Four persons jointly lift a $250 \,kg$ box to a height of $1 \,m$ and hold it.
$(a)$ Calculate the work done by the persons in lifting the box.
$(b)$ How much work do they do 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,$F = mg = 250 \,kg \times 10 \,m/s^2 = 2500 \,N$. The displacement is $S = 1 \,m$. The work done is $W = F \times S = 2500 \,N \times 1 \,m = 2500 \,J$.
$(b)$ When holding the box,there is no displacement $(S = 0)$. Therefore,the work done is $W = F \times 0 = 0 \,J$.
$(c)$ They get tired because of the continuous muscular effort required to counteract the gravitational force acting on the box,even though no mechanical work is performed.