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07_InstSolManual_PDF_Part18 - 7-18 Chapter 7 Reect Friction...

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Reflect: Friction does negative work and removes mechanical energy from the system. The tension does positive work on one person and the same magnitude of negative work on the other person and therefore does no net work on the system. 7.82. Set Up: Referring to the free-body diagram, a force balance in the horizontal direction results in The force applied by the worker can thus be obtained for part (a) and used in (b) to calculate the work as For parts (c) and (d), use the normal force to calculate the nonconservative work of friction as Also apply In (e), evaluate the net work as Figure 7.82 Solve: (a) From (b) (c) (d) The normal force, n , and the gravitational force, mg , do not contribute to the work since they both act perpendi- cular to the displacement, d . (e) 7.83. Set Up: For parts (a) and (b), consider the relations and In parts (c) and (d), use Also recall that for (d). Solve: (a) Identical springs are compressed the same distance, so is the same for both springs.
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