Two girls, each of weight 400 N climb up a rope through a height of 8 m. We name one of the girls A and the other B. Girl A takes 20 s while B takes 50 s to accomplish this task. What is the power expended by each girl?

Solution:
(i) Power expended by girl A:
Weight of the girl, mg = 400 N
Displacement (height), h = 8 m

Time taken, t = 20 s
Power, P = Work done / time taken

Power = mgh / t

P = 400 N × 8 m / 20 s
= 160 W.

(ii) Power expended by girl B:
Weight of the girl, mg = 400 N
Displacement (height), h = 8 m
Time taken, t = 50 s
Power, P = mgh /t /t

400 N × 8 m / 50s
= 64 W.
Power expended by girl A is 160 W.
Power expended by girl B is 64 W.