Chap 06 SM - Chapter 6 Work and Kinetic Energy Conceptual...

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539 Chapter 6 Work and Kinetic Energy Conceptual Problems 1 True or false: ( a ) If the net or total work done on a particle was not zero, then its speed must have changed. ( b ) If the net or total work done on a particle was not zero, then its velocity must have changed. ( c ) If the net or total work done on a particle was not zero, then its direction of motion could not have changed. ( d ) No work is done by the forces acting on a particle if it remains at rest. ( e) A force that is always perpendicular to the velocity of a particle never does work on the particle. Determine the Concept A force does work on an object when its point of application moves through some distance and there is a component of the force along the line of motion. ( a ) True. The total work done on a particle is equal to the change in its kinetic energy (the work-kinetic energy theorem). ( b ) True. The total work done on the particle changes the kinetic energy of the particle, which means that its velocity must change. ( c ) False. If the total work done on the particle is negative, it could be decreasing the particle’s speed and, eventually, reversing its direction of motion. ( d ) True. The object could be at rest in one reference frame and moving in another. If we consider only the frame in which the object is at rest, then, because it must undergo a displacement in order for work to be done on it, we would conclude that the statement is true. ( e ) True. A force that is always perpendicular to the velocity of a particle changes the direction the particle is moving but does no work on the particle. 2 You push a heavy box in a straight line along the top of a rough horizontal table. The box starts at rest and ends at rest. Describe the work done on it (including sign) by each force acting on it and the net work done on it. Determine the Concept The normal and gravitational forces do zero work because they act at right angles to the box’s direction of motion. You do positive work on the box and friction does negative work. Overall the net work is zero because its kinetic energy doesn’t change (zero to zero).
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Chapter 6 540 3 You are riding on a Ferris wheel that is rotating at constant speed. True or false: During any fraction of a revolution: ( a ) None of the forces acting on you does work on you. ( b ) The total work done by all forces acting on you is zero. ( c ) There is zero net force on you. ( d ) You are accelerating. ( a ) False. Both the force exerted by the surface on which you are sitting and the gravitational force acting on you do work on you. ( b ) False. Because you are rotating at constant speed, your kinetic energy does not change. However, since your height is continuously changing, your potential energy also changes continuously. ( c ) False. Because you are continuously changing direction, and therefore accelerating, there is a net force acting on you that does work on you.
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This note was uploaded on 02/28/2012 for the course PHYSICS 122 taught by Professor Goussiou during the Spring '12 term at University of Washington.

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Chap 06 SM - Chapter 6 Work and Kinetic Energy Conceptual...

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