Friction is a non-conservation force because work done by friction over closed path is not equal to zero.
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By eating two apples, a person starts pushing the wall of room. If food value of apple is 100 calories per apple, how much work he will do in pushing the wall in one hour?
Zero, because the displacement of the wall will be zero.
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What is the work done by gravitational force in taking the mass m with constant velocity through height h?