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Question
Four men lift a 250 kg box to a height of 1 m and hold it without raising or lowering it.
(a) How much work is done by the men in lifting the box?
(b) How much work do they do in just holding it?
(c) Why do they get tired while holding it? (g = 10 m s–2)
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Solution
Mass of the box, m = 250 kg
Height up to which it is raised, h = 1 m
(a) Work done by the men in lifting the box
= F.s = 250 × 10 × 1 J
(Here F = mg, s = h)
= 2500 J
(b) In just holding the box, there is no displacement, so no work is done.
(c) In order to hold the box at a certain height, men are applying a force that is equal and opposite to the gravitational force acting on the box. While applying the force, muscular effort is involved.
So, men get tired while holding it.
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