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Unformatted text preview: 8 Conservation of Energy CHAPTER OUTLINE 8.1 The Nonisolated System Conservation of Energy 8.2 The Isolated System 8.3 Situations Involving Kinetic Friction 8.4 Changes in Mechanical Energy for Nonconservative Forces 8.5 Power ANSWERS TO QUESTIONS *Q8.1 Not everything has energy. A rock stationary on the f loor, chosen as the y = 0 reference level, has no mechan- ical energy. In cosmic terms, think of the burnt-out core of a star far in the future after it has cooled nearly to absolute zero. *Q8.2 answer (c). Gravitational energy is proportional to the mass of the object in the Earths fi eld. *Q8.3 (i) answer b. Kinetic energy is proportional to mass. (ii) answer c. The slide is frictionless, so v = (2 gh ) 1 / 2 in both cases. (iii) answer a. g for the smaller child and g sin for the larger. *Q8.4 (a) yes: a block slides on the oor where we choose y = 0. (b) yes: a picture on the classroom wall high above the f loor. (c) yes: an eraser hurtling across the room. (d) yes: the block stationary on the oor. *Q8.5 answer (d). The energy is internal energy. Energy is never used up. The ball fi nally has no elevation and no compression, so it has no potential energy. There is no stove, so no heat is put in. The amount of sound energy is minuscule. *Q8.6 answer (a). We assume the light band of the slingshot puts equal amounts of kinetic energy into the missiles. With three times more speed, the bean has nine times more squared speed, so it must have one-ninth the mass. Q8.7 They will not agree on the original gravitational energy if they make different y = 0 choices. They see the same change in elevation, so they do agree on the change in gravitational energy and on the kinetic energy. Q8.8 Lift a book from a low shelf to place it on a high shelf. The net change in its kinetic energy is zero, but the book-Earth system increases in gravitational potential energy. Stretch a rubber band to encompass the ends of a ruler. It increases in elastic energy. Rub your hands together or let a pearl drift down at constant speed in a bottle of shampoo. Each system (two hands; pearl and shampoo) increases in internal energy. Q8.9 All the energy is supplied by foodstuffs that gained their energy from the sun. 175 13794_08_ch08_p175-208.indd 175 13794_08_ch08_p175-208.indd 175 12/2/06 1:24:39 PM 12/2/06 1:24:39 PM Q8.10 The total energy of the ball-Earth system is conserved. Since the system initially has gravitational energy mgh and no kinetic energy, the ball will again have zero kinetic energy when it returns to its original position. Air resistance will cause the ball to come back to a point slightly below its initial position. On the other hand, if anyone gives a forward push to the ball anywhere along its path, the demonstrator will have to duck....
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