A body of mass 20 kg is raised to the top of a building 15 m high and then dropped freely under gravity.

(i) Find out the work done in raising the body to the top of the building.

(ii) What will be the value of gravitational potential energy at the top of the building?

(iii) By what factor will the gravitational potential energy of the same body increases, if it is raised to the top of a multi-storey building 45 m high?

(iv) When will the kinetic energy of the body be maximum?

Given, m = 20 kg, h = 15 m, g = 10 ms”2

(i) Work done mgh = 20 X10 x 15 = 3000 J

(ii)Gravitational PE = Work done = 3000 J

(iii) Gravitational PE at a height of 45 m = mgh’ = 20 x 10 x 45 = 9000 J

Potential energy at height 45 m / Potential energy at height 15m = 9000/3000= 9

Therefore, PE increases by 3 times.

(iv) KE will be maximum just before the body strikes the ground.