In a children’s park, there is a slide which has a total length of 10 m and a height of 8.0 m (figure 8-E3). Vertical ladder are provided to reach the top. A boy weighing 200 N climbs up the ladder to the top of the slide and slides down to the ground, The average friction offered by the slide is three tenth of his weight. Find (a) the work done by the boy on the ladder as he goes up, (b) the work done by the slide on the boy as he comes down (c) the work done by forces inside the body by the boy.
(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