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Unformatted text preview: Quiz 5 Solutions Physics 2A November 5, 2012 1. A constant horizontal pull acts on a sled on a horizontal frictionless ice pond. The sled starts from rest. When the pull acts over a distance x , the sled acquires a speed v and kinetic energy k . If the same pull instead acts over twice this distance, what will happen to the sleds kinetic energy and velocity? This problem relies on the work-kinetic energy theorem K = W net and the definition of kinetic energy K = 1 2 mv 2 . Since the sled is pulled horizontally over a frictionless surface, the only force acting horizontally on the sled is the applied force F . Then the net work W net is equal to the work W F done by the force F . When the sled is pulled a horizontal distance x , the net work is W net = W F = Fx . By the work-kinetic energy theorem, W net = K = K f- K i where K i and K f are the final and initial kinetic energies. By definition of kinetic energy, K i = 1 2 mv 2 i = 0 since the initial velocity v i = 0. We are given that the final kinetic energy K f = k . Therefore K = k- 0 = Fx so k = Fx ....
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