(a) Law of conservation of energy : It states that energy can only be converted from one form to another. It can neither be created nor destroyed. The total energy before and after the transformation remains the same.
Example: During free fall of any object.
(b) (i) Power expended by girl A
Now, Weight of the girl A (mg) = 400 N
Height of girl A, h = 8 m
Time taken = 20 s
Power = work done/time taken
= mgh/t = 400 x 8/20 = 160 W
(ii) Power expended by girl B Now, Weight (mg) = 400 N
Displacement (h) = 8 m
Time (t) = 50 s
Power = mgh/t = 400 x 8/50 = 64 W
(c) P = 1500 W = 1.5 kW, t = 10h
Energy consumed = P x t = 1.5 x 10
= 15 kWh