Question

A ball of mass 200 gram falls from a height of 5m.What is its kinetic energy when it just reaches the ground?(g=9.8 m/sec^2)

Answer 1

height=5m.

mass=200g (0.2 kg)

g=9.8m/s^2

∴ potential energy=mgh

                        = 0.2*5*9.8

                        = 9.8 J

∵ P.E at the highest point= K.E at the lowest point (when it is just about to hit the ground)

∴ K.E energy will be= 9.8 J 

Answer 2

The sum of kinetic energy and potential energy is always constant.
When ball is at the height of 5 m, it has only potential energy which is = mgh
=0.2×10×5 = 10 J
(Mass in kg =0.2 kg , g=10)
When it reaches on ground,all its P.E converts in K.E
So,K.E when ball reaches to ground = 10 J