (b) What are the components of the force that the brick exerts on the wheel just as the wheel begins to lift over the brick? In both parts, assume the brick remains fixed and does not slide along the ground. Also assume the force applied by John is directed exactly toward the center of the wheel.

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Figure P12.21 Problems 21 and 22.

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25. A uniform plank of length 2.00 m and mass 30.0 kg is supported by three ropes as indicated by the blue vectors in Figure P12.25. Find the tension in each rope when a 700-N person is d 5 0.500 m from the left end.

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23. One end of a uniform 4.00-m-long rod of weight Fg is W supported by a cable at an angle of u 5 378 with the rod. The other end rests against the wall, where it is held by friction as shown in Figure P12.23. The coefficient of static friction between the wall and the rod is ms 5 0.500. Determine the minimum distance x from point A at which an additional object, also with the same weight Fg , can be hung without causing the rod to slip at point A.

A

T2 S

T3

S

T1

d

40.0

2.00 m

Figure P12.25

Section 12.4 Elastic Properties of Solids 26. A steel wire of diameter 1 mm can support a tension of 0.2 kN. A steel cable to support a tension of 20 kN should have diameter of what order of magnitude? 27. The deepest point in the ocean is in the Mariana Trench,

Q/C about 11 km deep, in the Pacific. The pressure at this

depth is huge, about 1.13 3 108 N/m2. (a) Calculate the change in volume of 1.00 m3 of seawater carried from the surface to this deepest point. (b) The density of seawater at the surface is 1.03 3 103 kg/m3. Find its density at the bottom. (c) Explain whether or when it is a good approximation to think of water as incompressible.

2 8. Assume Young’s modulus for bone is 1.50 3 1010 N/m2.

BIO The bone breaks if stress greater than 1.50 3 108 N/m2

u x

B

Fg

Figure P12.23 24. A 10.0-kg monkey climbs a uniform ladder with

Q/C weight 1.20 3 102 N and length L 5 3.00 m as shown

in Figure P12.24. The ladder rests against the wall

is imposed on it. (a) What is the maximum force that can be exerted on the femur bone in the leg if it has a minimum effective diameter of 2.50 cm? (b) If this much force is applied compressively, by how much does the 25.0-cm-long bone shorten?

29. A child slides across a floor in a pair of rubber-soled shoes. The friction force acting on each foot is 20.0 N. The footprint area of each shoe sole is 14.0 cm2, and the thickness of each sole is 5.00 mm. Find the horizontal distance by which the upper and lower surfaces of each sole are offset. The shear modulus of the rubber is 3.00 MN/m2.

Problems

30. Evaluate Young’s modulus for the material whose stress–strain curve is shown in Figure 12.12.

end exerts a normal force n1 on the beam, and the second pivot located a distance , 5 4.00 m from the left end exerts a normal force n2. A woman of mass m 5 55.0 kg steps onto the left end of the beam and begins walking to the right as in Figure P12.38. The goal is to find the woman’s position when the beam begins to tip. (a) What is the appropriate analysis model for the beam before it begins to tip? (b) Sketch a force diagram for the beam, labeling the gravitational and normal forces acting on the beam and placing the woman a distance x to the right of the first pivot, which is the origin. (c) Where is the woman when the normal force n1 is the greatest? (d) What is n1 when the beam is about to tip? (e) Use Equation 12.1 to find the value of n2 when the beam is about to tip. (f) Using the result of part (d) and Equation 12.2, with torques computed around the second pivot, find the woman’s position x when the beam is about to tip. (g) Check the answer to part (e) by computing torques around the first pivot point.

31. Assume if the shear stress in steel exceeds about 4.00 3 M 108 N/m2, the steel ruptures. Determine the shearing force necessary to (a) shear a steel bolt 1.00 cm in diameter and (b) punch a 1.00-cm-diameter hole in a steel plate 0.500 cm thick. 32. When water freezes, it expands by about 9.00%. What pressure increase would occur inside your automobile engine block if the water in it froze? (The bulk modulus of ice is 2.00 3 109 N/m2.)

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33. A 200-kg load is hung on a wire of length 4.00 m, crossM sectional area 0.200 3 1024 m2, and Young’s modulus 8.00 3 1010 N/m2. What is its increase in length?

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34. A walkway suspended across a hotel lobby is supported at numerous points along its edges by a vertical cable above each point and a vertical column underneath. The steel cable is 1.27 cm in diameter and is 5.75 m long before loading. The aluminum column is a hollow cylinder with an inside diameter of 16.14 cm, an outside diameter of 16.24 cm, and an unloaded length of 3.25 m. When the walkway exerts a load force of 8 500 N on one of the support points, how much does the point move down?

L

m M

s. w

x

ar ph

ys

ic

35. Review. A 2.00-m-long cylindrical steel wire with a cross-sectional diameter of 4.00 mm is placed over a light, frictionless pulley. An object of mass m1 5 5.00 kg is hung from one end of the wire and an object of mass m 2 5 3.00 kg from the other end as shown m1 in Figure P12.35. The objects are m2 released and allowed to move freely. Compared with its length before the Figure P12.35 objects were attached, by how much has the wire stretched while the objects are in motion?

Figure P12.38

39. In exercise physiology studies, it is sometimes imporW tant to determine the location of a person’s center BIO of mass. This determination can be done with the arrangement shown in Figure P12.39. A light plank rests on two scales, which read Fg 1 5 380 N and Fg 2 5 320 N. A distance of 1.65 m separates the scales. How far from the woman’s feet is her center of mass?

sw

36. Review. A 30.0-kg hammer, moving with speed 20.0 m/s,

383

AMT strikes a steel spike 2.30 cm in diameter. The hammer

1.65 m

.a

rebounds with speed 10.0 m/s after 0.110 s. What is the average strain in the spike during the impact?

Additional Problems

w

Fg1

w

w

kg is 37. A bridge of length 50.0 m and mass 8.00 3 M supported on a smooth pier at each end as shown in Figure P12.37. A truck of mass 3.00 3 104 kg is located 15.0 m from one end. What are the forces on the bridge at the points of support?

A

B

15.0 m

Fg2

104

50.0 m

Figure P12.37 38. A uniform beam resting on two pivots has a length L 5 GP 6.00 m and mass M 5 90.0 kg. The pivot under the left

Figure P12.39 40. The lintel of prestressed reinforced concrete in Figure P12.40 is 1.50 m long. The concrete encloses one steel reinforcing rod with cross-sectional area 1.50 cm2. The rod joins two strong end plates. The cross-sectional area of the concrete perpendicular to the rod is 50.0 cm2. Young’s modulus for the concrete is 30.0 3 109 N/m2. After the concrete cures and the original tension T1 in the rod is released, the concrete is to be under compres1.50 m sive stress 8.00 3 106 N/m2. (a) By what distance will the rod compress the concrete when the original tension in the rod is released? (b) What Figure P12.40

384 Chapter 12

Static Equilibrium and Elasticity

Ft 12.0

A

O

4 4. The following equations are obtained from a force diagram of a rectangular farm gate, supported by two hinges on the left-hand side. A bucket of grain is hanging from the latch.

om

S

form, weighs 200 N, and is 6.00 m long, and it is supported by a wire at an angle of u 5 60.0°. The basket weighs 80.0 N. (a) Draw a force diagram for the beam. (b) When the bear is at x 5 1.00 m, find the tension in the wire supporting the beam and the components of the force exerted by the wall on the left end of the beam. (c) What If? If the wire can withstand a maximum tension of 900 N, what is the maximum distance the bear can walk before the wire breaks?

2A 1 C 5 0

1B 2 392 N 2 50.0 N 5 0

.c

is the new tension T2 in the rod? (c) The rod will then be how much longer than its unstressed length? (d) When the concrete was poured, the rod should have been stretched by what extension distance from its unstressed length? (e) Find the required original tension T1 in the rod. 41. The arm in Figure P12.41 weighs 41.5 N. The gravitaBIO tional force on the arm acts through point A. DeterS mine the magnitudes of the S tension force F t in the deltoid muscle and the force F s exerted by the shoulder on the humerus (upper-arm bone) to hold the arm in the position shown.

Fs

A(0) 1 B(0) 1 C(1.80 m) 2 392 N(1.50 m)

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S

S

2 50.0 N(3.00 m) 5 0

Fg

Figure P12.41 42. When a person stands on tiptoe on one foot (a strenu-

BIO ous position), the position of the foot is as shown in S

4 5. A uniform sign of weight Fg and width 2L hangs from S a light, horizontal beam hinged at the wall and supported by a cable (Fig. P12.45). Determine (a) the tension in the cable and (b) the components of the reaction force exerted by the wall on the beam in terms of Fg , d, L, and u.

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ys

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Figure P12.42a. The total gravitational force F g on the n exerted by the body is supported by the normal force S floor on the toes of one foot. A mechanical model of S the situation is shown in Figure P12.42b, where T is the force exerted on the foot by the Achilles tendon S and R is the force exerted on the foot by the tibia. Find the values of T, R, and u when Fg 5 700 N.

(a) Draw the force diagram and complete the statement of the problem, specifying the unknowns. (b) Determine the values of the unknowns and state the physical meaning of each.

ee b

29.0 cm

s. w

8.00 cm

Achilles tendon

Tibia

u

S

R

sw

15.0

18.0 cm 25.0 cm

S

.a

n

b

w

a

w

Figure P12.42

43. A hungry bear weighing 700 N walks out on a beam

AMT in an attempt to retrieve a basket of goodies hanging

w

u

S

T

at the end of the beam (Fig. P12.43). The beam is uni-

d

Lulu and Lisa’s Cafe

2L

Figure P12.45 46. A 1 200-N uniform boom at f 5 658 to the vertical is supported by a cable at an angle u 5 25.08 to the horizontal as shown in Figure P12.46. The boom is pivoted at the bottom, and an object of weight m 5 2 000 N hangs from its top. Find (a) the tension in the support cable and (b) the components of the reaction force exerted by the floor on the boom.

u 3 4

x

m f

u

Figure P12.46 Goodies

Figure P12.43

47. A crane of mass m1 5 3 000 kg supports a load of mass m 2 5 10 000 kg as shown in Figure P12.47. The crane

385

Problems is pivoted with a frictionless pin at A and rests against a smooth support at B. Find the reaction forces at (a) point A and (b) point B.

shown in Figure P12.50a. The rope make an angle u 5 37.08 with the floor and is tied h1 5 10.0 cm from the bottom of the cabinet. The uniform rectangular cabinet has height , 5 100 cm and width w 5 60.0 cm, and it weighs 400 N. The cabinet slides with constant speed when a force F 5 300 N is applied through the rope. The worker tires of walking backward. He fastens the rope to a point on the cabinet h 2 5 65.0 cm off the floor and lays the rope over his shoulder so that he can walk forward and pull as shown in Figure P12.50b. In this way, the rope again makes an angle of u 5 37.08 with the horizontal and again has a tension of 300 N. Using this technique, the worker is able to slide the cabinet over a long distance on the floor without tiring.

A 1.00 m

S

B

m2

m1g

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2.00 m 6.00 m

Figure P12.47

ly

w

w

S

F

ee b

u

S

F

h2

u h1

s. w

pal supporting force provided by the erector spinalis muscle in the back. To see the magnitude of the forces involved, consider the model shown in Figure P12.48b for a person bending forward to lift a 200-N object. The spine and upper body are represented as a uniform horizontal rod of weight 350 N, pivoted at the base of the spine. The erector spinalis muscle, attached at a point two-thirds of the way up the spine, maintains the position of the back. The angle between the spine and this muscle is u 5 12.08. Find (a) the tension T in the back muscle and (b) the compressional force in the spine. (c) Is this method a good way to lift a load? Explain your answer, using the results of parts (a) and (b). (d) Can you suggest a better method to lift a load?

.c

48. Assume a person bends forward to lift a load “with his

BIO back” as shown in Figure P12.48a. The spine pivots Q/C mainly at the fifth lumbar vertebra, with the princi-

a

b

Back muscle

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ys

ic

Figure P12.50 Problems 50 and 62.

Ry

S T 12.0

sw

Pivot

w

w

a

.a

Rx

P

b

Figure P12.48

49. A 10 000-N shark is supported M by a rope attached to a 4.00-m rod that can pivot at the base. (a) Calculate the tension in the cable between the rod and the wall, assuming the cable is holding the system in the position shown in Figure P12.49. Find (b) the horizontal force and (c) the vertical force exerted on the base of the rod. Ignore the weight of the rod.

w

200 N

350 N

51. A uniform beam of mass m is inclined at an angle u to S the horizontal. Its upper end (point P) produces a 908 bend in a very rough rope tied to a wall, and its lower end rests on a rough floor (Fig. P12.51). Let ms represent the coefficient of static friction between beam and floor. Assume ms is less than the cotangent of u. (a) Find an expression for the maximum mass M that can be suspended from the top before the beam slips. Determine (b) the magnitude of the reaction force at the floor and (c) the magnitude of the force exerted by the beam on the rope at P in terms of m, M, and ms .

m

M u 20.0

Figure P12.51 52. The large quadriceps muscle in the upper leg terminates

BIO at its lower end in a tendon attached to the upper end of

60.0

10 000 N

Figure P12.49

50. Why is the following situation impossible? A worker in a factory pulls a cabinet across the floor using a rope as

the tibia (Fig. P12.52a, page 386). The forces on the lower leg when the legSis extended are modeled as in Figure S P12.52b, where T is the force in the tendon, F g,leg is the gravitational force acting on the lower leg, and S F g,foot is the gravitational force acting on the foot. Find T when the tendon is at an angle of f 5 25.08 with the tibia, assuming Fg,leg 5 30.0 N, Fg,foot 5 12.5 N, and the leg is extended at an angle u 5 40.08 with respect to the vertical. Also assume the center of gravity of the

Quadriceps

386 Chapter 12

Tendon

Static Equilibrium and Elasticity Tibia

tibia is at its geometric center and the tendon attaches to the lower leg at a position one-fifth of the way down a the leg. S

f Tendon

u

Tibia

S

S

Fg,leg

F

S

B

Fg,foot a

S

nA

b

Figure P12.52

S

f

T

A

u2

C

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BIO iron cross, he maintains the position at rest shown in Q/C Figure P12.53a. In this maneuver, the gymnast’s feet u

ee b

Figure P12.54

s. w

55. One side of a plant shelf is supported by a bracket 80.0 N 5.00 cm mounted on a vertical wall by a single screw as shown 3.00 cm in Figure P12.55. Ignore the weight of the bracket. 6.00 cm (a) Find the horizontal component of the force that the screw exerts on the bracket when an 80.0 N Figure P12.55 vertical force is applied as shown. (b) As your grandfather waters his geraniums, the 80.0-N load force is increasing at the rate 0.150 N/s. At what rate is the force exerted by the screw changing? Suggestion: Imagine that the bracket is slightly loose.

. Ed Bock/Surf/Corbis

ar ph

ys

ic

(not shown) are off the floor. The primary muscles S involved in supporting this position are the latissimus Fg,leg dorsi (“lats”) andFSthe pectoralis major (“pecs”). One S of the rings exertsg,foot an upward forceS F h on a hand as bshown in Figure P12.53b. The force F s is exerted by the shoulder joint on the arm. The latissimus S dorsi and pectoralis major muscles exert a total force F m on the arm. (a) Using the information in the figure, find the S magnitude of the force F m for an athlete of weight 750 N. (b) Suppose an athlete in training cannot perform the iron cross but can hold a position similar to the figure in which the arms make a 458 angle with the horizontal rather than being horizontal. Why is this position easier for the athlete?

nC

.c

53. When a gymnast performing on the rings executes the

u1

S

om

Quadriceps

T

are smooth. (a) Given u1 5 30.0° and u2 5 45.08, find nA and nC . (b) One can show that the force any strut exerts on a pin must be directed along the length of the strut as a force of tension or compression. Use that fact to identify the directions of the forces that the struts exert on the pins joining them. Find the force of tension or of compression in each of the three bars.

54. Figure P12.54 shows a light truss formed from three struts lying in a plane and joined by three smooth hinge pins at their ends. The truss supports a downS ward force of F 5 1 000 N applied at the point B. The truss has negligible weight. The piers at A and C

57. A stepladder of negligible weight is constructed as S shown in Figure P12.56, with AC 5 BC 5 ,. A painter of mass m stands on the ladder a distance d from the bottom. Assuming the floor is frictionless, find (a) the tension in the horizontal bar DE connecting the two

sw Figure P12.53

56. A stepladder of negligible M weight is constructed as m C shown in Figure P12.56, with AC 5 BC 5 , 5 4.00 m. A painter of mass m 5 2 70.0 kg stands on the ladder d 5 3.00 m from the D E bottom. Assuming the floor d is frictionless, find (a) the 2 tension in the horizontal bar DE connecting the two halves of the ladder, B A 2 (b) the normal forces at A and B, and (c) the comFigure P12.56 ponents of the reaction Problems 56 and 57. force at the single hinge C that the left half of the ladder exerts on the right half. Suggestion: Treat the ladder as a single object, but also treat each half of the ladder separately.

w

w

.a

a

Shoulder joint

S

w

Fs

4.00 cm S

S

Fh

70.0 cm

Fm 45.0

b

387

Problems

Challenge Problems 65. A uniform pole is propped between the floor and the ceiling of a room. The height of the room is 7.80 ft, and the coefficient of static friction between the pole and the ceiling is 0.576. The coefficient of static friction between the pole and the floor is greater than that between the pole and the ceiling. What is the length of the longest pole that can be propped between the floor and the ceiling?

45

S

P3

S

P2

66. In the What If? section of Example 12.2, let d represent the distance in meters between the person and the hinge at the left end of the beam. (a) Show that the cable tension is given by T 5 93.9d 1 125, with T in newtons. (b) Show that the direction angle u of the hinge force is described by

ly

P1

ee b

S

s. w

59. Two racquetballs, each having a mass of 170 g, are placed in a glass jar as shown in Figure P12.59. Their centers lie on a straight line that makes a 458 angle with the horizontal. (a) Assume the walls are frictionless and determine P 1, P 2, and P 3. (b) Determine the magnitude of the force exerted by the left ball on the right ball.

6 4. A steel cable 3.00 cm2 in cross-sectional area has a mass of 2.40 kg per meter of length. If 500 m of the cable is hung over a vertical cliff, how much does the cable stretch under its own weight? Take Ysteel 5 2.00 3 1011 N/m2.

.c

58. (a) Estimate the force with which a karate master strikes a board, assuming the hand’s speed at the moment of impact is 10.0 m/s and decreases to 1.00 m/s during a 0.002 00-s time interval of contact between the hand and the board. The mass of his hand and arm is 1.00 kg. (b) Estimate the shear stress, assuming this force is exerted on a 1.00-cm-thick pine board that is 10.0 cm wide. (c) If the maximum shear stress a pine board can support before breaking is 3.60 3 106 N/m2, will the board break?

nents of force exerted on the left end of the rod by the hinge.

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halves of the ladder, (b) the normal forces at A and B, and (c) the components of the reaction force at the single hinge C that the left half of the ladder exerts on the right half. Suggestion: Treat the ladder as a single object, but also treat each half of the ladder separately.

Figure P12.59

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ar ph

sw

61. Review. An aluminum wire is 0.850 m long and has a circular cross section of diameter 0.780 mm. Fixed at the top end, the wire supports a 1.20-kg object that swings in a horizontal circle. Determine the angular speed of the object required to produce a strain of 1.00 3 1023.

w

w

w

.a

62. Consider the rectangular cabinet ofSProblem 50 shown in Figure P12.50, but with a force F applied horizontally at the upper edge. (a) What is the minimum force required to start to tip the cabinet? (b) What is the minimum coefficient of static friction required for the cabinet not to slide with the application of a force of this magnitude? (c) Find the magnitude and direction of the minimum force required to tip the cabinet if the point of application can be chosen anywhere on the cabinet. 63. A 500-N uniform rectangular M sign 4.00 m wide and 3.00 m high is suspended from a horizontal, 6.00-m-long, uniform, 100-N rod as indicated in Figure P12.63. The left end of the rod is supported by a hinge, and the right end is supported by a thin cable making a 30.0° angle with the vertical. (a) Find the tension T in the cable. (b) Find the horizontal and vertical compo-

30.0°

T

32 2 1b tan 53.08 3d 1 4

(c) Show that the magnitude of the hinge force is given by

ic

60. Review. A wire of length L, S Young’s modulus Y, and cross-sectional area A is stretched elastically by an amount DL. By Hooke’s law, the restoring force is 2k DL. (a) Show that k 5 YA/L. (b) Show that the work done in stretching the wire by an amount DL is W 5 12YA 1 DL 2 2/L .

tan u 5 a

R 5 "8.82 3 103d2 2 9.65 3 104d 1 4.96 3 105

(d) Describe how the changes in T, u, and R as d increases differ from one another. 67. Figure P12.67 shows a vertical force

Q/C applied tangentially to a uniform cyl-

Figure P12.63

P

inder of weight Fg . The coefficient of static friction between the cylinder and all surfaces is 0.500. The force S P is increased in magnitude until the cylinder begins to rotate. In Figure P12.67 terms of Fg , find the maximum force magnitude P that can be applied without causing the cylinder to rotate. Suggestion: Show that both friction forces will be at their maximum values when the cylinder is on the verge of slipping.

68. A uniform rod of weight Fg and length L is supported at its ends by a frictionless trough as shown in Figure P12.68. (a) Show that the center of gravity of the rod must be vertically over point O when the rod is in equilibrium. (b) Determine the equilibrium value of the angle u. (c) Is the equilibrium of the rod stable or unstable?

u ICE CREAM SHOP

S

60.0

30.0 O

Figure P12.68

c h a p t e r

13

Universal Gravitation

13.1 Newton’s Law of Universal Gravitation

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13.2 Free-Fall Acceleration and the Gravitational Force

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13.3 Analysis Model: Particle in a Field (Gravitational)

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13.4 Kepler’s Laws and the Motion of Planets

ee b

13.5 Gravitational Potential Energy

ar ph

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s. w

13.6 Energy Considerations in Planetary and Satellite Motion

.a

sw

w

w

w

Hubble Space Telescope image of the Whirlpool Galaxy, M51, taken in 2005. The arms of this spiral galaxy compress hydrogen gas and create new clusters of stars. Some astronomers believe that the arms are prominent due to a close encounter with the small, yellow galaxy, NGC 5195, at the tip of one of its arms. (NASA, Hubble Heritage Team, (STScI/AURA), ESA, S. Beckwith (STScI). Additional Processing: Robert Gendler)

388

Before 1687, a large amount of data had been collected on the motions of the Moon and the planets, but a clear understanding of the forces related to these motions was not available. In that year, Isaac Newton provided the key that unlocked the secrets of the heavens. He knew, from his first law, that a net force had to be acting on the Moon because without such a force the Moon would move in a straight-line path rather than in its almost circular orbit. Newton reasoned that this force was the gravitational attraction exerted by the Earth on the Moon. He realized that the forces involved in the Earth–Moon attraction and in the Sun–planet attraction were not something special to those systems, but rather were particular cases of a general and universal attraction between objects. In other words, Newton saw that the same force of attraction that causes the Moon to follow its path around the Earth also causes an apple to fall from a tree. It was the first time that “earthly” and “heavenly” motions were unified. In this chapter, we study the law of universal gravitation. We emphasize a description of planetary motion because astronomical data provide an important test of this law’s validity. We then show that the laws of planetary motion developed by Johannes Kepler follow from

13.1 Newton’s Law of Universal Gravitation 389

the law of universal gravitation and the principle of conservation of angular momentum for an isolated system. We conclude by deriving a general expression for the gravitational potential energy of a system and examining the energetics of planetary and satellite motion.

13.1 Newton’s Law of Universal Gravitation

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You may have heard the legend that, while napping under a tree, Newton was struck on the head by a falling apple. This alleged accident supposedly prompted him to imagine that perhaps all objects in the Universe were attracted to each other in the same way the apple was attracted to the Earth. Newton analyzed astronomical data on the motion of the Moon around the Earth. From that analysis, he made the bold assertion that the force law governing the motion of planets was the same as the force law that attracted a falling apple to the Earth. In 1687, Newton published his work on the law of gravity in his treatise Mathematical Principles of Natural Philosophy. Newton’s law of universal gravitation states that

ee b

every particle in the Universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

WW The law of universal gravitation

Fg 5 G

m 1m 2 r2

(13.1)

ic

s. w

If the particles have masses m1 and m 2 and are separated by a distance r, the magnitude of this gravitational force is

ys

where G is a constant, called the universal gravitational constant. Its value in SI units is G 5 6.674 3 10211 N ? m2/kg2

The dashed line represents the original position of the rod.

(13.2)

Mirror

M

m

Figure 13.1 Cavendish apparatus for measuring gravitational forces.

w

Consistent with Newton’s S S third law, F21 F12. S

w

S

F 12 5 2G

m 1m 2 r2

Light source

r

w

.a

sw

ar ph

The universal gravitational constant G was first evaluated in the late nineteenth century, based on results of an important experiment by Sir Henry Cavendish (1731– 1810) in 1798. The law of universal gravitation was not expressed by Newton in the form of Equation 13.1, and Newton did not mention a constant such as G. In fact, even by the time of Cavendish, a unit of force had not yet been included in the existing system of units. Cavendish’s goal was to measure the density of the Earth. His results were then used by other scientists 100 years later to generate a value for G. Cavendish’s apparatus consists of two small spheres, each of mass m, fixed to the ends of a light, horizontal rod suspended by a fine fiber or thin metal wire as illustrated in Figure 13.1. When two large spheres, each of mass M, are placed near the smaller ones, the attractive force between smaller and larger spheres causes the rod to rotate and twist the wire suspension to a new equilibrium orientation. The angle of rotation is measured by the deflection of a light beam reflected from a mirror attached to the vertical suspension. The form of the force law given by Equation 13.1 is often referred to as an inverse-square law because the magnitude of the force varies as the inverse square of the separation of the particles.1 We shall see other examples of this type of force law in subsequent chapters. We can express this force in vector form by defining a unit vector r^ 12 (Fig. 13.2). Because this unit vector is directed from particle 1 toward particle 2, the force exerted by particle 1 on particle 2 is

F12

S

F21 r

m2

rˆ 12 m1

r^ 12

(13.3)

1An inverse proportionality between two quantities x and y is one in which y 5 k/x, where k is a constant. A direct proportion between x and y exists when y 5 kx.

Figure 13.2 The gravitational force between two particles is attractive. The unit vector r^ 12 is directed from particle 1 toward particle 2.

Universal Gravitation

represents the magnitude of the free-fall acceleration near a planet. At the surface of the Earth, g has an average value of 9.80 m/s2. On the other hand, G is a universal constant that has the same value everywhere in the Universe.

Fg 5 G

ME m R E2

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Pitfall Prevention 13.1 Be Clear on g and G The symbol g

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where the negative sign indicates that particle 2 is attracted to particle 1; hence, the force on particle 2 must be directed toward particle 1.SBy Newton’s third law, the forceS exerted by particle 2 on particle 1, designated F 21, is equal in magnitude to F12 and in Sthe opposite direction. That is, these forces form an action– S reaction pair, and F 21 5 2 F 12. Two features of Equation 13.3 deserve mention. First, the gravitational force is a field force that always exists between two particles, regardless of the medium that separates them. Second, because the force varies as the inverse square of the distance between the particles, it decreases rapidly with increasing separation. Equation 13.3 can also be used to show that the gravitational force exerted by a finite-size, spherically symmetric mass distribution on a particle outside the distribution is the same as if the entire mass of the distribution were concentrated at the center. For example, the magnitude of the force exerted by the Earth on a particle of mass m near the Earth’s surface is (13.4)

ly

390 Chapter 13

ee b

where M E is the Earth’s mass and R E its radius. This force is directed toward the center of the Earth.

ic

s. w

Q uick Quiz 13.1 A planet has two moons of equal mass. Moon 1 is in a circular orbit of radius r. Moon 2 is in a circular orbit of radius 2r. What is the magnitude of the gravitational force exerted by the planet on Moon 2? (a) four times as large as that on Moon 1 (b) twice as large as that on Moon 1 (c) equal to that on Moon 1 (d) half as large as that on Moon 1 (e) one-fourth as large as that on Moon 1

ys

Example 13.1 Billiards, Anyone?

ar ph

Three 0.300-kg billiard balls are placed on a table at the corners of a right triangle as shown in Figure 13.3. The sides of the triangle are of lengths a 5 0.400 m, b 5 0.300 m, and c 5 0.500 m. Calculate the gravitational force vector on the cue ball (designated m1) resulting from the other two balls as well as the magnitude and direction of this force.

m2

c

a

sw

Solution

y

Conceptualize Notice in Figure 13.3 that the cue ball is attracted to both other balls by the gravitational force. We can see graphically that the net force should point upward and toward the right. We locate our coordinate axes as shown in Figure 13.3, placing our origin at the position of the cue ball.

.a

Figure 13.3 (Example

w

w

13.1) The resultant gravitational force acting on the cue ball is the vector sum S S F 21 1 F 31.

S F21

m1

S

F

u

S F31

b

m3

x

w

Categorize This problem involves evaluating the gravitational forces on the cue ball using Equation 13.3. Once these forces are evaluated, it becomes a vector addition problem to find the net force. Analyze Find the force exerted by m 2 on the cue ball:

S

F 21 5 G

m 2m 1 a2

j^

5 1 6.674 3 10211 N # m2/kg2 2 5 3.75 3 10211 j^ N

Find the force exerted by m 3 on the cue ball:

S

F 31 5 G

m 3m 1 b2

1 0.300 kg 2 1 0.300 kg 2

^j

1 0.300 kg 2 1 0.300 kg 2

^i

1 0.400 m 2 2

i^

5 1 6.674 3 10211 N # m2/kg2 2 5 6.67 3 10211 i^ N

1 0.300 m 2 2

391

13.2 Free-Fall Acceleration and the Gravitational Force

▸ 13.1 c o n t i n u e d S

Find the net gravitational force on the cue ball by adding these force vectors:

S

S

F 5 F 31 1 F 21 5 1 6.67 i^ 1 3.75 j^ 2 3 10211 N

F 5 "F312 1 F212 5 " 1 6.67 2 2 1 1 3.75 2 2 3 10211 N

Find the magnitude of this force:

5 7.66 3 10211 N

Find the tangent of the angle u for the net force vector:

tan u 5

Evaluate the angle u:

Fy Fx

5

F21 F31

5

3.75 3 10211 N 5 0.562 6.67 3 10211 N

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u 5 tan21 (0.562) 5 29.4°

.c

Finalize The result for F shows that the gravitational forces between everyday objects have extremely small magnitudes.

ee b

13.2 Free-Fall Acceleration and the Gravitational Force

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g5G

M Em R E2

ic

mg 5 G

ys

s. w

We have called the magnitude of the gravitational force on an object near the Earth’s surface the weight of the object, where the weight is given by Equation 5.6. Equation 13.4 is another expression for this force. Therefore, we can set Equations 5.6 and 13.4 equal to each other to obtain

ar ph

ME R E2

(13.5)

.a

sw

Equation 13.5 relates the free-fall acceleration g to physical parameters of the Earth—its mass and radius—and explains the origin of the value of 9.80 m/s2 that we have used in earlier chapters. Now consider an object of mass m located a distance h above the Earth’s surface or a distance r from the Earth’s center, where r 5 R E 1 h. The magnitude of the gravitational force acting on this object is

w

Fg 5 G

Table 13.1 Free-Fall Acceleration g at Various Altitudes Above the Earth’s Surface Altitude h (km)

g (m/s2)

1 000 7.33 2 000 5.68 3 000 4.53 4 000 3.70 5 000 3.08 6 000 2.60 7 000 2.23 8 000 1.93 9 000 1.69 10 000 1.49 50 000 0.13 ` 0

M Em M Em 5G 2 1 r RE 1 h22

w

w

The magnitude of the gravitational force acting on the object at this position is also Fg 5 mg, where g is the value of the free-fall acceleration at the altitude h. Substituting this expression for Fg into the last equation shows that g is given by g5

GME GME 5 2 1 RE 1 h 2 2 r

(13.6)

Therefore, it follows that g decreases with increasing altitude. Values of g for the Earth at various altitudes are listed in Table 13.1. Because an object’s weight is mg, we see that as r S `, the weight of the object approaches zero. Q uick Quiz 13.2 Superman stands on top of a very tall mountain and throws a baseball horizontally with a speed such that the baseball goes into a circular orbit around the Earth. While the baseball is in orbit, what is the magnitude of the acceleration of the ball? (a) It depends on how fast the baseball is thrown. (b) It is zero because the ball does not fall to the ground. (c) It is slightly less than 9.80 m/s2. (d) It is equal to 9.80 m/s2.

WW Variation of g with altitude

392 Chapter 13

Universal Gravitation

Example 13.2 The Density of the Earth Using the known radius of the Earth and that g 5 9.80 m/s2 at the Earth’s surface, find the average density of the Earth. Solution

Conceptualize A ssume the Earth is a perfect sphere. The density of material in the Earth varies, but let’s adopt a simplified model in which we assume the density to be uniform throughout the Earth. The resulting density is the average density of the Earth.

Categorize This example is a relatively simple substitution problem.

om

rE 5

G gR E2/G g ME 5 4 5 34 3 VE pGR E 3 pR E

5 34

p 1 6.674 3 10

211

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Substitute this mass and the volume of a sphere into the definition of density (Eq. 1.1):

g R E2

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ME 5

9.80 m/s2 5 5.50 3 103 kg/m3 N # m2/kg 2 2 1 6.37 3 106 m 2

ee b

Using Equation 13.5, solve for the mass of the Earth:

What if you were told that a typical density of granite at the Earth’s surface is 2.75 3 103 kg/m3 ? What would you conclude about the density of the material in the Earth’s interior?

s. w

W h at If ?

Answer Because this value is about half the density we calculated as an average for the entire Earth, we would con-

ic

clude that the inner core of the Earth has a density much higher than the average value. It is most amazing that the Cavendish experiment—which can be used to determine G and can be done today on a tabletop—combined with simple free-fall measurements of g provides information about the core of the Earth!

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13.3 Analysis Model: Particle in a Field (Gravitational)

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w

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When Newton published his theory of universal gravitation, it was considered a success because it satisfactorily explained the motion of the planets. It represented strong evidence that the same laws that describe phenomena on the Earth can be used on large objects like planets and throughout the Universe. Since 1687, Newton’s theory has been used to account for the motions of comets, the deflection of a Cavendish balance, the orbits of binary stars, and the rotation of galaxies. Nevertheless, both Newton’s contemporaries and his successors found it difficult to accept the concept of a force that acts at a distance. They asked how it was possible for two objects such as the Sun and the Earth to interact when they were not in contact with each other. Newton himself could not answer that question. An approach to describing interactions between objects that are not in contact came well after Newton’s death. This approach enables us to look at the gravitational interaction in a different way, using the concept of a gravitational field that exists at every point in space. When a particle is placed at a point where the gravitational field exists, the particle experiences a gravitational force. In other words, we imagine that the field exerts a force on the particle rather than consider a direct S interaction between two particles. The gravitational field g is defined as S

Gravitational field

S

g ;

Fg

m0

(13.7) S

That is, the gravitational field at a point in space equals the gravitational force Fg experienced by a test particle placed at that point divided by the mass m 0 of the test particle. We call the object creating the field the source particle. (Although the Earth

393

13.3 Analysis Model: Particle in a Field (Gravitational)

The field vectors point in the direction of the acceleration a particle would experience if it were placed in the field. The magnitude of the field vector at any location is the magnitude of the free-fall acceleration at that location.

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is not a particle, it is possible to show that we can model the Earth as a particle for the purpose of finding the gravitational field that it creates.) Notice that the presence of the test particle is not necessary for the field to exist: the source particle creates the gravitational field. We can detect the presence of the field and measure its strength by placing a test particle in the field and noting the force exerted on it. In essence, we are describing the “effect” that any object (in this case, the Earth) has on the empty space around itself in terms of the force that would be present if a second object were somewhere in that space.2 The concept of a field is at the heart of the particle in a field analysis model. In the general version of this model, a particle resides in an area of space in which a field exists. Because of the existence of the field and a property of the particle, the particle experiences a force. In the gravitational version of the particle in a field model discussed here, the type of field is gravitational, and the property of the particle that results in the force is the particle’s mass m. The mathematical representation of the gravitational version of the particle in a field model is Equation 5.5: S

Fg 5 mS g

(5.5)

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ys

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In future chapters, we will see two other versions of the particle in a field model. In the electric version, the property of a particle that results in a force is electric charge: when a charged particle is placed in an electric field, it experiences a force. The magnitude of the force is the product of the electric charge and the field, in analogy with the gravitational force in Equation 5.5. In the magnetic version of the particle in a field model, a charged particle is placed in a magnetic field. One other property of this particle is required for the particle to experience a force: the particle must have a velocity at some nonzero angle to the magnetic field. The electric and magnetic versions of the particle in a field model are critical to the understanding of the principles of electromagnetism, which we will study in Chapters 23–34. Because the gravitational force acting on the object has a magnitude GM Em/r 2 (see Eq. 13.4), the gravitational field S g at a distance r from the center of the Earth is

a

S

S

g 5

Fg

m

52

GM E r^ r2

(13.8)

b

Figure 13.4 (a) The gravitational field vectors in the vicinity of a uniform spherical mass such as the Earth vary in both direction and magnitude. (b) The gravitational field vectors in a small region near the Earth’s surface are uniform in both direction and magnitude.

w

w

w

.a

sw

where r^ is a unit vector pointing radially outward from the Earth and the negative sign indicates that the field points toward the center of the Earth as illustrated in Figure 13.4a. The field vectors at different points surrounding the Earth vary in both direction and magnitude. In a small region near the Earth’s surface, the downward field S g is approximately constant and uniform as indicated in Figure 13.4b. Equation 13.8 is valid at all points outside the Earth’s surface, assuming the g has a magnitude of Earth is spherical. At the Earth’s surface, where r 5 R E , S 9.80 N/kg. (The unit N/kg is the same as m/s2.)

Analysis Model Particle in a Field (Gravitational)

Imagine an object with mass that we call a source particle. The source particle establishes a gravitational field S g throughout space. The gravitational field is evaluated by measuring the force on a test particle of mass m 0 and then using Equation 13.7. Now imagine a particle of mass m is placed in that field. The particle interacts with the gravitational field so that it experiences a gravitational force given by

S

Fg 5 mS g

S

g

m

S

S

Fg m g

(5.5) continued

2 We shall return to this idea of mass affecting the space around it when we discuss Einstein’s theory of gravitation in Chapter 39.

394 Chapter 13

Universal Gravitation

Analysis Model Particle in a Field (Gravitational) (continued) Examples:

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• an object of mass m near the surface of the Earth has a weight, which is the result of the gravitational field established in space by the Earth • a planet in the solar system is in orbit around the Sun, due to the gravitational force on the planet exerted by the gravitational field established by the Sun • an object near a black hole is drawn into the black hole, never to escape, due to the tremendous gravitational field established by the black hole (Section 13.6) • in the general theory of relativity, the gravitational field of a massive object is imagined to be described by a curvature of space–time (Chapter 39) • the gravitational field of a massive object is imagined to be mediated by particles called gravitons, which have never been detected (Chapter 46)

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Example 13.3 The Weight of the Space Station AM

ee b

The International Space Station operates at an altitude of 350 km. Plans for the final construction show that material of weight 4.22 3 106 N, measured at the Earth’s surface, will have been lifted off the surface by various spacecraft during the construction process. What is the weight of the space station when in orbit?

s. w

Solution

ic

Conceptualize The mass of the space station is fixed; it is independent of its location. Based on the discussions in this section and Section 13.2, we realize that the value of g will be reduced at the height of the space station’s orbit. Therefore, the weight of the Space Station will be smaller than that at the surface of the Earth.

m5

Fg

ar ph

Analyze From the particle in a field model,

ys

Categorize We model the Space Station as a particle in a gravitational field.

find the mass of the space station from its weight at the surface of the Earth:

.a

sw

Use Equation 13.6 with h 5 350 km to find the magnitude of the gravitational field at the orbital location:

5

4.22 3 106 N 5 4.31 3 105 kg 9.80 m/s2

GME 1RE 1 h22

1 6.674 3 10211 N # m2/kg2 2 1 5.97 3 1024 kg 2 5 8.82 m/s2 1 6.37 3 106 m 1 0.350 3 106 m 2 2

Fg 5 mg 5 (4.31 3 105 kg)(8.82 m/s2) 5 3.80 3 106 N

w

Use the particle in a field model again to find the space station’s weight in orbit:

g5

g

5

w

w

Finalize Notice that the weight of the Space Station is less when it is in orbit, as we expected. It has about 10% less weight than it has when on the Earth’s surface, representing a 10% decrease in the magnitude of the gravitational field.

13.4 Kepler’s Laws and the Motion of Planets Humans have observed the movements of the planets, stars, and other celestial objects for thousands of years. In early history, these observations led scientists to regard the Earth as the center of the Universe. This geocentric model was elaborated and formalized by the Greek astronomer Claudius Ptolemy (c. 100–c. 170) in the second century and was accepted for the next 1400 years. In 1543, Polish astronomer Nicolaus Copernicus (1473–1543) suggested that the Earth and the other planets revolved in circular orbits around the Sun (the heliocentric model). Danish astronomer Tycho Brahe (1546–1601) wanted to determine how the heavens were constructed and pursued a project to determine the positions of both

13.4 Kepler’s Laws and the Motion of Planets 395

WW Kepler’s laws

Kepler’s First Law

Johannes Kepler

German astronomer (1571–1630) Kepler is best known for developing the laws of planetary motion based on the careful observations of Tycho Brahe.

The semimajor axis has length a, and the semiminor axis has length b.

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s. w

The geocentric and original heliocentric models of the solar system both suggested circular orbits for heavenly bodies. Kepler’s first law indicates that the circular orbit is a very special case and elliptical orbits are the general situation. This notion was difficult for scientists of the time to accept because they believed that perfect circular orbits of the planets reflected the perfection of heaven. Figure 13.5 shows the geometry of an ellipse, which serves as our model for the elliptical orbit of a planet. An ellipse is mathematically defined by choosing two points F 1 and F 2, each of which is a called a focus, and then drawing a curve through points for which the sum of the distances r 1 and r 2 from F 1 and F 2, respectively, is a constant. The longest distance through the center between points on the ellipse (and passing through each focus) is called the major axis, and this distance is 2a. In Figure 13.5, the major axis is drawn along the x direction. The distance a is called the semimajor axis. Similarly, the shortest distance through the center between points on the ellipse is called the minor axis of length 2b, where the distance b is the semiminor axis. Either focus of the ellipse is located at a distance c from the center of the ellipse, where a 2 5 b 2 1 c 2. In the elliptical orbit of a planet around the Sun, the Sun is at one focus of the ellipse. There is nothing at the other focus. The eccentricity of an ellipse is defined as e 5 c/a, and it describes the general shape of the ellipse. For a circle, c 5 0, and the eccentricity is therefore zero. The smaller b is compared with a, the shorter the ellipse is along the y direction compared with its extent in the x direction in Figure 13.5. As b decreases, c increases and the eccentricity e increases. Therefore, higher values of eccentricity correspond to longer and thinner ellipses. The range of values of the eccentricity for an ellipse is 0 , e , 1. Eccentricities for planetary orbits vary widely in the solar system. The eccentricity of the Earth’s orbit is 0.017, which makes it nearly circular. On the other hand, the eccentricity of Mercury’s orbit is 0.21, the highest of the eight planets. Figure 13.6a on page 396 shows an ellipse with an eccentricity equal to that of Mercury’s orbit. Notice that even this highest-eccentricity orbit is difficult to distinguish from a circle, which is one reason Kepler’s first law is an admirable accomplishment. The eccentricity of the orbit of Comet Halley is 0.97, describing an orbit whose major axis is much longer than its minor axis, as shown in Figure 13.6b. As a result, Comet Halley spends much of its 76-year period far from the Sun and invisible from the Earth. It is only visible to the naked eye during a small part of its orbit when it is near the Sun. Now imagine a planet in an elliptical orbit such as that shown in Figure 13.5, with the Sun at focus F 2. When the planet is at the far left in the diagram, the distance

iStockphoto.com/GeorgiosArt

ee b

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1. All planets move in elliptical orbits with the Sun at one focus. 2. The radius vector drawn from the Sun to a planet sweeps out equal areas in equal time intervals. 3. The square of the orbital period of any planet is proportional to the cube of the semimajor axis of the elliptical orbit.

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stars and planets. Those observations of the planets and 777 stars visible to the naked eye were carried out with only a large sextant and a compass. (The telescope had not yet been invented.) German astronomer Johannes Kepler was Brahe’s assistant for a short while before Brahe’s death, whereupon he acquired his mentor’s astronomical data and spent 16 years trying to deduce a mathematical model for the motion of the planets. Such data are difficult to sort out because the moving planets are observed from a moving Earth. After many laborious calculations, Kepler found that Brahe’s data on the revolution of Mars around the Sun led to a successful model. Kepler’s complete analysis of planetary motion is summarized in three statements known as Kepler’s laws:

r2

r1 F1

a

b

c F2

x

w

w

w

.a

y

Each focus is located at a distance c from the center.

Figure 13.5 Plot of an ellipse.

Pitfall Prevention 13.2 Where Is the Sun? The Sun is located at one focus of the elliptical orbit of a planet. It is not located at the center of the ellipse.

396 Chapter 13

Universal Gravitation

Figure 13.6 (a) The shape of

The Sun is located at a focus of the ellipse. There is nothing physical located at the center (the black dot) or the other focus (the blue dot).

the orbit of Mercury, which has the highest eccentricity (e 5 0.21) among the eight planets in the solar system. (b) The shape of the orbit of Comet Halley. The shape of the orbit is correct; the comet and the Sun are shown larger than in reality for clarity.

Orbit of Comet Halley

Sun

Sun

Center Orbit of Mercury

Center

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Comet Halley b

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a

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between the planet and the Sun is a 1 c. At this point, called the aphelion, the planet is at its maximum distance from the Sun. (For an object in orbit around the Earth, this point is called the apogee.) Conversely, when the planet is at the right end of the ellipse, the distance between the planet and the Sun is a 2 c. At this point, called the perihelion (for an Earth orbit, the perigee), the planet is at its minimum distance from the Sun. Kepler’s first law is a direct result of the inverse-square nature of the gravitational force. Circular and elliptical orbits correspond to objects that are bound to the gravitational force center. These objects include planets, asteroids, and comets that move repeatedly around the Sun as well as moons orbiting a planet. There are also unbound objects, such as a meteoroid from deep space that might pass by the Sun once and then never return. The gravitational force between the Sun and these objects also varies as the inverse square of the separation distance, and the allowed paths for these objects include parabolas (e 5 1) and hyperbolas (e . 1).

Kepler’s Second Law

Mp S

Sun

S

v

Fg

.a

sw

MS

Kepler’s second law can be shown to be a result of the isolated system model for angular momentum. Consider a planet of mass Mp moving about the Sun in an elliptical orbit (Fig. 13.7a). Let’s consider the planet as a system. We model the Sun to be so much more massive than the planet that the Sun does not move. The gravitational force exerted by the Sun on the planet is a central force, always along the radius vector, directed toward the Sun (Fig. 13.7a). The torque onSthe planet due to S this central force about an axis through the Sun is zero because Fg is parallel to r . Therefore, because the external torque on the planet is zero, it is modeled as S an isolated system for angular momentum, and the angular momentum L of the planet is a constant of the motion:

r

S

S

d r v dt

w

Sun

S

w

a

w

dA

S

S

D L 5 0 S L 5 constant

S

Evaluating L for the planet, S

The area swept out by r in a time interval dt is half the area of the parallelogram. b

Figure 13.7 (a) The gravitational force acting on a planet is directed toward the Sun. (b) During a time interval dt, a parallelogram is formed by the vectors S r and d S r 5S v dt.

S

L 5S r 3S p 5 Mp S r 3S v S L 5 Mp 0 S v 0 r 3 S

(13.9)

We can relate this result to the following geometric consideration. In a time interval dt, the radius vector S r in Figure 13.7b sweeps out the area dA, which equals half the area 0 S r 0 of the parallelogram formed by the vectors S r and d S r . Because r 3 dS the displacement of the planet in the time interval dt is given by d S r 5S v dt,

dA 5 12 0 S r 3 dS r 0 5 12 0 S r 3 S v dt 0 5 12 0 S r 3 S v 0 dt

Substitute for the absolute value of the cross product from Equation 13.9:

dA 5 12 a

L bdt Mp

13.4 Kepler’s Laws and the Motion of Planets 397

Divide both sides by dt to obtain dA L 5 dt 2Mp

(13.10)

where L and Mp are both constants. This result shows that that the derivative dA/dt is constant—the radius vector from the Sun to any planet sweeps out equal areas in equal time intervals as stated in Kepler’s second law. This conclusion is a result of the gravitational force being a central force, which in turn implies that angular momentum of the planet is constant. Therefore, the law applies to any situation that involves a central force, whether inverse square or not.

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Kepler’s Third Law

Fg 5 M p a

S

GMS M p r2

v2 5 M p a b r

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Kepler’s third law can be predicted from the inverse-square law for circular orbits and our analysis models. Consider a planet of mass Mp that is assumed to be moving about the Sun (mass MS) in a circular orbit as in Figure 13.8. Because the gravitational force provides the centripetal acceleration of the planet as it moves in a circle, we model the planet as a particle under a net force and as a particle in uniform circular motion and incorporate Newton’s law of universal gravitation,

1 2pr/T 2 GMS 5 2 r r T2 5 a

Mp r MS

Figure 13.8 A planet of mass Mp

moving in a circular orbit around the Sun. The orbits of all planets except Mercury are nearly circular.

4p 2 3 br 5 K S r 3 GMS

ar ph

where K S is a constant given by

v

ys

ic

2

s. w

The orbital speed of the planet is 2pr/T, where T is the period; therefore, the preceding expression becomes

S

KS 5

4p2 5 2.97 3 10219 s2 /m3 GMS

sw

This equation is also valid for elliptical orbits if we replace r with the length a of the semimajor axis (Fig. 13.5):

.a

T2 5 a

4p2 3 ba 5 K S a 3 GMS

(13.11)

w

w

w

Equation 13.11 is Kepler’s third law: the square of the period is proportional to the cube of the semimajor axis. Because the semimajor axis of a circular orbit is its radius, this equation is valid for both circular and elliptical orbits. Notice that the constant of proportionality K S is independent of the mass of the planet.3 Equation 13.11 is therefore valid for any planet. If we were to consider the orbit of a satellite such as the Moon about the Earth, the constant would have a different value, with the Sun’s mass replaced by the Earth’s mass; that is, K E 5 4p2/GM E . Table 13.2 on page 398 is a collection of useful data for planets and other objects in the solar system. The far-right column verifies that the ratio T 2/r 3 is constant for all objects orbiting the Sun. The small variations in the values in this column are the result of uncertainties in the data measured for the periods and semimajor axes of the objects. Recent astronomical work has revealed the existence of a large number of solar system objects beyond the orbit of Neptune. In general, these objects lie in the Kuiper belt, a region that extends from about 30 AU (the orbital radius of Neptune) to 50 AU. (An AU is an astronomical unit, equal to the radius of the Earth’s orbit.) Current 3Equation

13.11 is indeed a proportion because the ratio of the two quantities T 2 and a 3 is a constant. The variables in a proportion are not required to be limited to the first power only.

WW Kepler’s third law

398 Chapter 13 Table 13.2

Universal Gravitation

Useful Planetary Data

Body Mass (kg)

Mean Radius (m)

Period of Revolution (s)

Mean Distance from the Sun (m)

T2 2 3 1 s /m 2 r3

om

Mercury 3.30 3 1023 2.44 3 106 7.60 3 106 5.79 3 1010 2.98 3 10219 Venus 4.87 3 1024 6.05 3 106 1.94 3 107 1.08 3 1011 2.99 3 10219 Earth 5.97 3 1024 6.37 3 106 3.156 3 107 1.496 3 1011 2.97 3 10219 Mars 6.42 3 1023 3.39 3 106 5.94 3 107 2.28 3 1011 2.98 3 10219 Jupiter 1.90 3 1027 6.99 3 107 3.74 3 108 7.78 3 1011 2.97 3 10219 Saturn 5.68 3 1026 5.82 3 107 9.29 3 108 1.43 3 1012 2.95 3 10219 Uranus 8.68 3 1025 2.54 3 107 2.65 3 109 2.87 3 1012 2.97 3 10219 Neptune 1.02 3 1026 2.46 3 107 5.18 3 109 4.50 3 1012 2.94 3 10219 Plutoa 1.25 3 1022 1.20 3 106 7.82 3 109 5.91 3 1012 2.96 3 10219 Moon 7.35 3 1022 1.74 3 106 — — — Sun 1.989 3 1030 6.96 3 108 — — — a In

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August 2006, the International Astronomical Union adopted a definition of a planet that separates Pluto from the other eight planets. Pluto is now defined as a “dwarf planet” like the asteroid Ceres.

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estimates identify at least 70 000 objects in this region with diameters larger than 100 km. The first Kuiper belt object (KBO) is Pluto, discovered in 1930 and formerly classified as a planet. Starting in 1992, many more have been detected. Several have diameters in the 1 000-km range, such as Varuna (discovered in 2000), Ixion (2001), Quaoar (2002), Sedna (2003), Haumea (2004), Orcus (2004), and Makemake (2005). One KBO, Eris, discovered in 2005, is believed to be significantly larger than Pluto. Other KBOs do not yet have names, but are currently indicated by their year of discovery and a code, such as 2009 YE7 and 2010 EK139. A subset of about 1 400 KBOs are called “Plutinos” because, like Pluto, they exhibit a resonance phenomenon, orbiting the Sun two times in the same time interval as Neptune revolves three times. The contemporary application of Kepler’s laws and such exotic proposals as planetary angular momentum exchange and migrating planets suggest the excitement of this active area of current research.

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Q uick Quiz 13.3 An asteroid is in a highly eccentric elliptical orbit around the Sun. The period of the asteroid’s orbit is 90 days. Which of the following statements is true about the possibility of a collision between this asteroid and the Earth? (a) There is no possible danger of a collision. (b) There is a possibility of a collision. (c) There is not enough information to determine whether there is danger of a collision.

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Example 13.4 The Mass of the Sun

w

Calculate the mass of the Sun, noting that the period of the Earth’s orbit around the Sun is 3.156 3 107 s and its distance from the Sun is 1.496 3 1011 m. Solution

Conceptualize Based on the mathematical representation of Kepler’s third law expressed in Equation 13.11, we realize that the mass of the central object in a gravitational system is related to the orbital size and period of objects in orbit around the central object. Categorize This example is a relatively simple substitution problem. Solve Equation 13.11 for the mass of the Sun:

MS 5

Substitute the known values:

MS 5

4p2r 3 GT 2 4p2 1 1.496 3 1011 m 2 3 5 1.99 3 1030 kg 1 6.674 3 10211 N # m2/kg2 2 1 3.156 3 107 s 2 2

13.4 Kepler’s Laws and the Motion of Planets 399

▸ 13.4 c o n t i n u e d In Example 13.2, an understanding of gravitational forces enabled us to find out something about the density of the Earth’s core, and now we have used this understanding to determine the mass of the Sun!

Example 13.5 A Geosynchronous Satellite AM

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Consider a satellite of mass m moving in a circular orbit around the Earth at a constant speed v and at an altitude h above the Earth’s surface as illustrated in Figure 13.9.

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(A) Determine the speed of satellite in terms of G, h, R E (the radius of the Earth), and M E (the mass of the Earth).

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Solution

Conceptualize Imagine the satellite moving around the Earth in a circular orbit

ee b

under the influence of the gravitational force. This motion is similar to that of the International Space Station, the Hubble Space Telescope, and other objects in orbit around the Earth.

s. w

Categorize The satellite moves in a circular orbit at a constant speed. Therefore, we categorize the satellite as a particle in uniform circular motion as well as a particle under a net force.

ic

Analyze The only external force acting on the satellite is the gravitational force

RE

r h S

Fg S

v

m

Figure 13.9 (Example 13.5) A satellite of mass m moving around the Earth in a circular orbit of radius r with constant speed v. The only force acting on the satelS lite is the gravitational force F g . (Not drawn to scale.)

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from the Earth, which acts toward the center of the Earth and keeps the satellite in its circular orbit. MEm v2 Apply the particle under a net force and particle in uniFg 5 ma S G 2 5 m a b r r form circular motion models to the satellite: Solve for v, noting that the distance r from the center of the Earth to the satellite is r 5 R E 1 h:

sw

(1) v 5

GME GME 5 Å r Å RE 1 h

Solution

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(B) I f the satellite is to be geosynchronous (that is, appearing to remain over a fixed position on the Earth), how fast is it moving through space?

w

w

To appear to remain over a fixed position on the Earth, the period of the satellite must be 24 h 5 86 400 s and the satellite must be in orbit directly over the equator. r5a

Substitute numerical values:

r5 c

w

Solve Kepler’s third law (Equation 13.11, with a 5 r and MS S M E ) for r :

GMET 2 1/3 b 4p2

1 6.674 3 10211 N # m2/kg 2 2 1 5.97 3 1024 kg 2 1 86 400 s 2 2 4p 2

5 4.22 3 107 m

Use Equation (1) to find the speed of the satellite:

v5

Å

1/3

d

1 6.674 3 10211 N # m2/kg 2 2 1 5.97 3 10 24 kg 2 4.22 3 107 m

5 3.07 3 103 m/s

Finalize The value of r calculated here translates to a height of the satellite above the surface of the Earth of almost 36 000 km. Therefore, geosynchronous satellites have the advantage of allowing an earthbound antenna to be aimed continued

400 Chapter 13

Universal Gravitation

▸ 13.5 c o n t i n u e d in a fixed direction, but there is a disadvantage in that the signals between the Earth and the satellite must travel a long distance. It is difficult to use geosynchronous satellites for optical observation of the Earth’s surface because of their high altitude. W hat if the satellite motion in part (A) were taking place at height h above the surface of another planet more massive than the Earth but of the same radius? Would the satellite be moving at a higher speed or a lower speed than it does around the Earth?

W h at If ?

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Answer If the planet exerts a larger gravitational force on the satellite due to its larger mass, the satellite must move with a higher speed to avoid moving toward the surface. This conclusion is consistent with the predictions of Equation (1), which shows that because the speed v is proportional to the square root of the mass of the planet, the speed increases as the mass of the planet increases.

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13.5 Gravitational Potential Energy

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A

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S

ri

DU 5 Uf 2 Ui 5 23 F 1 r 2 dr rf

(13.12)

ri

We can use this result to evaluate the general gravitational potential energy function. Consider a particle of mass m moving between two points A and B above the Earth’s surface (Fig. 13.10). The particle is subject to the gravitational force given by Equation 13.1. We can express this force as

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m Fg

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ee b

In Chapter 8, we introduced the concept of gravitational potential energy, which is the energy associated with the configuration of a system of objects interacting via the gravitational force. We emphasized that the gravitational potential energy function U 5 mgy for a particle–Earth system is valid only when the particle of mass m is near the Earth’s surface, where the gravitational force is independent of y. This expression for the gravitational potential energy is also restricted to situations where a very massive object (such as the Earth) establishes a gravitational field of magnitude g and a particle of much smaller mass m resides in that field. Because the gravitational force between two particles varies as 1/r 2, we expect that a more general potential energy function—one that is valid without the restrictions mentioned above—will be different from U 5 mgy. Recall from Equation 7.27 that the change in the potential energy of a system associated with a given displacement of a member of the system is defined as the negative of the internal work done by the force on that member during the displacement:

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GME m r2 Fg B rf where the negative sign indicates that the force is attractive. Substituting this ME expression for F(r) into Equation 13.12, we can compute the change in the gravitational potential energy function for the particle–Earth system as the separation Figure 13.10 A s a particle of distance r changes: mass m moves from A to B above S

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RE

the Earth’s surface, the gravitational potential energy of the particle–Earth system changes according to Equation 13.12.

F 1r2 5 2

Uf 2 Ui 5 GM E m 3

rf

ri

dr 1 rf 5 GM m 2 c d E r ri r2

1 1 Uf 2 Ui 5 2GM E m a 2 b rf ri

(13.13)

As always, the choice of a reference configuration for the potential energy is completely arbitrary. It is customary to choose the reference configuration for zero

13.5 Gravitational Potential Energy 401

potential energy to be the same as that for which the force is zero. Taking Ui 5 0 at ri 5 `, we obtain the important result U 1r 2 5 2

GMEm r

WW Gravitational potential energy of the Earth–particle system

(13.14)

This expression applies when the particle is separated from the center of the Earth by a distance r, provided that r $ R E . The result is not valid for particles inside the Earth, where r , R E . Because of our choice of Ui , the function U is always negative (Fig. 13.11). Although Equation 13.14 was derived for the particle–Earth system, a similar form of the equation can be applied to any two particles. That is, the gravitational potential energy associated with any pair of particles of masses m 1 and m 2 separated by a distance r is

ME

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Gm 1m 2 U52 r

U

(13.15)

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Earth

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RE

Figure 13.11 Graph of the gravitational potential energy U versus r for the system of an object above the Earth’s surface.

2

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Utotal 5 U12 1 U13 1 U23 5 2G a

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r

GME m

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This expression shows that the gravitational potential energy for any pair of particles varies as 1/r, whereas the force between them varies as 1/r 2. Furthermore, the potential energy is negative because the force is attractive and we have chosen the potential energy as zero when the particle separation is infinite. Because the force between the particles is attractive, an external agent must do positive work to increase the separation between the particles. The work done by the external agent produces an increase in the potential energy as the two particles are separated. That is, U becomes less negative as r increases. When two particles are at rest and separated by a distance r, an external agent has to supply an energy at least equal to 1Gm1m 2/r to separate the particles to an infinite distance. It is therefore convenient to think of the absolute value of the potential energy as the binding energy of the system. If the external agent supplies an energy greater than the binding energy, the excess energy of the system is in the form of kinetic energy of the particles when the particles are at an infinite separation. We can extend this concept to three or more particles. In this case, the total potential energy of the system is the sum over all pairs of particles. Each pair contributes a term of the form given by Equation 13.15. For example, if the system contains three particles as in Figure 13.12,

The potential energy goes to zero as r approaches infinity.

r 12

r 23

1 r 13

3

Figure 13.12 Three interacting particles.

m 2m 3 m 1m 2 m 1m 3 1 1 b r 12 r 13 r 23

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The absolute value of Utotal represents the work needed to separate the particles by an infinite distance.

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Example 13.6 The Change in Potential Energy A particle of mass m is displaced through a small vertical distance Dy near the Earth’s surface. Show that in this situation the general expression for the change in gravitational potential energy given by Equation 13.13 reduces to the familiar relationship DU 5 mg Dy. Solution

Conceptualize Compare the two different situations for which we have developed expressions for gravitational potential energy: (1) a planet and an object that are far apart for which the energy expression is Equation 13.14 and (2) a small object at the surface of a planet for which the energy expression is Equation 7.19. We wish to show that these two expressions are equivalent.

continued

402 Chapter 13

Universal Gravitation

▸ 13.6 c o n t i n u e d Categorize This example is a substitution problem. rf 2 ri 1 1 (1) DU 5 2GMEm a 2 b 5 GMEm a b rf ri ri rf

Combine the fractions in Equation 13.13: Evaluate rf 2 ri and rirf if both the initial and final positions of the particle are close to the Earth’s surface:

rf 2 ri 5 Dy ri rf < R E2

Substitute these expressions into Equation (1):

DU <

om

where g 5 GM E /R E 2 (Eq. 13.5).

GM E m Dy 5 mg Dy R E2

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W h at If ? Suppose you are performing upper-atmosphere studies and are asked by your supervisor to find the height in the Earth’s atmosphere at which the “surface equation” DU 5 mg Dy gives a 1.0% error in the change in the potential energy. What is this height?

g ri rf mg Dy 5 5 1.010 GME GME m 1 Dy/ri rf 2 1 GM E/R E2 2 R E 1 R E 1 Dy 2 GME

5

R E 1 Dy

ic

Substitute for ri , rf , and g from Equation 13.5:

ee b

Substitute the expressions for each of these changes DU:

DUsurface 5 1.010 DUgeneral

s. w

Set up a ratio reflecting a 1.0% error:

ly

Answer Because the surface equation assumes a constant value for g, it will give a DU value that is larger than the value given by the general equation, Equation 13.13.

RE

511

Dy RE

5 1.010

Dy 5 0.010RE 5 0.010 1 6.37 3 106 m 2 5 6.37 3 104 m 5 63.7 km

ys

Solve for Dy:

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13.6 E nergy Considerations in Planetary and Satellite Motion

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Given the general expression for gravitational potential energy developed in Section 13.5, we can now apply our energy analysis models to gravitational systems. Consider an object of mass m moving with a speed v in the vicinity of a massive object of mass M, where M .. m. The system might be a planet moving around the Sun, a satellite in orbit around the Earth, or a comet making a one-time flyby of the Sun. If we assume the object of mass M is at rest in an inertial reference frame, the total mechanical energy E of the two-object system when the objects are separated by a distance r is the sum of the kinetic energy of the object of mass m and the potential energy of the system, given by Equation 13.15:

E 5 K 1 U E 5 12 mv 2 2

GMm r

(13.16)

If the system of objects of mass m and M is isolated, and there are no nonconservative forces acting within the system, the mechanical energy of the system given by Equation 13.16 is the total energy of the system and this energy is conserved:

DE system 5 0 S DK 1 DUg 5 0 S E i 5 E f

Therefore, as the object of mass m moves from A to B in Figure 13.10, the total energy remains constant and Equation 13.16 gives

1 2 2 mv i

2

GMm GMm 5 12mv f 2 2 ri rf

(13.17)

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13.6 Energy Considerations in Planetary and Satellite Motion

Combining this statement of energy conservation with our earlier discussion of conservation of angular momentum, we see that both the total energy and the total angular momentum of a gravitationally bound, two-object system are constants of the motion. Equation 13.16 shows that E may be positive, negative, or zero, depending on the value of v. For a bound system such as the Earth–Sun system, however, E is necessarily less than zero because we have chosen the convention that U S 0 as r S `. We can easily establish that E , 0 for the system consisting of an object of mass m moving in a circular orbit about an object of mass M .. m (Fig. 13.13). Modeling the object of mass m as a particle under a net force and a particle in uniform circular motion gives GMm mv 2 5 2 r r

S

Fg 5 ma

m moving in a circular orbit about a much larger object of mass M.

GMm 2r

(13.18)

E52

GMm GMm 2 r 2r

GMm 2r

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E5

ee b

Substituting this equation into Equation 13.16, we obtain

M

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5

r

ly

1 2 2 mv

m

Figure 13.13 A n object of mass

Multiplying both sides by r and dividing by 2 gives

v

om

S

1 circular orbits 2

(13.19)

ar ph

ys

ic

This result shows that the total mechanical energy is negative in the case of circular orbits. Notice that the kinetic energy is positive and equal to half the absolute value of the potential energy. The absolute value of E is also equal to the binding energy of the system because this amount of energy must be provided to the system to move the two objects infinitely far apart. The total mechanical energy is also negative in the case of elliptical orbits. The expression for E for elliptical orbits is the same as Equation 13.19 with r replaced by the semimajor axis length a:

WW Total energy for circular orbits of an object of mass m around an object of mass M g m

GMm 2a

sw E52

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1 elliptical orbits 2

(13.20)

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w

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Q uick Quiz 13.4 A comet moves in an elliptical orbit around the Sun. Which point in its orbit (perihelion or aphelion) represents the highest value of (a) the speed of the comet, (b) the potential energy of the comet–Sun system, (c) the kinetic energy of the comet, and (d) the total energy of the comet–Sun system?

WW Total energy for elliptical orbits of an object of mass m around an object of mass M g m

Example 13.7 Changing the Orbit of a Satellite A space transportation vehicle releases a 470-kg communications satellite while in an orbit 280 km above the surface of the Earth. A rocket engine on the satellite boosts it into a geosynchronous orbit. How much energy does the engine have to provide? Solution

Conceptualize Notice that the height of 280 km is much lower than that for a geosynchronous satellite, 36 000 km, as mentioned in Example 13.5. Therefore, energy must be expended to raise the satellite to this much higher position. Categorize This example is a substitution problem. Find the initial radius of the satellite’s orbit when it is still in the vehicle’s cargo bay:

ri 5 R E 1 280 km 5 6.65 3 106 m

continued

404 Chapter 13

Universal Gravitation

▸ 13.7 c o n t i n u e d Use Equation 13.19 to find the difference in energies for the satellite–Earth system with the satellite at the initial and final radii:

DE 5 Ef 2 Ei 5 2

Substitute numerical values, using rf 5 4.22 3 107 m from Example 13.5:

DE 5 2 a

GME m GME m GME m 1 1 2 a2 b52 a 2 b rf ri 2rf 2ri 2

1 6.674 3 10211 N # m2/kg2 2 1 5.97 3 1024 kg 2 1 470 kg 2 2

3

1 1 2 b 5 1.19 3 1010 J 7 4.22 3 10 m 6.65 3 106 m

om

which is the energy equivalent of 89 gal of gasoline. NASA engineers must account for the changing mass of the spacecraft as it ejects burned fuel, something we have not done here. Would you expect the calculation that includes the effect of this changing mass to yield a greater or a lesser amount of energy required from the engine?

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Escape Speed

Suppose an object of mass m is projected vertically upward from the Earth’s surface with an initial speed vi as illustrated in Figure 13.14. We can use energy considerations to find the value of the initial speed needed to allow the object to reach a certain distance away from the center of the Earth. Equation 13.16 gives the total energy of the system for any configuration. As the object is projected upward from the surface of the Earth, v 5 vi and r 5 ri 5 R E . When the object reaches its maximum altitude, v 5 vf 5 0 and r 5 rf 5 rmax. Because the object–Earth system is isolated, we substitute these values into the isolated-system model expression given by Equation 13.17:

S

s. w

ee b

vf 0

rmax

S

vi

1 2 2 mv i

ys

m

GME m GMEm 52 rmax RE

ic

h

2

RE

w

w

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Escape speed from the Earth

Pitfall Prevention 13.3

You Can’t Really Escape Although Equation 13.22 provides the “escape speed” from the Earth, complete escape from the Earth’s gravitational influence is impossible because the gravitational force is of infinite range.

1 1 2 b rmax RE

(13.21)

For a given maximum altitude h 5 rmax 2 R E , we can use this equation to find the required initial speed. We are now in a position to calculate the escape speed, which is the minimum speed the object must have at the Earth’s surface to approach an infinite separation distance from the Earth. Traveling at this minimum speed, the object continues to move farther and farther away from the Earth as its speed asymptotically approaches zero. Letting rmax S ` in Equation 13.21 and identifying vi as vesc gives

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Figure 13.14 A n object of mass m projected upward from the Earth’s surface with an initial speed vi reaches a maximum altitude h.

v i 2 5 2GM E a

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ME

ar ph

Solving for vi2 gives

v esc 5

2GME Å RE

(13.22)

This expression for vesc is independent of the mass of the object. In other words, a spacecraft has the same escape speed as a molecule. Furthermore, the result is independent of the direction of the velocity and ignores air resistance. If the object is given an initial speed equal to vesc, the total energy of the system is equal to zero. Notice that when r S `, the object’s kinetic energy and the potential energy of the system are both zero. If vi is greater than vesc, however, the total energy of the system is greater than zero and the object has some residual kinetic energy as r S `.

Example 13.8 Escape Speed of a Rocket Calculate the escape speed from the Earth for a 5 000-kg spacecraft and determine the kinetic energy it must have at the Earth’s surface to move infinitely far away from the Earth.

13.6 Energy Considerations in Planetary and Satellite Motion

405

▸ 13.8 c o n t i n u e d Solution

Conceptualize Imagine projecting the spacecraft from the Earth’s surface so that it moves farther and farther away, traveling more and more slowly, with its speed approaching zero. Its speed will never reach zero, however, so the object will never turn around and come back. Categorize This example is a substitution problem. Use Equation 13.22 to find the escape speed:

v esc 5

2GME 2 1 6.674 3 10211 N # m2/kg 2 2 1 5.97 3 1024 kg 2 5 Å RE Å 6.37 3 106 m

om

5 1.12 3 104 m/s

K 5 12mv 2esc 5 12 1 5.00 3 103 kg 2 1 1.12 3 104 m/s 2 2

Evaluate the kinetic energy of the spacecraft from Equation 7.16:

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5 3.13 3 1011 J

W h at If ?

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The calculated escape speed corresponds to about 25 000 mi/h. The kinetic energy of the spacecraft is equivalent to the energy released by the combustion of about 2 300 gal of gasoline. What if you want to launch a 1 000-kg spacecraft at the escape speed? How much energy would that

require?

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Answer In Equation 13.22, the mass of the object moving with the escape speed does not appear. Therefore, the escape speed for the 1 000-kg spacecraft is the same as that for the 5 000-kg spacecraft. The only change in the kinetic energy is due to the mass, so the 1 000-kg spacecraft requires one-fifth of the energy of the 5 000-kg spacecraft: K 5 15 1 3.13 3 1011 J 2 5 6.25 3 1010 J

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Equations 13.21 and 13.22 can be applied to objects projected from any planet. That is, in general, the escape speed from the surface of any planet of mass M and radius R is

v esc 5

2GM Å R

(13.23)

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Escape speeds for the planets, the Moon, and the Sun are provided in Table 13.3. The values vary from 2.3 km/s for the Moon to about 618 km/s for the Sun. These results, together with some ideas from the kinetic theory of gases (see Chapter 21), explain why some planets have atmospheres and others do not. As we shall see later, at a given temperature the average kinetic energy of a gas molecule depends only on the mass of the molecule. Lighter molecules, such as hydrogen and helium, have a higher average speed than heavier molecules at the same temperature. When the average speed of the lighter molecules is not much less than the escape speed of a planet, a significant fraction of them have a chance to escape. This mechanism also explains why the Earth does not retain hydrogen molecules and helium atoms in its atmosphere but does retain heavier molecules, such as oxygen and nitrogen. On the other hand, the very large escape speed for Jupiter enables that planet to retain hydrogen, the primary constituent of its atmosphere.

WW Escape speed from the surface of a planet of mass M and radius R

Black Holes In Example 11.7, we briefly described a rare event called a supernova, the catastrophic explosion of a very massive star. The material that remains in the central core of such an object continues to collapse, and the core’s ultimate fate depends on its mass. If the core has a mass less than 1.4 times the mass of our Sun, it gradually cools down and ends its life as a white dwarf star. If the core’s mass is greater than this value, however, it may collapse further due to gravitational forces. What

Table 13.3 Escape Speeds from the Surfaces of the Planets, Moon, and Sun Planet

vesc (km/s)

Mercury 4.3 Venus 10.3 Earth 11.2 Mars 5.0 Jupiter 60 Saturn 36 Uranus 22 Neptune 24 Moon 2.3 Sun 618

Universal Gravitation

remains is a neutron star, discussed in Example 11.7, in which the mass of a star is compressed to a radius of about 10 km. (On the Earth, a teaspoon of this material would weigh about 5 billion tons!) An even more unusual star death may occur when the core has a mass greater than about three solar masses. The collapse may continue until the star becomes a very small object in space, commonly referred to as a black hole. In effect, black holes are remains of stars that have collapsed under their own gravitational force. If an object such as a spacecraft comes close to a black hole, the object experiences an extremely strong gravitational force and is trapped forever. The escape speed for a black hole is very high because of the concentration of the star’s mass into a sphere of very small radius (see Eq. 13.23). If the escape speed exceeds the speed of light c, radiation from the object (such as visible light) cannot escape and the object appears to be black (hence the origin of the terminology “black hole”). The critical radius R S at which the escape speed is c is called the Schwarzschild radius (Fig. 13.15). The imaginary surface of a sphere of this radius surrounding the black hole is called the event horizon, which is the limit of how close you can approach the black hole and hope to escape. There is evidence that supermassive black holes exist at the centers of galaxies, with masses very much larger than the Sun. (There is strong evidence of a supermassive black hole of mass 2–3 million solar masses at the center of our galaxy.)

Event horizon

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406 Chapter 13

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Dark Matter

Equation (1) in Example 13.5 shows that the speed of an object in orbit around the Earth decreases as the object is moved farther away from the Earth:

Black hole

v5

ic

ys

RS

Figure 13.15 A black hole. The

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Using data in Table 13.2 to find the speeds of planets in their orbits around the Sun, we find the same behavior for the planets. Figure 13.16 shows this behavior for the eight planets of our solar system. The theoretical prediction of the planet speed as a function of distance from the Sun is shown by the red-brown curve, using Equation 13.24 with the mass of the Earth replaced by the mass of the Sun. Data for the individual planets lie right on this curve. This behavior results from the vast majority of the mass of the solar system being concentrated in a small space, i.e., the Sun. Extending this concept further, we might expect the same behavior in a galaxy. Much of the visible galactic mass, including that of a supermassive black hole, is near the central core of a galaxy. The opening photograph for this chapter shows the central core of the Whirlpool galaxy as a very bright area surrounded by the “arms” of the galaxy, which contain material in orbit around the central core. Based on this distribution of matter in the galaxy, the speed of an object in the outer part of the galaxy would be smaller than that for objects closer to the center, just like for the planets of the solar system. That is not what is observed, however. Figure 13.17 shows the results of measurements of the speeds of objects in the Andromeda galaxy as a function of distance from the galaxy’s center.4 The red-brown curve shows the expected speeds for these objects if they were traveling in circular orbits around the mass concentrated in the central core. The data for the individual objects in the galaxy shown by the black dots are all well above the theoretical curve. These data, as well as an extensive amount of data taken over the past half century, show that for objects outside the central core of the galaxy, the curve of speed versus distance from the center of the galaxy is approximately flat rather than decreasing at larger distances. Therefore, these objects (including our own Solar System in the Milky Way) are rotating faster than can be accounted for by gravity due to the visible galaxy! This surprising

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distance R S equals the Schwarzschild radius.

(13.24)

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Any event occurring within the event horizon is invisible to an outside observer.

GME Å r

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v (km/s) Mercury 40 Venus Earth Mars Jupiter 20 Saturn Uranus Neptune r (1012 m) 2 4 0

Figure 13.16 The orbital speed v as a function of distance r from the Sun for the eight planets of the solar system. The theoretical curve is in red-brown, and the data points for the planets are in black.

4V.

C. Rubin and W. K. Ford, “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions,” Astrophysical Journal 159: 379–403 (1970).

Summary 407 v (km/s) Central 600 core 400 200 0

20

40

60

r (1019 m) 80

Figure 13.17 The orbital speed v of a galaxy object as a function of distance r from the center of the central core of the Andromeda galaxy. The theoretical curve is in red-brown, and the data points for the galaxy objects are in black. No data are provided on the left because the behavior inside the central core of the galaxy is more complicated.

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result means that there must be additional mass in a more extended distribution, causing these objects to orbit so fast, and has led scientists to propose the existence of dark matter. This matter is proposed to exist in a large halo around each galaxy (with a radius up to 10 times as large as the visible galaxy’s radius). Because it is not luminous (i.e., does not emit electromagnetic radiation) it must be either very cold or electrically neutral. Therefore, we cannot “see” dark matter, except through its gravitational effects. The proposed existence of dark matter is also implied by earlier observations made on larger gravitationally bound structures known as galaxy clusters. 5 These observations show that the orbital speeds of galaxies in a cluster are, on average, too large to be explained by the luminous matter in the cluster alone. The speeds of the individual galaxies are so high, they suggest that there is 50 times as much dark matter in galaxy clusters as in the galaxies themselves! Why doesn’t dark matter affect the orbital speeds of planets like it does those of a galaxy? It seems that a solar system is too small a structure to contain enough dark matter to affect the behavior of orbital speeds. A galaxy or galaxy cluster, on the other hand, contains huge amounts of dark matter, resulting in the surprising behavior. What, though, is dark matter? At this time, no one knows. One theory claims that dark matter is based on a particle called a weakly interacting massive particle, or WIMP. If this theory is correct, calculations show that about 200 WIMPs pass through a human body at any given time. The new Large Hadron Collider in Europe (see Chapter 46) is the first particle accelerator with enough energy to possibly generate and detect the existence of WIMPs, which has generated much current interest in dark matter. Keeping an eye on this research in the future should be exciting.

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Summary Definitions

S

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The gravitational field at a point in space is defined as the gravitational force Fg experienced by any test particle located at that point divided by the mass m 0 of the test particle: g ;

Fg

m0

(13.7)

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S S

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Concepts and Principles

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Newton’s law of universal gravitation states that the gravitational force of attraction between any two particles of masses m1 and m 2 separated by a distance r has the magnitude m 1m 2 Fg 5 G (13.1) r2

where G 5 6.674 3 10211 N ? m2/kg2 is the universal gravitational constant. This equation enables us to calculate the force of attraction between masses under many circumstances.

5F.

An object at a distance h above the Earth’s surface experiences a gravitational force of magnitude mg, where g is the free-fall acceleration at that elevation:

g5

GME GME 5 2 1 RE 1 h 2 2 r

(13.6)

In this expression, M E is the mass of the Earth and R E is its radius. Therefore, the weight of an object decreases as the object moves away from the Earth’s surface.

Zwicky, “On the Masses of Nebulae and of Clusters of Nebulae,” Astrophysical Journal 86: 217–246 (1937).

Universal Gravitation

Kepler’s laws of planetary motion state:

If an isolated system consists of an object of mass m moving with a speed v in the vicinity of a massive object of mass M, the total energy E of the system is the sum of the kinetic and potential energies:

1. All planets move in elliptical orbits with the Sun at one focus. 2. The radius vector drawn from the Sun to a planet sweeps out equal areas in equal time intervals. 3. The square of the orbital period of any planet is proportional to the cube of the semimajor axis of the elliptical orbit.

GMm (13.16) r The total energy of the system is a constant of the motion. If the object moves in an elliptical orbit of semimajor axis a around the massive object and M .. m, the total energy of the system is

T2 5 a

4p 2 3 ba GMS

(13.11)

GMm (13.20) 2a For a circular orbit, this same equation applies with a 5 r.

ly The escape speed for an object projected from the surface of a planet of mass M and radius R is

The gravitational potential energy associated with a system of two particles of mass m 1 and m 2 separated by a distance r is

v esc 5

s. w

G m 1m 2 r where U is taken to be zero as r S `.

(13.15)

2GM Å R

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Analysis Model for Problem Solving

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Particle in a Field (Gravitational) A source particle with some mass establishes a gravitational field S g throughout space. When a particle of mass m is placed in that field, it experiences a gravitational force given by S

g Fg 5 mS

(5.5)

S

g

m

S

S

Fg m g

w

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(13.23)

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U52

E52

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where MS is the mass of the Sun and a is the semimajor axis. For a circular orbit, a can be replaced in Equation 13.11 by the radius r. Most planets have nearly circular orbits around the Sun.

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Kepler’s third law can be expressed as

E 5 12mv 2 2

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408 Chapter 13

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Objective Questions

1. denotes answer available in Student Solutions Manual/Study Guide

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1. A system consists of five particles. How many terms appear in the expression for the total gravitational potential energy of the system? (a) 4 (b) 5 (c) 10 (d) 20 (e) 25 2. Rank the following quantities of energy from largest to smallest. State if any are equal. (a) the absolute value of the average potential energy of the Sun–Earth system (b) the average kinetic energy of the Earth in its orbital motion relative to the Sun (c) the absolute value of the total energy of the Sun–Earth system 3. A satellite moves in a circular orbit at a constant speed around the Earth. Which of the following statements is

true? (a) No force acts on the satellite. (b) The satellite moves at constant speed and hence doesn’t accelerate. (c) The satellite has an acceleration directed away from the Earth. (d) The satellite has an acceleration directed toward the Earth. (e) Work is done on the satellite by the gravitational force. 4. Suppose the gravitational acceleration at the surface of a certain moon A of Jupiter is 2 m/s2. Moon B has twice the mass and twice the radius of moon A. What is the gravitational acceleration at its surface? Neglect the gravitational acceleration due to Jupiter. (a) 8 m/s2 (b) 4 m/s2 (c) 2 m/s2 (d) 1 m/s2 (e) 0.5 m/s2

Conceptual Questions

5. Imagine that nitrogen and other atmospheric gases were more soluble in water so that the atmosphere of the Earth is entirely absorbed by the oceans. Atmospheric pressure would then be zero, and outer space would start at the planet’s surface. Would the Earth then have a gravitational field? (a) Yes, and at the surface it would be larger in magnitude than 9.8 N/kg. (b) Yes, and it would be essentially the same as the current value. (c) Yes, and it would be somewhat less than 9.8 N/kg. (d) Yes, and it would be much less than 9.8 N/kg. (e) No, it would not.

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9. Rank the magnitudes of the following gravitational forces from largest to smallest. If two forces are equal, show their equality in your list. (a) the force exerted by a 2-kg object on a 3-kg object 1 m away (b) the force exerted by a 2-kg object on a 9-kg object 1 m away (c) the force exerted by a 2-kg object on a 9-kg object 2 m away (d) the force exerted by a 9-kg object on a 2-kg object 2 m away (e) the force exerted by a 4-kg object on another 4-kg object 2 m away 10. The gravitational force exerted on an astronaut on the Earth’s surface is 650 N directed downward. When she is in the space station in orbit around the Earth, is the gravitational force on her (a) larger, (b) exactly the same, (c) smaller, (d) nearly but not exactly zero, or (e) exactly zero?

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7. A satellite originally moves in a circular orbit of radius R around the Earth. Suppose it is moved into a circular orbit of radius 4R. (i) What does the force exerted on the satellite then become? (a) eight times larger (b) four times larger (c) one-half as large (d) oneeighth as large (e) one-sixteenth as large (ii) What happens to the satellite’s speed? Choose from the same possibilities (a) through (e). (iii) What happens to its period? Choose from the same possibilities (a) through (e).

September (autumnal) equinox (which contains the summer solstice) longer than the interval from the September to the March equinox rather than being equal to that interval? Choose one of the following reasons. (a) They are really the same, but the Earth spins faster during the “summer” interval, so the days are shorter. (b) Over the “summer” interval, the Earth moves slower because it is farther from the Sun. (c) Over the March-to-September interval, the Earth moves slower because it is closer to the Sun. (d) The Earth has less kinetic energy when it is warmer. (e) The Earth has less orbital angular momentum when it is warmer.

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6. An object of mass m is located on the surface of a spherical planet of mass M and radius R. The escape speed from the planet does not depend on which of the following? (a) M (b) m (c) the density of the planet (d) R (e) the acceleration due to gravity on that planet

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11. Halley’s comet has a period of approximately 76 years, and it moves in an elliptical orbit in which its distance from the Sun at closest approach is a small fraction of its maximum distance. Estimate the comet’s maximum distance from the Sun in astronomical units (AUs) (the distance from the Earth to the Sun). (a) 6 AU (b) 12 AU (c) 20 AU (d) 28 AU (e) 35 AU

1. denotes answer available in Student Solutions Manual/Study Guide

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Conceptual Questions

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8. The vernal equinox and the autumnal equinox are associated with two points 180° apart in the Earth’s orbit. That is, the Earth is on precisely opposite sides of the Sun when it passes through these two points. From the vernal equinox, 185.4 days elapse before the autumnal equinox. Only 179.8 days elapse from the autumnal equinox until the next vernal equinox. Why is the interval from the March (vernal) to the

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1. Each Voyager spacecraft was accelerated toward escape speed from the Sun by the gravitational force exerted by Jupiter on the spacecraft. (a) Is the gravitational force a conservative or a nonconservative force? (b) Does the interaction of the spacecraft with Jupiter meet the definition of an elastic collision? (c) How could the spacecraft be moving faster after the collision? 2. In his 1798 experiment, Cavendish was said to have “weighed the Earth.” Explain this statement. 3. Why don’t we put a geosynchronous weather satellite in orbit around the 45th parallel? Wouldn’t such a satellite be more useful in the United States than one in orbit around the equator? 4. (a) Explain why the force exerted on a particle by a uniform sphere must be directed toward the center of the sphere. (b) Would this statement be true if the mass distribution of the sphere were not spherically symmetric? Explain.

5. (a) At what position in its elliptical orbit is the speed of a planet a maximum? (b) At what position is the speed a minimum? 6. You are given the mass and radius of planet X. How would you calculate the free-fall acceleration on this planet’s surface? 7. (a) If a hole could be dug to the center of the Earth, would the force on an object of mass m still obey Equation 13.1 there? (b) What do you think the force on m would be at the center of the Earth? 8. Explain why it takes more fuel for a spacecraft to travel from the Earth to the Moon than for the return trip. Estimate the difference. 9. A satellite in low-Earth orbit is not truly traveling through a vacuum. Rather, it moves through very thin air. Does the resulting air friction cause the satellite to slow down?

410 Chapter 13

Universal Gravitation

Problems AMT Analysis Model tutorial available in

The problems found in this chapter may be assigned online in Enhanced WebAssign

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GP Guided Problem M Master It tutorial available in Enhanced WebAssign

1. straightforward; 2. intermediate; 3. challenging

W Watch It video solution available in Enhanced WebAssign

1. full solution available in the Student Solutions Manual/Study Guide

BIO Q/C

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Section 13.1 Newton’s Law of Universal Gravitation

magnitude of the gravitational force exerted by one particle on the other?

2. Determine the order of magnitude of the gravitational force that you exert on another person 2 m away. In your solution, state the quantities you measure or estimate and their values.

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9. Two objects attract each other with a gravitational 28 W force of magnitude 1.00 3 10 N when separated by 20.0 cm. If the total mass of the two objects is 5.00 kg, what is the mass of each? 10. Review. A student proposes to study the gravitational force by suspending two 100.0-kg spherical objects at the lower ends of cables from the ceiling of a tall cathedral and measuring the deflection of the cables from the vertical. The 45.00-m-long cables are attached to the ceiling 1.000 m apart. The first object is suspended, and its position is carefully measured. The second object is suspended, and the two objects attract each other gravitationally. By what distance has the first object moved horizontally from its initial position due to the gravitational attraction to the other object? Suggestion: Keep in mind that this distance will be very small and make appropriate approximations.

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3. A 200-kg object and a 500-kg object are separated by W 4.00 m. (a) Find the net gravitational force exerted by these objects on a 50.0-kg object placed midway between them. (b) At what position (other than an infinitely remote one) can the 50.0-kg object be placed so as to experience a net force of zero from the other two objects?

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1. In introductory physics laboratories, a typical CavenM dish balance for measuring the gravitational constant G uses lead spheres with masses of 1.50 kg and 15.0 g whose centers are separated by about 4.50 cm. Calculate the gravitational force between these spheres, treating each as a particle located at the sphere’s center.

8. Why is the following situation impossible? The centers of two homogeneous spheres are 1.00 m apart. The spheres are each made of the same element from the periodic table. The gravitational force between the spheres is 1.00 N.

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Problem 12 in Chapter 1 can also be assigned with this section.

4. During a solar eclipse, the Moon, the Earth, and the

Q/C Sun all lie on the same line, with the Moon between

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the Earth and the Sun. (a) What force is exerted by the Sun on the Moon? (b) What force is exerted by the Earth on the Moon? (c) What force is exerted by the Sun on the Earth? (d) Compare the answers to parts (a) and (b). Why doesn’t the Sun capture the Moon away from the Earth?

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5. Two ocean liners, each with a mass of 40 000 metric tons, are moving on parallel courses 100 m apart. What is the magnitude of the acceleration of one of the liners toward the other due to their mutual gravitational attraction? Model the ships as particles. 6. Three uniform spheres of W masses m1 5 2.00 kg, m 2 5 4.00 kg, and m 3 5 6.00 kg are placed at the corners of a right triangle as shown in Figure P13.6. Calculate the resultant gravitational force on the object of mass m 2, assuming the spheres are isolated from the rest of the Universe.

y (0, 3.00) m

m1 S

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(– 4.00, 0) m

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Figure P13.6

7. Two identical isolated particles, each of mass 2.00 kg, are separated by a distance of 30.0 cm. What is the

Section 13.2 Free-Fall Acceleration and the Gravitational Force 11. When a falling meteoroid is at a distance above the M Earth’s surface of 3.00 times the Earth’s radius, what is its acceleration due to the Earth’s gravitation? 12. The free-fall acceleration on the surface of the Moon W is about one-sixth that on the surface of the Earth. The radius of the Moon is about 0.250R E (R E 5 Earth’s radius 5 6.37 3 106 m). Find the ratio of their average densities, rMoon/rEarth. 13. Review. Miranda, a satellite of Uranus, is shown in Figure P13.13a. It can be modeled as a sphere of radius 242 km and mass 6.68 3 1019 kg. (a) Find the free-fall acceleration on its surface. (b) A cliff on Miranda is 5.00 km high. It appears on the limb at the 11 o’clock position in Figure P13.13a and is magnified in Figure P13.13b. If a devotee of extreme sports runs horizontally off the top of the cliff at 8.50 m/s, for what time interval is he in flight? (c) How far from the base of the vertical cliff does he strike the icy surface of Miranda? (d) What will be his vector impact velocity?

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Problems

tional fields acting on the occupants in the nose of the ship and on those in the rear of the ship, farthest from the black hole? (This difference in accelerations grows rapidly as the ship approaches the black hole. It puts the body of the ship under extreme tension and eventually tears it apart.) Black hole

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Figure P13.16 Section 13.4 Kepler’s Laws and the Motion of Planets

Section 13.3 Analysis Model: Particle in a Field (Gravitational)

18. Io, a satellite of Jupiter, has an orbital period of 1.77 days and an orbital radius of 4.22 3 105 km. From these data, determine the mass of Jupiter. 19. A minimum-energy transfer orbit to an outer planet

Q/C consists of putting a spacecraft on an elliptical trajec-

tory with the departure planet corresponding to the perihelion of the ellipse, or the closest point to the Sun, and the arrival planet at the aphelion, or the farthest point from the Sun. (a) Use Kepler’s third law to calculate how long it would take to go from Earth to Mars on such an orbit as shown in Figure P13.19. (b) Can such an orbit be undertaken at any time? Explain.

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14. (a) Compute the vector gravitational field at a point P Q/C on the perpendicular bisector of the line joining two S objects of equal mass separated by a distance 2a as shown in Figure P13.14. (b) Explain physically why the field should approach zero as r S 0. (c) Prove mathematically that the answer to part (a) behaves in this way. (d) Explain physically why the magnitude of the field should approach 2GM/r 2 as r S `. (e) Prove mathematically that the answer to part (a) behaves correctly in this limit.

17. An artificial satellite circles the Earth in a circular orbit at a location where the acceleration due to gravity is 9.00 m/s2. Determine the orbital period of the satellite.

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Figure P13.13

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Transfer orbit Arrival at Mars

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Launch from the Earth

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Mars orbit

Figure P13.14

Earth orbit

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15. Three objects of equal mass are located at three corS ners of a square of edge length , as shown in Figure P13.15. Find the magnitude and direction of the gravitational field at the fourth corner due to these objects.

Figure P13.19

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20. A particle of mass m moves along a straight line with

m

Q/C constant velocity S v 0 in the x direction, a distance b from S the x axis (Fig. P13.20). (a) Does the particle possess any

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Figure P13.15

angular momentum about the origin? (b) Explain why the amount of its angular momentum should change or should stay constant. (c) Show that Kepler’s second law is satisfied by showing that the two shaded triangles in the figure have the same area when t D 2 t C 5 t B 2 t A.

16. A spacecraft in the shape of a long cylinder has a length

AMT of 100 m, and its mass with occupants is 1 000 kg. W It has strayed too close to a black hole having a mass

100 times that of the Sun (Fig. P13.16). The nose of the spacecraft points toward the black hole, and the distance between the nose and the center of the black hole is 10.0 km. (a) Determine the total force on the spacecraft. (b) What is the difference in the gravita-

y S

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B

C

D

b x

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Figure P13.20

412 Chapter 13

Universal Gravitation

21. Plaskett’s binary system consists of two stars that revolve M in a circular orbit about a center of mass midway between them. This statement implies that the masses of the two stars are equal (Fig. P13.21). Assume the orbital speed v 0 5 220 km/s and the orbital period of each star is 0 S of each is 14.4 days. Find the mass M of each star. (For comparison, the mass of our Sun is 1.99 3 1030 kg.)

25.

S

26.

M

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Figure P13.21

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between the center of the Earth and the center of the Moon is 3.84 3 108 m, use Equation 13.11 to calculate the mass of the Earth. (b) Why is the value you calculate a bit too large?

29. Suppose the Sun’s gravity were switched off. The plan-

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2 8. (a) Given that the period of the Moon’s orbit about the

Q/C Earth is 27.32 days and the nearly constant distance

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22. Two planets X and Y travel counterclockwise in circular orbits about a star as shown in Figure P13.22. The radii of their orbits are in the ratio 3:1. At one moment, they are aligned as shown in Figure P13.22a, making a straight line with the star. During the next five years, the angular displacement of planet X is 90.0° as shown in Figure P13.22b. What is the angular displacement of planet Y at this moment?

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orbit parameters: perigee, 459 km; apogee, 2 289 km (both distances above the Earth’s surface); period, 112.7 min. Find the ratio vp /va of the speed at perigee to that at apogee. Use Kepler’s third law to determine how many days it takes a spacecraft to travel in an elliptical orbit from a point 6 670 km from the Earth’s center to the Moon, 385 000 km from the Earth’s center. Neutron stars are extremely dense objects formed from the remnants of supernova explosions. Many rotate very rapidly. Suppose the mass of a certain spherical neutron star is twice the mass of the Sun and its radius is 10.0 km. Determine the greatest possible angular speed it can have so that the matter at the surface of the star on its equator is just held in orbit by the gravitational force. A synchronous satellite, which always remains above the same point on a planet’s equator, is put in orbit around Jupiter to study that planet’s famous red spot. Jupiter rotates once every 9.84 h. Use the data of Table 13.2 to find the altitude of the satellite above the surface of the planet.

Q/C ets would leave their orbits and fly away in straight lines

a

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Figure P13.22

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23. Comet Halley (Fig. P13.23) approaches the Sun to W within 0.570 AU, and its orbital period is 75.6 yr. (AU is the symbol for astronomical unit, where 1 AU 5 1.50 3 1011 m is the mean Earth–Sun distance.) How far from the Sun will Halley’s comet travel before it starts its return journey?

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as described by Newton’s first law. (a) Would Mercury ever be farther from the Sun than Pluto? (b) If so, find how long it would take Mercury to achieve this passage. If not, give a convincing argument that Pluto is always farther from the Sun than is Mercury.

Section 13.5 Gravitational Potential Energy Note: In Problems 30 through 50, assume U 5 0 at r 5 `.

30. A satellite in Earth orbit has a mass of 100 kg and is 6 W at an altitude of 2.00 3 10 m. (a) What is the potential energy of the satellite–Earth system? (b) What is the magnitude of the gravitational force exerted by the Earth on the satellite? (c) What If? What force, if any, does the satellite exert on the Earth? 31. How much work is done by the Moon’s gravitational field on a 1 000-kg meteor as it comes in from outer space and impacts on the Moon’s surface?

0.570 AU

x 2a

Figure P13.23 (Orbit is not drawn to scale.)

24. The Explorer VIII satellite, placed into orbit November 3, 1960, to investigate the ionosphere, had the following

32. How much energy is required to move a 1 000-kg object from the Earth’s surface to an altitude twice the Earth’s radius? 33. After the Sun exhausts its nuclear fuel, its ultimate fate will be to collapse to a white dwarf state. In this state, it would have approximately the same mass as it has now, but its radius would be equal to the radius of the Earth. Calculate (a) the average density of the white dwarf, (b) the surface free-fall acceleration, and (c) the

413

Problems gravitational potential energy associated with a 1.00-kg object at the surface of the white dwarf.

sphere will produce only a beautiful meteor shower. The astronaut finds that the density of the spherical asteroid is equal to the average density of the Earth. To ensure its pulverization, she incorporates into the explosives the rocket fuel and oxidizer intended for her return journey. What maximum radius can the asteroid have for her to be able to leave it entirely simply by jumping straight up? On Earth she can jump to a height of 0.500 m.

34. An object is released from rest at an altitude h above the surface of the Earth. (a) Show that its speed at a distance r from the Earth’s center, where R E # r # R E 1 h, is

Dt 5 3 dt 5 2 3 f

i

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f

dr v

to find the time of fall as the object moves from the release point to the Earth’s surface. The negative sign appears because the object is moving opposite to the radial direction, so its speed is v 5 2dr/dt. Perform the integral numerically. 35. A system consists of three particles, each of mass 5.00 g,

Q/C located at the corners of an equilateral triangle with

4 4. (a) What is the minimum speed, relative to the Sun, necessary for a spacecraft to escape the solar system if it starts at the Earth’s orbit? (b) Voyager 1 achieved a maximum speed of 125 000 km/h on its way to photograph Jupiter. Beyond what distance from the Sun is this speed sufficient to escape the solar system? 45. A satellite of mass 200 kg is placed into Earth orbit W at a height of 200 km above the surface. (a) Assuming a circular orbit, how long does the satellite take to complete one orbit? (b) What is the satellite’s speed? (c) Starting from the satellite on the Earth’s surface, what is the minimum energy input necessary to place this satellite in orbit? Ignore air resistance but include the effect of the planet’s daily rotation.

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Section 13.6 Energy Considerations in Planetary and Satellite Motion

43. (a) Determine the amount of work that must be done on a 100-kg payload to elevate it to a height of 1 000 km above the Earth’s surface. (b) Determine the amount of additional work that is required to put the payload into circular orbit at this elevation.

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sides of 30.0 cm. (a) Calculate the potential energy of the system. (b) Assume the particles are released simultaneously. Describe the subsequent motion of each. Will any collisions take place? Explain.

42. Derive an expression for the work required to move an S Earth satellite of mass m from a circular orbit of radius 2R E to one of radius 3R E .

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(b) Assume the release altitude is 500 km. Perform the integral

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1 1 2GM E a 2 b r RE 1 h

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36. A space probe is fired as a projectile from the Earth’s

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4 AMT surface with an initial speed of 2.00 3 10 m/s. What will M its speed be when it is very far from the Earth? Ignore

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37. A 500-kg satellite is in a circular orbit at an altitude of 500 km above the Earth’s surface. Because of air friction, the satellite eventually falls to the Earth’s surface, where it hits the ground with a speed of 2.00 km/s. How much energy was transformed into internal energy by means of air friction?

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38. A “treetop satellite” moves in a circular orbit just above S the surface of a planet, assumed to offer no air resistance. Show that its orbital speed v and the escape speed from the planet are related by the expression v esc 5 !2v.

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39. A 1 000-kg satellite orbits the Earth at a constant altiW tude of 100 km. (a) How much energy must be added Q/C to the system to move the satellite into a circular orbit with altitude 200 km? What are the changes in the system’s (b) kinetic energy and (c) potential energy?

40. A comet of mass 1.20 3 1010 kg moves in an elliptical orbit around the Sun. Its distance from the Sun ranges between 0.500 AU and 50.0 AU. (a) What is the eccentricity of its orbit? (b) What is its period? (c) At aphelion, what is the potential energy of the comet–Sun system? Note: 1 AU 5 one astronomical unit 5 the average distance from the Sun to the Earth 5 1.496 3 1011 m. 41. An asteroid is on a collision course with Earth. An astronaut lands on the rock to bury explosive charges that will blow the asteroid apart. Most of the small fragments will miss the Earth, and those that fall into the atmo-

46. A satellite of mass m, originally on the surface of S the Earth, is placed into Earth orbit at an altitude h. (a) Assuming a circular orbit, how long does the satellite take to complete one orbit? (b) What is the satellite’s speed? (c) What is the minimum energy input necessary to place this satellite in orbit? Ignore air resistance but include the effect of the planet’s daily rotation. Represent the mass and radius of the Earth as M E and R E , respectively. 47. Ganymede is the largest of Jupiter’s moons. Consider

Q/C a rocket on the surface of Ganymede, at the point far-

thest from the planet (Fig. P13.47). Model the rocket as a particle. (a) Does the presence of Ganymede make Jupiter exert a larger, smaller, or same size force on the rocket compared with the force it would exert if Ganymede were not interposed? (b) Determine the escape speed for the rocket from the planet–satellite system. The radius of Ganymede is 2.64 3 106 m, and its mass

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Ganymede Jupiter

Figure P13.47

Universal Gravitation

is 1.495 3 1023 kg. The distance between Jupiter and Ganymede is 1.071 3 109 m, and the mass of Jupiter is 1.90 3 1027 kg. Ignore the motion of Jupiter and Ganymede as they revolve about their center of mass. 48. A satellite moves around the Earth in a circular orbit S of radius r. (a) What is the speed vi of the satellite? (b) Suddenly, an explosion breaks the satellite into two pieces, with masses m and 4m. Immediately after the explosion, the smaller piece of mass m is stationary with respect to the Earth and falls directly toward the Earth. What is the speed v of the larger piece immediately after the explosion? (c) Because of the increase in its speed, this larger piece now moves in a new elliptical orbit. Find its distance away from the center of the Earth when it reaches the other end of the ellipse.

around the hub axis, which is connected to the rest of the spacecraft to generate artificial gravity in the cabins. A space traveler lies in a bed parallel to the outer wall as shown in Figure P13.56. (a) With r 5 10.0 m, what would the angular speed of the 60.0-kg traveler need to be if he is to experience half his normal Earth weight? (b) If the astronaut stands up perpendicular to the bed, without holding on to anything with his hands, will his head be moving at a faster, a slower, or the same tangential speed as his feet? Why? (c) Why is the action in part (b) dangerous?

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49. At the Earth’s surface, a projectile is launched straight up at a speed of 10.0 km/s. To what height will it rise? Ignore air resistance.

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Figure P13.56

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250 km when traveling at 6.00 km/s. (a) What maximum distance from the Earth’s surface does it travel before falling back to the Earth? (b) Would its maximum distance from the surface be larger if the same rocket were fired with the same fuel load from a launch site on the equator? Why or why not?

57. (a) A space vehicle is launched vertically upward from the Earth’s surface with an initial speed of 8.76 km/s, which is less than the escape speed of 11.2 km/s. What maximum height does it attain? (b) A meteoroid falls toward the Earth. It is essentially at rest with respect to the Earth when it is at a height of 2.51 3 107 m above the Earth’s surface. With what speed does the meteorite (a meteoroid that survives to impact the Earth’s surface) strike the Earth?

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50. A rocket is fired straight up through the atmosphere

Q/C from the South Pole, burning out at an altitude of

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Additional Problems

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51. Review. A cylindrical habitat in space 6.00 km in diamW eter and 30.0 km long has been proposed (by G. K. O’Neill, 1974). Such a habitat would have cities, land, and lakes on the inside surface and air and clouds in the center. They would all be held in place by rotation of the cylinder about its long axis. How fast would the cylinder have to rotate to imitate the Earth’s gravitational field at the walls of the cylinder?

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52. Voyager 1 and Voyager 2 surveyed the surface of Jupiter’s moon Io and photographed active volcanoes spewing liquid sulfur to heights of 70 km above the surface of this moon. Find the speed with which the liquid sulfur left the volcano. Io’s mass is 8.9 3 1022 kg, and its radius is 1 820 km.

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53. A satellite is in a circular orbit around the Earth at an M altitude of 2.80 3 10 6 m. Find (a) the period of the orbit, (b) the speed of the satellite, and (c) the acceleration of the satellite. 54. Why is the following situation impossible? A spacecraft is launched into a circular orbit around the Earth and circles the Earth once an hour. 55. Let DgM represent the difference in the gravitational fields produced by the Moon at the points on the Earth’s surface nearest to and farthest from the Moon. Find the fraction DgM /g, where g is the Earth’s gravitational field. (This difference is responsible for the occurrence of the lunar tides on the Earth.) 56. A sleeping area for a long space voyage consists of two

Q/C cabins each connected by a cable to a central hub as

shown in Figure P13.56. The cabins are set spinning

58. (a) A space vehicle is launched vertically upward from S the Earth’s surface with an initial speed of vi that is comparable to but less than the escape speed vesc. What maximum height does it attain? (b) A meteoroid falls toward the Earth. It is essentially at rest with respect to the Earth when it is at a height h above the Earth’s surface. With what speed does the meteorite (a meteoroid that survives to impact the Earth’s surface) strike the Earth? (c) What If? Assume a baseball is tossed up with an initial speed that is very small compared to the escape speed. Show that the result from part (a) is consistent with Equation 4.12.

59. Assume you are agile enough to run across a horizon-

Q/C tal surface at 8.50 m/s, independently of the value of

the gravitational field. What would be (a) the radius and (b) the mass of an airless spherical asteroid of uniform density 1.10 3 103 kg/m3 on which you could launch yourself into orbit by running? (c) What would be your period? (d) Would your running significantly affect the rotation of the asteroid? Explain.

60. Two spheres having masses M and 2M and radii R and GP 3R, respectively, are simultaneously released from S rest when the distance between their centers is 12R. Assume the two spheres interact only with each other and we wish to find the speeds with which they collide. (a) What two isolated system models are appropriate for this system? (b) Write an equation from one of the modv 1, the velocity of the sphere of mass els and solve it for S v 2, the velocM at any time after release in terms of S

Problems

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potential energy of the object–ring system when the object is at A. (b) Calculate the gravitational potential energy of the system when the object is at B. (c) Calculate the speed of the object as it passes through B.

61. Two hypothetical planets of masses m1 and m 2 and AMT radii r 1 and r 2, respectively, are nearly at rest when they M are an infinite distance apart. Because of their gravitational attraction, they head toward each other on a collision course. (a) When their center-to-center separation is d, find expressions for the speed of each planet and for their relative speed. (b) Find the kinetic energy of each planet just before they collide, taking m1 5 2.00 3 1024 kg, m 2 5 8.00 3 1024 kg, r 1 5 3.00 3 106 m, and r 2 5 5.00 3 106 m. Note: Both the energy and momentum of the isolated two-planet system are constant.

6 4. A spacecraft of mass 1.00 3 104 kg is in a circular orbit at an altitude of 500 km above the Earth’s surface. Mission Control wants to fire the engines in a direction tangent to the orbit so as to put the spacecraft in an elliptical orbit around the Earth with an apogee of 2.00 3 104 km, measured from the Earth’s center. How much energy must be used from the fuel to achieve this orbit? (Assume that all the fuel energy goes into increasing the orbital energy. This model will give a lower limit to the required energy because some of the energy from the fuel will appear as internal energy in the hot exhaust gases and engine parts.) 65. Review. As an astronaut, you observe a small planet AMT to be spherical. After landing on the planet, you set off, walking always straight ahead, and find yourself returning to your spacecraft from the opposite side after completing a lap of 25.0 km. You hold a hammer and a falcon feather at a height of 1.40 m, release them, and observe that they fall together to the surface in 29.2 s. Determine the mass of the planet.

dr

52

2GM E R E3

2GM E h R E3

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(c) Evaluate this difference for h 5 6.00 m, a typical height for a two-story building.

NASA

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63. A ring of matter is a familiar structure in planetary and stellar astronomy. Examples include Saturn’s rings and a ring nebula. Consider a uniform ring of mass 2.36 3 1020 kg and radius 1.00 3 108 m. An object of mass 1 000 kg is placed at a point A on the axis of the ring, 2.00 3 108 m from the center of the ring (Fig. P13.63). When the object is released, the attraction of the ring makes the object move along the axis toward the center of the ring (point B). (a) Calculate the gravitational

B

A

Figure P13.63

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66. A certain quaternary star system consists of three stars, S each of mass m, moving in the same circular orbit of radius r about a central star of mass M. The stars orbit in the same sense and are positioned one-third of a revolution apart from one another. Show that the period of each of the three stars is given by

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0 Dg 0 5

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This rate of change with position is called a gradient. (b) Assuming h is small in comparison to the radius of the Earth, show that the difference in free-fall acceleration between two points separated by vertical distance h is

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dg

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62. (a) Show that the rate of change of the free-fall acceleration with vertical position near the Earth’s surface is

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ity of 2M. (c) Write an equation from the other model and solve it for speed v1 in terms of speed v 2 when the spheres collide. (d) Combine the two equations to find the two speeds v1 and v 2 when the spheres collide.

T 5 2p

r3 Å G 1 M 1 m/"3 2

67. Studies of the relationship of the Sun to our galaxy— the Milky Way—have revealed that the Sun is located near the outer edge of the galactic disc, about 30 000 ly (1 ly 5 9.46 3 1015 m) from the center. The Sun has an orbital speed of approximately 250 km/s around the galactic center. (a) What is the period of the Sun’s galactic motion? (b) What is the order of magnitude of the mass of the Milky Way galaxy? (c) Suppose the galaxy is made mostly of stars of which the Sun is typical. What is the order of magnitude of the number of stars in the Milky Way? 68. Review. Two identical hard spheres, each of mass m S and radius r, are released from rest in otherwise empty space with their centers separated by the distance R. They are allowed to collide under the influence of their gravitational attraction. (a) Show that the magnitude of the impulse received by each sphere before they make contact is given by [Gm 3(1/2r 2 1/R)]1/2. (b) What If? Find the magnitude of the impulse each receives during their contact if they collide elastically. 69. The maximum distance from the Earth to the Sun (at aphelion) is 1.521 3 1011 m, and the distance of closest approach (at perihelion) is 1.471 3 1011 m. The Earth’s orbital speed at perihelion is 3.027 3 104 m/s. Determine (a) the Earth’s orbital speed at aphelion and the kinetic and potential energies of the Earth–Sun system

Universal Gravitation

71. X-ray pulses from Cygnus X-1, the first black hole to be identified and a celestial x-ray source, have been recorded during high-altitude rocket flights. The signals can be interpreted as originating when a blob of ionized matter orbits a black hole with a period of 5.0 ms. If the blob is in a circular orbit about a black hole whose mass is 20M Sun, what is the orbit radius?

3p ÅGr

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independent of the planet’s radius.

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73. Astronomers detect a distant meteoroid moving along S a straight line that, if extended, would pass at a distance 3R E from the center of the Earth, where R E is the Earth’s radius. What minimum speed must the meteoroid have if it is not to collide with the Earth? m

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74. Two stars of masses M and S m, separated by a distance d, revolve in circular orbits about their center of mass (Fig. P13.74). Show that each star has a period given by

S

v1

CM

S

M

v2

r1 d

r2

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4p 2d 3 T 5 G 1M 1 m 2 2

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78. The Solar and Heliospheric Observatory (SOHO) spacecraft has a special orbit, located between the Earth and the Sun along the line joining them, and it is always close enough to the Earth to transmit data easily. Both objects exert gravitational forces on the observatory. It moves around the Sun in a near-circular orbit that is smaller than the Earth’s circular orbit. Its period, however, is not less than 1 yr but just equal to 1 yr. Show that its distance from the Earth must be 1.48 3 109 m. In 1772, Joseph Louis Lagrange determined theoretically the special location allowing this orbit. Suggestions: Use data that are precise to four digits. The mass of the Earth is 5.974 3 1024 kg. You will not be able to easily solve the equation you generate; instead, use a computer to verify that 1.48 3 109 m is the correct value.

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Tmin 5

Challenge Problems

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72. Show that the minimum period for a satellite in orbit S around a spherical planet of uniform density r is

77. As thermonuclear fusion proceeds in its core, the Sun loses mass at a rate of 3.64 3 109 kg/s. During the 5 000-yr period of recorded history, by how much has the length of the year changed due to the loss of mass from the Sun? Suggestions: Assume the Earth’s orbit is circular. No external torque acts on the Earth–Sun system, so the angular momentum of the Earth is constant.

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can, however, actually be responsible for making the object speed up. Consider a 100-kg Earth satellite in a circular orbit at an altitude of 200 km. A small force of air resistance makes the satellite drop into a circular orbit with an altitude of 100 km. (a) Calculate the satellite’s initial speed. (b) Calculate its final speed in this process. (c) Calculate the initial energy of the satellite–Earth system. (d) Calculate the final energy of the system. (e) Show that the system has lost mechanical energy and find the amount of the loss due to friction. (f) What force makes the satellite’s speed increase? Hint: You will find a free-body diagram useful in explaining your answer.

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70. Many people assume air resistance acting on a mov-

Q/C ing object will always make the object slow down. It

particles, isolated from the rest of the Universe. (a) Find the magnitude of the acceleration a rel with which each starts to move relative to the other as a function of m. Evaluate the acceleration (b) for m 5 5.00 kg, (c) for m 5 2 000 kg, and (d) for m 5 2.00 3 1024 kg. (e) Describe the pattern of variation of a rel with m.

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(b) at perihelion and (c) at aphelion. (d) Is the total energy of the system constant? Explain. Ignore the effect of the Moon and other planets.

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416 Chapter 13

75. Two identical particles, each Figure P13.74 of mass 1 000 kg, are coasting in free space along the same path, one in front of the other by 20.0 m. At the instant their separation distance has this value, each particle has precisely the same velocity of 800 i^ m/s. What are their precise velocities when they are 2.00 m apart? 76. Consider an object of mass m, not necessarily small compared with the mass of the Earth, released at a distance of 1.20 3 107 m from the center of the Earth. Assume the Earth and the object behave as a pair of

79. The oldest artificial satellite still in orbit is Vanguard I, launched March 3, 1958. Its mass is 1.60 kg. Neglecting atmospheric drag, the satellite would still be in its initial orbit, with a minimum distance from the center of the Earth of 7.02 Mm and a speed at this perigee point of 8.23 km/s. For this orbit, find (a) the total energy of the satellite–Earth system and (b) the magnitude of the angular momentum of the satellite. (c) At apogee, find the satellite’s speed and its distance from the center of the Earth. (d) Find the semimajor axis of its orbit. (e) Determine its period. 80. A spacecraft is approaching Mars after a long trip from the Earth. Its velocity is such that it is traveling along a parabolic trajectory under the influence of the gravitational force from Mars. The distance of closest approach will be 300 km above the Martian surface. At this point of closest approach, the engines will be fired to slow down the spacecraft and place it in a circular orbit 300 km above the surface. (a) By what percentage must the speed of the spacecraft be reduced to achieve the desired orbit? (b) How would the answer to part (a) change if the distance of closest approach and the desired circular orbit altitude were 600 km instead of 300 km? (Note: The energy of the spacecraft–Mars system for a parabolic orbit is E 5 0.)

c h a p t e r

14

Fluid Mechanics

14.1 Pressure

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14.2 Variation of Pressure with Depth 14.3 Pressure Measurements

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14.4 Buoyant Forces and Archimedes’s Principle

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14.5 Fluid Dynamics 14.7 Other Applications of Fluid Dynamics

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14.6 Bernoulli’s Equation

Fish congregate around a reef in Hawaii searching for food. How do fish such as the lined butterflyfish (Chaetodon lineolatus) at the upper left control their movements up and down in the water? We’ll find out in this chapter. (Vlad61/Shutterstock.com)

At any point on the surface of the object, the force exerted by the fluid is perpendicular to the surface of the object.

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Matter is normally classified as being in one of three states: solid, liquid, or gas. From everyday experience we know that a solid has a definite volume and shape, a liquid has a definite volume but no definite shape, and an unconfined gas has neither a definite volume nor a definite shape. These descriptions help us picture the states of matter, but they are somewhat artificial. For example, asphalt and plastics are normally considered solids, but over long time intervals they tend to flow like liquids. Likewise, most substances can be a solid, a liquid, or a gas (or a combination of any of these three), depending on the temperature and pressure. In general, the time interval required for a particular substance to change its shape in response to an external force determines whether we treat the substance as a solid, a liquid, or a gas. A fluid is a collection of molecules that are randomly arranged and held together by weak cohesive forces and by forces exerted by the walls of a container. Both liquids and gases are fluids. In our treatment of the mechanics of fluids, we’ll be applying principles and analysis models that we have already discussed. First, we consider the mechanics of a fluid at rest, that is, fluid statics, and then study fluids in motion, that is, fluid dynamics.

14.1 Pressure Fluids do not sustain shearing stresses or tensile stresses such as those discussed in Chapter 12; therefore, the only stress that can be exerted on an object submerged in a static fluid is one that tends to compress the object from all sides. In other words, the force exerted by a static fluid on an object is always perpendicular to the surfaces of the object as shown in Figure 14.1. We discussed this situation in Section 12.4.

Figure 14.1 The forces exerted by a fluid on the surfaces of a submerged object.

417

418 Chapter 14

Fluid Mechanics

The pressure in a fluid can be measured with the device pictured in Figure 14.2. The device consists of an evacuated cylinder that encloses a light piston connected to a spring. As the device is submerged in a fluid, the fluid presses on the top of the piston and compresses the spring until the inward force exerted by the fluid is balanced by the outward force exerted by the spring. The fluid pressure can be measured directly if the spring is calibrated in advance. If F is the magnitude of the force exerted on the piston and A is the surface area of the piston, the pressure P of the fluid at the level to which the device has been submerged is defined as the ratio of the force to the area: F P; (14.1) A

A S

F

Vacuum

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Figure 14.2 A simple device for measuring the pressure exerted by a fluid.

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Pressure is a scalar quantity because it is proportional to the magnitude of the force on the piston. If the pressure varies over an area, the infinitesimal force dF on an infinitesimal surface element of area dA is dF 5 P dA (14.2)

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where P is the pressure at the location of the area dA. To calculate the total force exerted on a surface of a container, we must integrate Equation 14.2 over the surface. The units of pressure are newtons per square meter (N/m2) in the SI system. Another name for the SI unit of pressure is the pascal (Pa): 1 Pa ; 1 N/m2

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(14.3)

For a tactile demonstration of the definition of pressure, hold a tack between your thumb and forefinger, with the point of the tack on your thumb and the head of the tack on your forefinger. Now gently press your thumb and forefinger together. Your thumb will begin to feel pain immediately while your forefinger will not. The tack is exerting the same force on both your thumb and forefinger, but the pressure on your thumb is much larger because of the small area over which the force is applied.

Pitfall Prevention 14.1

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Force and Pressure Equations 14.1 and 14.2 make a clear distinction between force and pressure. Another important distinction is that force is a vector and pressure is a scalar. There is no direction associated with pressure, but the direction of the force associated with the pressure is perpendicular to the surface on which the pressure acts.

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Q uick Quiz 14.1 Suppose you are standing directly behind someone who steps back and accidentally stomps on your foot with the heel of one shoe. Would you be better off if that person were (a) a large, male professional basketball player wearing sneakers or (b) a petite woman wearing spike-heeled shoes?

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Example 14.1 The Water Bed

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The mattress of a water bed is 2.00 m long by 2.00 m wide and 30.0 cm deep.

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(A) Find the weight of the water in the mattress. Solution

Conceptualize Think about carrying a jug of water and how heavy it is. Now imagine a sample of water the size of a water bed. We expect the weight to be relatively large. Categorize This example is a substitution problem. Find the volume of the water filling the mattress:

V 5 (2.00 m)(2.00 m)(0.300 m) 5 1.20 m3

Use Equation 1.1 and the density of fresh water (see Table 14.1) to find the mass of the water bed:

M 5 rV 5 (1 000 kg/m3)(1.20 m3) 5 1.20 3 103 kg

Find the weight of the bed:

Mg 5 (1.20 3 103 kg)(9.80 m/s2) 5 1.18 3 104 N

which is approximately 2 650 lb. (A regular bed, including mattress, box spring, and metal frame, weighs approximately 300 lb.) Because this load is so great, it is best to place a water bed in the basement or on a sturdy, well- supported floor.

14.2 Variation of Pressure with Depth 419

▸ 14.1 c o n t i n u e d (B) Find the pressure exerted by the water bed on the floor when the bed rests in its normal position. Assume the entire lower surface of the bed makes contact with the floor. Solution

When the water bed is in its normal position, the area in contact with the floor is 4.00 m2. Use Equation 14.1 to find the pressure:

P5

1.18 3 104 N 5 2.94 3 103 Pa 4.00 m2

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W h at If ? W hat if the water bed is replaced by a 300-lb regular bed that is supported by four legs? Each leg has a circular cross section of radius 2.00 cm. What pressure does this bed exert on the floor?

Answer The weight of the regular bed is distributed over four circular cross sections at the bottom of the legs. There-

5 2.65 3 105 Pa

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mg F 300 lb 1N 5 5 a b A 4 1 pr 2 2 4p 1 0.020 0 m 2 2 0.225 lb

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P5

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fore, the pressure is

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This result is almost 100 times larger than the pressure due to the water bed! The weight of the regular bed, even though it is much less than the weight of the water bed, is applied over the very small area of the four legs. The high pressure on the floor at the feet of a regular bed could cause dents in wood floors or permanently crush carpet pile.

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14.2 Variation of Pressure with Depth

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As divers well know, water pressure increases with depth. Likewise, atmospheric pressure decreases with increasing altitude; for this reason, aircraft flying at high altitudes must have pressurized cabins for the comfort of the passengers. We now show how the pressure in a liquid increases with depth. As Equation 1.1 describes, the density of a substance is defined as its mass per unit volume; Table 14.1 lists the densities of various substances. These values vary slightly with temperature because the volume of a substance is dependent on temperature (as shown in Chapter 19). Under standard conditions (at 08C and at atmospheric pressure), the 1 densities of gases are about 1 000 the densities of solids and liquids. This difference in densities implies that the average molecular spacing in a gas under these conditions is about ten times greater than that in a solid or liquid.

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Table 14.1 Densities of Some Common Substances at Standard Temperature (08C) and Pressure (Atmospheric)

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Substance

r (kg/m 3) Substance

r (kg/m3)

Air 1.29 Iron 7.86 3 103 Air (at 20°C and Lead 11.3 3 103 atmospheric pressure) 1.20 Mercury 13.6 3 103 3 Aluminum 2.70 3 10 Nitrogen gas 1.25 Benzene 0.879 3 103 Oak 0.710 3 103 Brass 8.4 3 103 Osmium 22.6 3 103 3 Copper 8.92 3 10 Oxygen gas 1.43 Ethyl alcohol 0.806 3 103 Pine 0.373 3 103 Fresh water 1.00 3 103 Platinum 21.4 3 103 Glycerin 1.26 3 103 Seawater 1.03 3 103 3 Gold 19.3 3 10 Silver 10.5 3 103 21 Helium gas 1.79 3 10 Tin 7.30 3 103 Hydrogen gas 8.99 3 1022 Uranium 19.1 3 103 Ice 0.917 3 103

420 Chapter 14

Fluid Mechanics

d dh

Mg ˆj

PAˆj

^ ^ ^ a F 5 PA j 2 P0 A j 2 Mg j 5 0 S

or

PA 2 P 0A 2 rAhg 5 0

P 5 P 0 1 rgh

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Variation of pressure with depth

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P 0 5 1.00 atm 5 1.013 3 105 Pa

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Equation 14.4 implies that the pressure is the same at all points having the same depth, independent of the shape of the container. Because the pressure in a fluid depends on depth and on the value of P 0, any increase in pressure at the surface must be transmitted to every other point in the fluid. This concept was first recognized by French scientist Blaise Pascal (1623– 1662) and is called Pascal’s law: a change in the pressure applied to a fluid is transmitted undiminished to every point of the fluid and to the walls of the container. An important application of Pascal’s law is the hydraulic press illustrated in Figure 14.4a. A force of magnitude F 1 is applied to a small piston of surface area A1. The pressure is transmitted through an incompressible liquid to a larger piston of surface area A 2. Because the pressure must be the same on both sides, P 5 F 1/A1 5 F 2/A 2. Therefore, the force F 2 is greater than the force F 1 by a factor of A 2/A1. By designing a hydraulic press with appropriate areas A1 and A 2, a large outBecause the increase in pressure is the same on the two sides, a small S force F1 at the left produces a much greater S force F2 at the right.

S

x 1

F1

A1

x 2

A2 S

F2

Figure 14.4 (a) Diagram of a

Sam Jordash/Digital Vision/Getty Images

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Pascal’s law

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(14.4)

That is, the pressure P at a depth h below a point in the liquid at which the pressure is P 0 is greater by an amount rgh. If the liquid is open to the atmosphere and P 0 is the pressure at the surface of the liquid, then P 0 is atmospheric pressure. In our calculations and working of end-of-chapter problems, we usually take atmospheric pressure to be

a hydraulic press. (b) A vehicle undergoing repair is supported by a hydraulic lift in a garage.

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Figure 14.3 A parcel of fluid in a larger volume of fluid is singled out.

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P0A ˆj

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Now consider a liquid of density r at rest as shown in Figure 14.3. We assume r is uniform throughout the liquid, which means the liquid is incompressible. Let us select a parcel of the liquid contained within an imaginary block of cross-sectional area A extending from depth d to depth d 1 h. The liquid external to our parcel exerts forces at all points on the surface of the parcel, perpendicular to the surface. The pressure exerted by the liquid on the bottom face of the parcel is P, and the pressure on the top face is P 0. Therefore, the upward force exerted by the outside fluid on the bottom of the parcel has a magnitude PA, and the downward force exerted on the top has a magnitude P 0 A. The mass of liquid in the parcel is M 5 rV 5 rAh; therefore, the weight of the liquid in the parcel is Mg 5 rAhg. Because the parcel is at rest and remains at rest, it can be modeled as a particle in equilibrium, so that the net force acting on it must be zero. Choosing upward to be the positive y direction, we see that

The parcel of fluid is in equilibrium, so the net force on it is zero.

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14.2 Variation of Pressure with Depth 421

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put force can be applied by means of a small input force. Hydraulic brakes, car lifts, hydraulic jacks, and forklifts all make use of this principle (Fig. 14.4b). Because liquid is neither added to nor removed from the system, the volume of liquid pushed down on the left in Figure 14.4a as the piston moves downward through a displacement Dx1 equals the volume of liquid pushed up on the right as the right piston moves upward through a displacement Dx 2. That is, A1 Dx1 5 A2 Dx 2; therefore, A 2 /A1 5 Dx1/Dx 2. We have already shown that A2 /A1 5 F 2 /F 1. Therefore, F 2 /F 1 5 Dx1/Dx 2, so F 1 Dx1 5 F 2 Dx 2. Each side of this equationSis the work done by the force on its respective piston. Therefore, the work done by F 1 on the input piston equals S the work done by F 2 on the output piston, as it must to conserve energy. (The process can be modeled as a special case of the nonisolated system model: the nonisolated system in steady state. There is energy transfer into and out of the system, but these energy transfers balance, so that there is no net change in the energy of the system.)

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Q uick Quiz 14.2 The pressure at the bottom of a filled glass of water (r 5 1 000 kg/m3) is P. The water is poured out, and the glass is filled with ethyl alcohol (r 5 806 kg/m3). What is the pressure at the bottom of the glass? (a) smaller than P (b) equal to P (c) larger than P (d) indeterminate

Example 14.2 The Car Lift

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In a car lift used in a service station, compressed air exerts a force on a small piston that has a circular cross section of radius 5.00 cm. This pressure is transmitted by a liquid to a piston that has a radius of 15.0 cm.

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(A) What force must the compressed air exert to lift a car weighing 13 300 N?

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Solution

Conceptualize Review the material just discussed about Pascal’s law to understand the operation of a car lift.

F1 5 a

A1 p 1 5.00 3 1022 m 2 2 1 1.33 3 104 N 2 b F2 5 A2 p 1 15.0 3 1022 m 2 2

5 1.48 3 103 N

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Solve F 1/A1 5 F 2/A 2 for F 1:

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Categorize This example is a substitution problem.

Solution

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(B) What air pressure produces this force?

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Use Equation 14.1 to find the air pressure that produces this force:

P5

F1 1.48 3 103 N 5 A1 p 1 5.00 3 1022 m 2 2

5 1.88 3 105 Pa

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This pressure is approximately twice atmospheric pressure.

Example 14.3 A Pain in Your Ear Estimate the force exerted on your eardrum due to the water when you are swimming at the bottom of a pool that is 5.0 m deep. Solution

Conceptualize A s you descend in the water, the pressure increases. You may have noticed this increased pressure in your ears while diving in a swimming pool, a lake, or the ocean. We can find the pressure difference exerted on the continued

422 Chapter 14

Fluid Mechanics

▸ 14.3 c o n t i n u e d eardrum from the depth given in the problem; then, after estimating the ear drum’s surface area, we can determine the net force the water exerts on it.

Categorize This example is a substitution problem. The air inside the middle ear is normally at atmospheric pressure P 0. Therefore, to find the net force on the eardrum, we must consider the difference between the total pressure at the bottom of the pool and atmospheric pressure. Let’s estimate the surface area of the eardrum to be approximately 1 cm2 5 1 3 1024 m2. Use Equation 14.4 to find this pressure difference:

P bot 2 P 0 5 rgh

Use Equation 14.1 to find the magnitude of the net force on the ear:

F 5 (P bot 2 P 0)A 5 (4.9 3 104 Pa)(1 3 1024 m2) < 5 N

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5 (1.00 3 103 kg/m3)(9.80 m/s2)(5.0 m) 5 4.9 3 104 Pa

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Because a force of this magnitude on the eardrum is extremely uncomfortable, swimmers often “pop their ears” while under water, an action that pushes air from the lungs into the middle ear. Using this technique equalizes the pressure on the two sides of the eardrum and relieves the discomfort.

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Example 14.4 The Force on a Dam

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Solution

y

h

Conceptualize Because pressure varies with depth, we cannot calculate the

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force simply by multiplying the area by the pressure. As the pressure in the water increases with depth, the force on the adjacent portion of the dam also increases. dy

Categorize Because of the variation of pressure with depth, we must use integration to solve this example, so we categorize it as an analysis problem.

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Analyze Let’s imagine a vertical y axis, with y 5 0 at the bottom of the dam. We divide the face of the dam into narrow horizontal strips at a distance y above the bottom, such as the red strip in Figure 14.5. The pressure on each such strip is due only to the water; atmospheric pressure acts on both sides of the dam.

y O

x

Figure 14.5 (Example 14.4) Water exerts a force on a dam.

P 5 rgh 5 rg(H 2 y)

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Use Equation 14.4 to calculate the pressure due to the water at the depth h:

dF 5 P dA 5 rg(H 2 y)w dy

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Use Equation 14.2 to find the force exerted on the shaded strip of area dA 5 w dy:

F 5 3 P dA 5 3 rg 1 H 2 y 2 w dy 5 H

Integrate to find the total force on the dam:

0

1 2 2 rg wH

Finalize Notice that the thickness of the dam shown in Figure 14.5 increases with depth. This design accounts for the greater force the water exerts on the dam at greater depths. W h at If ?

What if you were asked to find this force without using calculus? How could you determine its value?

Answer We know from Equation 14.4 that pressure varies linearly with depth. Therefore, the average pressure due to the water over the face of the dam is the average of the pressure at the top and the pressure at the bottom: Pavg 5

Ptop 1 Pbottom 2

5

0 1 rgH 2

5 12 rgH

14.4 Buoyant Forces and Archimedes’s Principle 423

▸ 14.4 c o n t i n u e d The total force on the dam is equal to the product of the average pressure and the area of the face of the dam: F 5 Pavg A 5 1 12 rgH 2 1 Hw 2 5 12 rg wH 2

which is the same result we obtained using calculus.

14.3 Pressure Measurements

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During the weather report on a television news program, the barometric pressure is often provided. This reading is the current local pressure of the atmosphere, which varies over a small range from the standard value provided earlier. How is this pressure measured? One instrument used to measure atmospheric pressure is the common barometer, invented by Evangelista Torricelli (1608–1647). A long tube closed at one end is filled with mercury and then inverted into a dish of mercury (Fig. 14.6a). The closed end of the tube is nearly a vacuum, so the pressure at the top of the mercury column can be taken as zero. In Figure 14.6a, the pressure at point A, due to the column of mercury, must equal the pressure at point B, due to the atmosphere. If that were not the case, there would be a net force that would move mercury from one point to the other until equilibrium is established. Therefore, P 0 5 rHg gh, where rHg is the density of the mercury and h is the height of the mercury column. As atmospheric pressure varies, the height of the mercury column varies, so the height can be calibrated to measure atmospheric pressure. Let us determine the height of a mercury column for one atmosphere of pressure, P 0 5 1 atm 5 1.013 3 105 Pa:

P0 A B

a P0

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P0 5 r Hg gh

P0 1.013 3 105 Pa 5 0.760 m 5 r Hg g 1 13.6 3 103 kg/m3 2 1 9.80 m/s2 2

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Based on such a calculation, one atmosphere of pressure is defined to be the pressure equivalent of a column of mercury that is exactly 0.760 0 m in height at 08C. A device for measuring the pressure of a gas contained in a vessel is the opentube manometer illustrated in Figure 14.6b. One end of a U-shaped tube containing a liquid is open to the atmosphere, and the other end is connected to a container of gas at pressure P. In an equilibrium situation, the pressures at points A and B must be the same (otherwise, the curved portion of the liquid would experience a net force and would accelerate), and the pressure at A is the unknown pressure of the gas. Therefore, equating the unknown pressure P to the pressure at point B, we see that P 5 P 0 1 rgh. Again, we can calibrate the height h to the pressure P. The difference in the pressures in each part of Figure 14.6 (that is, P 2 P 0) is equal to rgh. The pressure P is called the absolute pressure, and the difference P 2 P 0 is called the gauge pressure. For example, the pressure you measure in your bicycle tire is gauge pressure. Q uick Quiz 14.3 Several common barometers are built, with a variety of fluids. For which of the following fluids will the column of fluid in the barometer be the highest? (a) mercury (b) water (c) ethyl alcohol (d) benzene

14.4 Buoyant Forces and Archimedes’s Principle Have you ever tried to push a beach ball down under water (Fig. 14.7a, p. 424)? It is extremely difficult to do because of the large upward force exerted by the water on the ball. The upward force exerted by a fluid on any immersed object is called

h

P

B

b

Figure 14.6 Two devices for measuring pressure: (a) a mercury barometer and (b) an open-tube manometer.

424 Chapter 14

Fluid Mechanics

S

The buoyant force B on a beach ball that replaces this parcel of water is exactly the same as the buoyant force on the parcel.

S

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Fg

a

b

Figure 14.7 (a) A swimmer pushes a beach ball under water. (b) The forces on a beach ball–sized

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B

parcel of water.

Archimedes

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Archimedes was perhaps the greatest scientist of antiquity. He was the first to compute accurately the ratio of a circle’s circumference to its diameter, and he also showed how to calculate the volume and surface area of spheres, cylinders, and other geometric shapes. He is well known for discovering the nature of the buoyant force and was also a gifted inventor. One of his practical inventions, still in use today, is Archimedes’s screw, an inclined, rotating, coiled tube used originally to lift water from the holds of ships. He also invented the catapult and devised systems of levers, pulleys, and weights for raising heavy loads. Such inventions were successfully used to defend his native city, Syracuse, during a two-year siege by Romans.

The buoyant force on the cube is the resultant of the forces exerted on its top and bottom faces by the liquid.

a buoyant force. We can determine the magnitude of a buoyant force by applying some logic. Imagine a beach ball–sized parcel of water beneath the water surface as in Figure 14.7b. Because this parcel is in equilibrium, there must be an upward force that balances the downward gravitational force on the parcel. This upward force is the buoyant force, and its magnitude is equal to the weight of the water in the parcel. The buoyant force is the resultant force on the parcel due to all forces applied by the fluid surrounding the parcel. Now imagine replacing the beach ball–sized parcel of water with a beach ball of the same size. The net force applied by the fluid surrounding the beach ball is the same, regardless of whether it is applied to a beach ball or to a parcel of water. Consequently, the magnitude of the buoyant force on an object always equals the weight of the fluid displaced by the object. This statement is known as Archimedes’s principle. With the beach ball under water, the buoyant force, equal to the weight of a beach ball–sized parcel of water, is much larger than the weight of the beach ball. Therefore, there is a large net upward force, which explains why it is so hard to hold the beach ball under the water. Note that Archimedes’s principle does not refer to the makeup of the object experiencing the buoyant force. The object’s composition is not a factor in the buoyant force because the buoyant force is exerted by the surrounding fluid. To better understand the origin of the buoyant force, consider a cube of solid material immersed in a liquid as in Figure 14.8. According to Equation 14.4, the pressure P bot at the bottom of the cube is greater than the pressure Ptop at the top by an amount rfluid gh, where h is the height of the cube and rfluid is the density of the fluid. The pressure at the bottom of the cube causes an upward force equal to P bot A, where A is the area of the bottom face. The pressure at the top of the cube causes a downward force equal to PtopA. The resultant of these two forces is the S buoyant force B with magnitude

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Greek Mathematician, Physicist, and Engineer (c. 287–212 BC)

S

w

B

h

B 5 (P bot 2 Ptop)A 5 (rfluid gh)A

B 5 rfluid gVdisp

(14.5)

S

Fg

Figure 14.8 The external forces acting on an immersed cube are S the gravitational force Fg and the S buoyant force B .

where Vdisp 5 Ah is the volume of the fluid displaced by the cube. Because the product rfluidVdisp is equal to the mass of fluid displaced by the object,

B 5 Mg

where Mg is the weight of the fluid displaced by the cube. This result is consistent with our initial statement about Archimedes’s principle above, based on the discussion of the beach ball. Under normal conditions, the weight of a fish in the opening photograph for this chapter is slightly greater than the buoyant force on the fish. Hence, the fish would sink if it did not have some mechanism for adjusting the buoyant force. The

14.4 Buoyant Forces and Archimedes’s Principle 425 Pitfall Prevention 14.2 Buoyant Force Is Exerted by the Fluid Remember that the buoyant force is exerted by the fluid. It is not determined by properties of the object except for the amount of fluid displaced by the object. Therefore, if several objects of different densities but the same volume are immersed in a fluid, they will all experience the same buoyant force. Whether they sink or float is determined by the relationship between the buoyant force and the gravitational force.

Vobj

5

robj

ys

Vdisp

ic

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ee b

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fish accomplishes that by internally regulating the size of its air-filled swim bladder to increase its volume and the magnitude of the buoyant force acting on it, according to Equation 14.5. In this manner, fish are able to swim to various depths. Before we proceed with a few examples, it is instructive to discuss two common situations: a totally submerged object and a floating (partly submerged) object. Case 1: Totally Submerged Object When an object is totally submerged in a fluid of density rfluid, the volume Vdisp of the displaced fluid is equal to the volume Vobj of the object; so, from Equation 14.5, the magnitude of the upward buoyant force is B 5 rfluid gVobj. If the object has a mass M and density robj, its weight is equal to Fg 5 Mg 5 robj gVobj, and the net force on the object is B 2 Fg 5 (rfluid 2 robj)gVobj. Hence, if the density of the object is less than the density of the fluid, the downward gravitational force is less than the buoyant force and the unsupported object accelerates upward (Fig. 14.9a). If the density of the object is greater than the density of the fluid, the upward buoyant force is less than the downward gravitational force and the unsupported object sinks (Fig. 14.9b). If the density of the submerged object equals the density of the fluid, the net force on the object is zero and the object remains in equilibrium. Therefore, the direction of motion of an object submerged in a fluid is determined only by the densities of the object and the fluid. Case 2: Floating Object Now consider an object of volume Vobj and density robj , rfluid in static equilibrium floating on the surface of a fluid, that is, an object that is only partially submerged (Fig. 14.10). In this case, the upward buoyant force is balanced by the downward gravitational force acting on the object. If Vdisp is the volume of the fluid displaced by the object (this volume is the same as the volume of that part of the object beneath the surface of the fluid), the buoyant force has a magnitude B 5 rfluid gVdisp. Because the weight of the object is Fg 5 Mg 5 robj gVobj and because Fg 5 B, we see that rfluid gVdisp 5 robj gVobj, or rfluid

(14.6)

ar ph

This equation shows that the fraction of the volume of a floating object that is below the fluid surface is equal to the ratio of the density of the object to that of the fluid.

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sw

Q uick Quiz 14.4 You are shipwrecked and floating in the middle of the ocean on a raft. Your cargo on the raft includes a treasure chest full of gold that you found before your ship sank, and the raft is just barely afloat. To keep you floating as high as possible in the water, should you (a) leave the treasure chest on top of the raft, (b) secure the treasure chest to the underside of the raft, or (c) hang the treasure chest in the water with a rope attached to the raft? (Assume throwing the treasure chest overboard is not an option you wish to consider.)

robj rfluid

robj rfluid

S

B S

a

a

S

S

B

a

S

Fg

Because the object floats in equilibrium, B Fg . S

B S

Fg

b

Figure 14.9 (a) A totally submerged object that is less dense than the fluid in which it is submerged experiences a net upward force and rises to the surface after it is released. (b) A totally submerged object that is denser than the fluid experiences a net downward force and sinks.

S

Fg

Figure 14.10 An object floating on the surface of a fluid experiences two forces, S the gravitational force F g and the buoyant S force B .

426 Chapter 14

Fluid Mechanics

Example 14.5 Eureka! AM Archimedes supposedly was asked to determine whether a crown made for the king consisted of pure gold. According to legend, he solved this problem by weighing the crown first in air and then in water as shown in Figure 14.11. Suppose the scale read 7.84 N when the crown was in air and 6.84 N when it was in water. What should Archimedes have told the king? S

T1

Solution

S S

om

Conceptualize Figure 14.11 helps us imagine what is happening

Analyze W hen the crown is suspended in air, the scale reads the

ly

Fg

a

S

Fg

b

Figure 14.11 (Example 14.5) (a) When the crown is suspended in air, the scale reads its true weight because T1 5 Fg (the buoyancy of air is negligible). (b) When the crown is immersed in water, the buoyant S force B changes the scale reading to a lower value T 2 5 Fg 2 B.

o F 5 B 1 T

ys

Apply the particle in equilibrium model to the crown in water:

ic

s. w

true weight T1 5 Fg (neglecting the small buoyant force due to the surrounding air). When the crown is immersed in water, the buoyS ant force B reduces the scale reading to an apparent weight of T2 5 Fg 2 B.

S

ee b

Categorize This problem is an example of Case 1 discussed earlier because the crown is completely submerged. The scale reading is a measure of one of the forces on the crown, and the crown is stationary. Therefore, we can categorize the crown as a particle in equilibrium.

.c

in this example. Because of the buoyant force, the scale reading is smaller in Figure 14.11b than in Figure 14.11a.

B T2

ar ph

Solve for B :

2

2 Fg 5 0

B 5 Fg 2 T2

sw

Because this buoyant force is equal in magnitude to the weight of the displaced water, B 5 rw gVdisp , where Vdisp is the volume of the displaced water and rw is its density. Also, the volume of the crown Vc is equal to the volume of the displaced water because the crown is completely submerged, so B 5 rw gVc .

.a

Find the density of the crown from Equation 1.1:

rc 5

mc g mc g mc g rw mc g rw mc 5 5 5 5 1 2 Vc Vc g B Fg 2 T2 B/rw 1 7.84 N 2 1 1 000 kg/m3 2 7.84 N 2 6.84 N

5 7.84 3 103 kg/m3

w

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Substitute numerical values:

rc 5

Finalize From Table 14.1, we see that the density of gold is 19.3 3 103 kg/m3. Therefore, Archimedes should have

w

reported that the king had been cheated. Either the crown was hollow, or it was not made of pure gold. W h at If ? Suppose the crown has the same weight but is indeed pure gold and not hollow. What would the scale reading be when the crown is immersed in water?

Answer Find the buoyant force on the crown:

B 5 rw gVw 5 rw gVc 5 rw g a

Substitute numerical values:

B 5 1 1.00 3 10 3 kg/m3 2

Find the tension in the string hanging from the scale:

mc g mc b 5 rw a b rc rc

7.84 N 5 0.406 N 19.3 3 10 3 kg/m3

T2 5 Fg 2 B 5 7.84 N 2 0.406 N 5 7.43 N

14.5 Fluid Dynamics 427

An iceberg floating in seawater as shown in Figure 14.12a is extremely dangerous because most of the ice is below the surface. This hidden ice can damage a ship that is still a considerable distance from the visible ice. What fraction of the iceberg lies below the water level? Solution

. Mark Karrass/Corbis

Example 14.6 A Titanic Surprise

a

Conceptualize You are likely familiar with the phrase,

b

Figure 14.12 (Example 14.6) (a) Much of the volume of this

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“That’s only the tip of the iceberg.” The origin of this popular saying is that most of the volume of a floating iceberg is beneath the surface of the water (Fig. 14.12b).

iceberg is beneath the water. (b) A ship can be damaged even when it is not near the visible ice.

Evaluate Equation 14.6 using the densities of ice and seawater (Table 14.1):

Vdisp Vice

5

r ice

r seawater

5

917 kg/m3

1 030 kg/m3

ee b

f5

ly

.c

Categorize This example corresponds to Case 2 because only part of the iceberg is underneath the water. It is also a simple substitution problem involving Equation 14.6. 5 0.890 or 89.0%

ys

14.5 Fluid Dynamics

ic

Figure 14.13 Laminar flow around an automobile in a test wind tunnel.

1. The fluid is nonviscous. In a nonviscous fluid, internal friction is neglected. An object moving through the fluid experiences no viscous force. 2. The flow is steady. In steady (laminar) flow, all particles passing through a point have the same velocity. 3. The fluid is incompressible. The density of an incompressible fluid is constant. 4. The flow is irrotational. In irrotational flow, the fluid has no angular momentum about any point. If a small paddle wheel placed anywhere in the fluid does not rotate about the wheel’s center of mass, the flow is irrotational.

© Cengage Learning/Charles D. Winters

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Thus far, our study of fluids has been restricted to fluids at rest. We now turn our attention to fluids in motion. When fluid is in motion, its flow can be characterized as being one of two main types. The flow is said to be steady, or laminar, if each particle of the fluid follows a smooth path such that the paths of different particles never cross each other as shown in Figure 14.13. In steady flow, every fluid particle arriving at a given point in space has the same velocity. Above a certain critical speed, fluid flow becomes turbulent. Turbulent flow is irregular flow characterized by small whirlpool-like regions as shown in Figure 14.14. The term viscosity is commonly used in the description of fluid flow to characterize the degree of internal friction in the fluid. This internal friction, or viscous force, is associated with the resistance that two adjacent layers of fluid have to moving relative to each other. Viscosity causes part of the fluid’s kinetic energy to be transformed to internal energy. This mechanism is similar to the one by which the kinetic energy of an object sliding over a rough, horizontal surface decreases as discussed in Sections 8.3 and 8.4. Because the motion of real fluids is very complex and not fully understood, we make some simplifying assumptions in our approach. In our simplification model of ideal fluid flow, we make the following four assumptions:

Andy Sacks/Stone/Getty Images

s. w

Therefore, the visible fraction of ice above the water’s surface is about 11%. It is the unseen 89% below the water that represents the danger to a passing ship.

Figure 14.14 Hot gases from a cigarette made visible by smoke particles. The smoke first moves in laminar flow at the bottom and then in turbulent flow above.

428 Chapter 14

Fluid Mechanics

At t 0, fluid in the blue portion is moving past S point 1 at velocity v1.

Figure 14.15 A particle in laminar flow follows a streamline.

At each point along its path, the particle’s velocity is tangent to the streamline.

S

v

A1

S

v1

x1

Point 2

om

a A2 S

v2

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x2

ee b

After a time interval t, the fluid in the blue portion is moving past S point 2 at velocity v2.

s. w

b

Figure 14.16 A fluid moving

ys

ic

with steady flow through a pipe of varying cross-sectional area. (a) At t 5 0, the small bluecolored portion of the fluid at the left is moving through area A1. (b) After a time interval Dt, the blue-colored portion shown here is that fluid that has moved through area A 2.

sw

A1v1 5 A 2v 2 5 constant

(14.7)

This expression is called the equation of continuity for fluids. It states that the product of the area and the fluid speed at all points along a pipe is constant for an incompressible fluid. Equation 14.7 shows that the speed is high where the tube is constricted (small A) and low where the tube is wide (large A). The product Av, which has the dimensions of volume per unit time, is called either the volume flux or the flow rate. The condition Av 5 constant is equivalent to the statement that the volume of fluid that enters one end of a tube in a given time interval equals the volume leaving the other end of the tube in the same time interval if no leaks are present. You demonstrate the equation of continuity each time you water your garden with your thumb over the end of a garden hose as in Figure 14.17. By partially block-

© Cengage Learning/George Semple

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Equation of Continuity for Fluids

The path taken by a fluid particle under steady flow is called a streamline. The velocity of the particle is always tangent to the streamline as shown in Figure 14.15. A set of streamlines like the ones shown in Figure 14.15 form a tube of flow. Fluid particles cannot flow into or out of the sides of this tube; if they could, the streamlines would cross one another. Consider ideal fluid flow through a pipe of nonuniform size as illustrated in Figure 14.16. Let’s focus our attention on a segment of fluid in the pipe. Figure 14.16a shows the segment at time t 5 0 consisting of the gray portion between point 1 and point 2 and the short blue portion to the left of point 1. At this time, the fluid in the short blue portion is flowing through a cross section of area A1 at speed v1. During the time interval Dt, the small length Dx 1 of fluid in the blue portion moves past point 1. During the same time interval, fluid at the right end of the segment moves past point 2 in the pipe. Figure 14.16b shows the situation at the end of the time interval Dt. The blue portion at the right end represents the fluid that has moved past point 2 through an area A 2 at a speed v 2. The mass of fluid contained in the blue portion in Figure 14.16a is given by m1 5 rA1 Dx 1 5 rA1v1 Dt, where r is the (unchanging) density of the ideal fluid. Similarly, the fluid in the blue portion in Figure 14.16b has a mass m 2 5 rA2 Dx 2 5 rA2v 2 Dt. Because the fluid is incompressible and the flow is steady, however, the mass of fluid that passes point 1 in a time interval Dt must equal the mass that passes point 2 in the same time interval. That is, m1 5 m 2 or rA1v1 Dt 5 rA2v 2 Dt, which means that

ar ph

Point 1

Figure 14.17 The speed of water spraying from the end of a garden hose increases as the size of the opening is decreased with the thumb.

14.5 Fluid Dynamics 429

ing the opening with your thumb, you reduce the cross-sectional area through which the water passes. As a result, the speed of the water increases as it exits the hose, and the water can be sprayed over a long distance.

Example 14.7 Watering a Garden AM A gardener uses a water hose to fill a 30.0-L bucket. The gardener notes that it takes 1.00 min to fill the bucket. A nozzle with an opening of cross-sectional area 0.500 cm2 is then attached to the hose. The nozzle is held so that water is projected horizontally from a point 1.00 m above the ground. Over what horizontal distance can the water be projected?

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Solution

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Conceptualize Imagine any past experience you have with projecting water from a horizontal hose or a pipe using either your thumb or a nozzle, which can be attached to the end of the hose. The faster the water is traveling as it leaves the hose, the farther it will land on the ground from the end of the hose.

Analyze

R 5 A1v1

ic

Express the volume flow rate R in terms of area and speed of the water in the hose:

s. w

ee b

ly

Categorize Once the water leaves the hose, it is in free fall. Therefore, we categorize a given element of the water as a projectile. The element is modeled as a particle under constant acceleration (due to gravity) in the vertical direction and a particle under constant velocity in the horizontal direction. The horizontal distance over which the element is projected depends on the speed with which it is projected. This example involves a change in area for the pipe, so we also categorize it as one in which we use the continuity equation for fluids.

v1 5

ys

Solve for the speed of the water in the hose:

R A1

ar ph

We have labeled this speed v1 because we identify point 1 within the hose. We identify point 2 in the air just outside the nozzle. We must find the speed v 2 5 vxi with which the water exits the nozzle. The subscript i anticipates that it will be the initial velocity component of the water projected from the hose, and the subscript x indicates that the initial velocity vector of the projected water is horizontal. (1) v 2 5 v xi 5

sw

Solve the continuity equation for fluids for v 2:

A1 A1 R R v1 5 a b5 A2 A 2 A1 A2

.a

We now shift our thinking away from fluids and to projectile motion. In the vertical direction, an element of the water starts from rest and falls through a vertical distance of 1.00 m. yf 5 yi 1 v yi t 2 12g t 2

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Write Equation 2.16 for the vertical position of an element of water, modeled as a particle under constant acceleration:

(2) y f 5 0 1 0 2 12 g t 2 S t 5

Use Equation 2.7 to find the horizontal position of the element at this time, modeled as a particle under constant velocity:

xf 5 xi 1 vxi t 5 0 1 v 2 t 5 v 2 t

Substitute from Equations (1) and (2):

xf 5

w

Call the initial position of the water yi 5 0 and recognize that the water begins with a vertical velocity component of zero. Solve for the time at which the water reaches the ground:

Substitute numerical values:

xf 5

22y f Å g

22y f R A2 Å g

30.0 L/min 22 1 21.00 m 2 10 3 cm3 1 min a ba b 5 452 cm 5 4.52 m 1L 60 s 0.500 cm2 Å 9.80 m/s2

continued