Answer :

(a.) Work done by the boy on the ladder = 0 J

All the work done by the boy in climbing up the ladder is done against gravity. His displacement with respect to the horizontal is zero.

(b.) Weight of the boy = 200 N

Frictional force = (3/10) of the weight of the boy

= 60 N

Work done by frictional force = Frictional force × Length of the ladder × cosθ

(where θ = angle between force of friction and displacement of the boy as he comes down)

= 60 × 10 × cos180°

= -600 J

(c.) Work done in this case is done against gravity = Weight of the boy × height of the ladder

= 200 × 8

= 1600 J

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