(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