question_answer 57)

  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? [given = 10 ms-2].

Answer:

  Given, m = 250 kg, height (h) = 1 m and acceleration due to gravity (a) Work done by the man in lifting the box W = Potential energy of box W = mgh W = 250 x 1 x 10 = 2500 J (b) Work done is zero in holding a box, because displacement is zero. (c) In holding the box, the energy of man loses. Due to loss of energy he felt tired.