Question

62. A ball at rest is dropped freely from a height of20 m. It loses 30% of its energy on striking theground and bounces back. The height to which it bounces back is
1) 14 m 2) 12 m 3)9 m 4) 6 m​

Answer 1

When it hits the floor it has no potential energy, but lots of kinetic energy. Another interesting thing happens when the ball hits the floor. Remember that the ball bounces back up to a height lower than it started, so after one bounce it has less potential energy than it started with.

Answer 2

Answer:

The kinetic energy on reaching the ground will be equal to the initial potential energy, i.e. 

KE=mg(20). Since the remaining kinetic energy (after loss) will be used to rise the block to height h, 

mg(20)(1−0.3)=mgh

Hence, h=14 m