Question

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Pretest: Work and the work-energy theorem Name ce blocks. A,B, and care placed on an elevator platform. Blocks s, and block A has less massin an ime the following questions, the positive y direction is upward. Blocks B and C have the same ons, use the convention that Consider the following motions of the elevator platform 1. The platform moves upward with constant speed a + y direction s they component of the force exerted on block the platform positive negative, or zero? Explain by U constant b. Is the work done on block C by the platform positive. negative, or zero? Explain there is no such work, state so explicitly.) platform C. is the work done on block B by block A positivenerative, or sero? Explain. If there is no such work, state so explicitly.) d. Is the absolute value of the work done on block B by the platform greater than, less than, or equal to the absolute value of the work done on block C by the platform? Explain. (If there is no such work in either case, state so explicitly.) 2. The platform moves downward with constant speed. + y direction a. Is the y component of the force exerted on block C by the platform positive, negative, or zero? Explain. constant Platform b. Is the work done on block C by the platform positive, negative, or zero? Explain. (If there is no such work, state so explicitly.) c. Is the work done on block B by block A positive, negative, or zero? Explain. (If there is no such work, state so explicitly.) Tutorials in Introductory Physics Prentice Hall, Inc.

Answer 1

1a) The net force on the block C in Y direction is zero since it is travelling with constant speed . There are two forces being applied on block C one is its weight which is acting downwards and the other one is force exerted by the platform. For the net force to be zero the force applied by platform on block C should be equal and opposite to the weight of block C.Therefore the force applied on block C by platform is in upward direction which is positive direction and hence positive force is applied on block C by the platform.

b)The work done by platform on block C is positive .Since force applied by platform on block C is in upward direction and displacement of the block C is also in upward direction . Since both force and displacement are in same direction hence work done is positive.

c) Block A applies a force on block B in downward direction whereas the displacement of the block B is in upwards direction .Since force and displacement are in opposite directions (theta =180) hence work done by Block A on block B is negative.

d)The absolute value of work done by platform on block B is greater than absolute work done by platform on block C ,Since the force applied by platform on Block B is equal to sum of weights of block B aand Block A which is greater than force applied by platform on block C (equal to weight of block C).

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