Chap07-Solution

# Chap07-Solution - Chap. 7. HW Questions 4. Figure 7-18a...

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1 Chap. 7. HW Questions 4. Figure 7-18a shows two horizontal forces that act on a block that is sliding to the right across a frictionless floor. Figure 7-18b shows three plots of the block's kinetic energy K versus time t. Which of the plots best corresponds to the following three situations: (a) F 1 = F 2 , (b) F 1 > F 2 , (c) F 1 < F 2 ? a) plot 2 (because net force = 0, a = 0 and so v will not change) b) plot 3 (because the net force is to the left and so the block will slow down and stop and reverse direction) c) plot 1 (because the net force is to the right and so the block will speed up). Problems 9. A 3.0 kg body is at rest on a frictionless horizontal air track when a constant horizontal force F v acting in the positive direction of an x axis along the track is applied to the body. A stroboscopic graph of the position of the body as it slides to the right is shown in Figure 7- 26. The force F v is applied to the body at t = 0, and the graph records the position of the body at 0.50 s intervals. How much work is done on the body by the applied force F v between t = 0 and t = 2.0 s? Hint: You can start out with K W Δ = and then find the initial ( v 0 ) and final ( v ) velocities. To find the v , first find the acceleration from the information given, then find v . Since K W Δ = we need to find v 0 and v. From the question v 0 is zero. To find v we consider the motion in the diagram. We know that the body is at the origin when F v is applied. This implies that x o = 0. We also know that the body is at rest when F v is applied. This implies that v o = 0. Since it is constant acceleration motion we can use 2 0 2 1 at t v x x o + + = . Applying this to the point at t = 1.0 s we get 2 / 0 0 2 . 0 a + + = Therefore a = 040 .m s 2 . The velocity at t = 2.0 s is from m/s 8 . 0 2 4 . 0 0 0 = × + = + = at v v .

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## This note was uploaded on 04/04/2008 for the course PHYS 213 taught by Professor Oshea during the Spring '08 term at Kansas State University.

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Chap07-Solution - Chap. 7. HW Questions 4. Figure 7-18a...

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