(a) State law of conservation of energy. Explain it with one example.
(b) Two girls each of weight 400 N (Four hundred N) climb up a rope through a height of 8 metres (eight metres). Girl A takes 20 (Twenty) seconds while Girl B takes 50 (Fifty) seconds to accomplish the ’ task. What is the power expended by each girl ?
© An electric heater is rated 1500 watt. How much energy does it use in 10 hours (Ten hours) ?
(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
© P = 1500 W = 1.5 kW, t = 10h
Energy consumed = P x t = 1.5 x 10
= 15 kWh