chapter 5

# chapter 5 - Chapter 5 Energy Quick Quizzes 1(c The work...

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Chapter 5 Energy Quick Quizzes 1. (c). The work done by the force is ( ) cos W F x θ = , where θ is the angle between the direction of the force and the direction of the displacement (positive x -direction). Thus, the work has its largest positive value in (c) where 0 θ = ° , the work done in (a) is zero since 90 θ = ° , the work done in (d) is negative since 90 270 θ ° < < ° , and the work done is most negative in (b) where 180 θ = ° . 2. (d). All three balls have the same speed the moment they hit the ground because all start with the same kinetic energy and undergo the same change in gravitational potential energy. 3. (c). They both start from rest, so the initial kinetic energy is zero for each of them. They have the same mass and start from the same height, so they have the same initial potential energy. Since neither spends energy overcoming friction, all of their original potential energy will be converted into kinetic energy as they move downward. Thus, they will have equal kinetic energies when they reach the ground. 4. (c). The decrease in mechanical energy of the system is f k x . This has a smaller value on the tilted surface for two reasons: (1) the force of kinetic friction f k is smaller because the normal force is smaller, and (2) the displacement x is smaller because a component of the gravitational force is pulling on the book in the direction opposite to its velocity. 143

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144 CHAPTER 5 Answers to Even Numbered Conceptual Questions 2. (a) The chicken does positive work on the ground. (b) No work is done. (c) The crane does positive work on the bucket. (d) The force of gravity does negative work on the bucket. (e) The leg muscles do negative work on the individual. 4. (a) Kinetic energy is always positive. Mass and speed squared are both positive. (b) Gravitational potential energy can be negative when the object is lower than the chosen reference level. 6. (a) Kinetic energy is proportional to the speed squared. Doubling the speed makes the object’s kinetic energy four times larger. (b) If the total work done on an object in some process is zero, its speed must be the same at the final point as it was at the initial point. 8. The total energy of the bowling ball is conserved. Because the ball initially has gravitational potential energy mgh and no kinetic energy, it will again have zero kinetic energy when it returns to its original position. Air resistance and friction at the support 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. 10. (a) The effects are the same except for such features as having to overcome air resistance outside. (b) The person must lift his body slightly with each step on the tilted treadmill.
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