How is the power related to the speed at which a body can be lifted? How many kilograms will a man working at the power of 400 W, be able to lift at constant speed of 1 ms^–1 vertically? (g = 10 ms^–2)

Four men lift a 300 kg box to a height of 1 m and hold it without raising or lowering it.

(a) How much work is done by the men in lifting the box?

(b) How much work do they do in just holding it?

(c) Why do they get tired while holding it? (g = 10 m s^–2)

What is power? How do you differentiate kilowatt from kilowatt hour? The Jog Falls in Karnataka state are nearly 40 m high. 2000 tones of water falls from it in a minute. Calculate the equivalent power if all this energy can be utilized? (g = 10 m s^–2)

Define watt. Express kilowatt in terms of joule per second. A 200 kg car engine develops 500 W for each kg. What force does it exert in moving the car at a speed of 20 m s^–1?

Compare the power at which each of the following is moving upwards against the force of gravity? (given g = 10 m s^–2)

(i) a butterfly of mass 1.0 g that flies upward at a rate of 0.5 m s^–1.

(ii) a 250 g squirrel climbing up on a tree at a rate of 0.5 m s^–1.