Question

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 )

  • Work and energy CBSE Class 9
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Answer

(a) F = 250 kg × 10 ms-2           (g = 10 ms-2 )

= 2500 N

s = 1 m

W = F. s

= 2500 N x 1 m

= 2500 Nm = 2500 J

(b) Zero, as the box does not move at all while holding it.

(c) In order to hold the box, men are applying a force which is opposite and equal to the gravitational force acting on the box. While applying the force, muscular effort is involved. So, they feel tired.

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