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EPChap08-Solution2

# EPChap08-Solution2 - Chap 8 HW2 6 In Figure 8-25 a block...

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6 Chap. 8 HW2 6. In Figure 8-25, a block slides along a track that descends through distance h . The track is frictionless except for the lower section. There the block slides to a stop in a certain distance D because of friction. (a) If we decrease h , will the block now slide to a stop in a distance that is greater than, less than, or equal to D ? Decreasing h will decrease the KE of the block just before it hits the friction section.. Therefore it will take less work to stop it. So the block will slide a smaller distance than D (since W = Fd ) . (b) If, instead, we increase the mass of the block, will the stopping distance now be greater than, less than, or equal to D ? Increasing the mass of the block will increase the KE after it slides by a certain factor (KE mass). The friction force (and therefore work done) is increased by the same factor since F N KE mass. Therefore the block will slide the same distance. Problems 28. Figure 8-40 shows an 8.00 kg stone at rest on a spring. The spring is compressed 10.0 cm by the stone. (a) What is the spring constant? (b) The stone is pushed down an additional 30.0 cm and released. What is the elastic potential energy of the compressed spring just before that release? (c) What is the change in the gravitational potential energy of the stone–Earth system when the stone moves from the release point to its maximum height? (d) What is that maximum height, measured from the release point? a) Know that kx F = . The spring force F supporting the stone is mg = 78.4 N and the compression x is -0.10 m. Therefore k = 78.4/0.10 = 784 N/m.

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EPChap08-Solution2 - Chap 8 HW2 6 In Figure 8-25 a block...

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