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? $(g=10\ ms^{-2})$

Mass of the box, $m=250\ kg$

Height $h=1\ m$

$(a)$. Work done by the men in lifting the box $=mgh=250\times 10\times 1$

$=2500\ J$

$(b)$. When the men just hold the box, then 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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