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? $(g = 10 \,m \,s^{-2})$

(N/A) The force applied to lift the box is equal to its weight: $F = m \times g = 250 \,kg \times 10 \,m \,s^{-2} = 2500 \,N$.
Work done in lifting the box is $W = F \times s = 2500 \,N \times 1 \,m = 2500 \,J$.
$(b)$ Work done in holding the box is zero because the displacement $s = 0$.
$(c)$ They get tired because,to hold the box,their muscles must continuously exert a force to counteract the gravitational force,which involves internal muscular effort and energy expenditure.