Question
Can the work by kinetic friction on an object be positive? Zero?
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Answer
Yes. Let us consider a block A which is resting on another block B. Block B is resting on a smooth horizontal surface. Let the coefficient of friction between the blocks be $\mu.$
When a force F is applied on block B in the forward direction as shown in the above figure, block A moves with block B in the direction of the applied force. The frictional force on block A and the displacement will be in the forward direction. Therefore, work done by the frictional force is positive. If we consider the reference frame of block B, then displacement of block A will be zero. Therefore, work done by the frictional force is zero.
When a force F is applied on block B in the forward direction as shown in the above figure, block A moves with block B in the direction of the applied force. The frictional force on block A and the displacement will be in the forward direction. Therefore, work done by the frictional force is positive. If we consider the reference frame of block B, then displacement of block A will be zero. Therefore, work done by the frictional force is zero.
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