A boy pushes his friend across a skating rink. Since the frictional forces are very small, the force exerted by the boy on his friend's back is the only significant forece acting on the friend in the horizontal direction. Is the change in kinetic energy of the friend greater than, equal to, or less than the work done by the force exerted by the boy?
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It depends upon whether the person who is exerting the force on the friend is also slipping in the opposite direction. If he is on skates and pushing off edges (to use a skating term), he does not slip and the work he performs is converted to kinetic energy of the friend, assuming the friend's friction is negligible.
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