11_P44InstructorSolution

11_P44InstructorSolution - 11.44. Model: Model Sam strapped...

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11.44. Model: Model Sam strapped with skis as a particle, and apply the law of conservation of energy. Visualize: Solve: (a) The conservation of energy equation is 1g 1 t h 0g 0e x t K UE K U W ++ Δ=++ The snow is frictionless, so th 0 J. E Δ= However, the wind is an external force doing work on Sam as he moves down the hill. Thus, ext wind 1 g1 0 g0 22 2 2 11 00 1 0 10 wind () ( ) 1 1 0 J (0 J ) 2 2 2 2 WW K U K U mv mgy mv mgy mv mgy mv mgy W vg y m == + + ⎛⎞ =+ −+ + = ⎜⎟ ⎝⎠ ⇒= + We compute the work done by the wind as follows: wind wind wind cos160 (200 N)(146 m)cos160 27,400 J WFr F r =⋅ Δ ° = ° = G G where we have used /sin 20 146 m. rh °= Now we can compute 2 1 2( 27,400 J) 2(9.8 m/s )(50 m) 15.7 m/s 75 kg v = (b) We will use a tilted coordinate system, with the
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