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? Why do they get tired while holding it? (g = 10 m s–2)

• Soln:

(a)

F= 250kg×g= 2500 N S= 1m

W=F.s= 250Nm= 2500 J

(b)

While holding box there is no displacement hence workdone is zero

1. c) While holding the box they apply muscular force which is equal and opposite to the gravitational force hence they feel tired while holding box.