Question

Give an example in which a force does work on a body but fails to change its kinetic energy.

Answer 1

Answer:

If you have a block on a rough surface, and you keep pushing it such that it moves with a constant speed, then your force is doing a positive work on the block, and at the same time friction (kinetic friction) is doing negative work. Thus the net work done on the block is zero, hence its kinetic energy will not change.  

Another example is if you take a small steel ball and drop it in a thick syrup, you would see that the ball descends with a uniform speed (terminal speed). In this case, gravity is doing positive work, and the syrup does a negative work. Again, total work on the ball is zero, hence there is no change in kinetic energy.  

So in conclusion, just because a force does work on an object, doesn't necessarily mean that the kinetic energy must increase, there could be other forces doing work, which could be negative. Thus the work energy theorem states

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Answer 2

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When a body is ppulled over a rough horizontal surface with a uniform velocity. Work is done ono the body by the applied force, but its KE remains unchanges.