Question

A ball after falling a distance of 10 metre from rest hits elastically the ceiling of a lift and rebounds time of impact the lift was moving up with a velocity of 1 m/sec. The velocity with which the ball record just after impact is (g = $10m/se{c}^2$) -

• A. 10 m/sec
• B. 12 m/sec
• C. 126 m/sec
• D. 13 m/sec

Answer 1

The correct option is B 12 m/sec

Velocity of the ball at impact$,$

$v=\sqrt{2gh}$

Here h is the height and its value given $10m.$

$\begin{array}{c}v=\sqrt{2\times 9.8\times 10m/s}\\ v=14m/s\end{array}$

If a lighter object moving with velocity v collide with heavy object which moving in the same direction with velocity $V,$

then after collision lighter object recoil in the opposite direction with velocity$.$

$v-2v.$

Given velocity of lift$,$ $v=1m/s$

Therefore$,$ recoil velocity of the ball $=14-2\times 1$ $=12m/s$

Hence,

option $\left(B\right)$ is correct answer.