A ball is moving with velocity $2\text{m/s}$ towards a heavy wall approaching the ball with a speed $1\text{m/s}$ as shown in figure. Assuming collision to be elastic, find the velocity of the ball immediately after the collision.
(Considering the initial direction of motion of ball to be $+ve$).

The correct option is B $-4\text{m/s}$
The speed of wall will not change after after the collision. So, let $v$ be the velocity of the ball after collision in the direction shown in figure. Collision is elastic $(e=1)$,


$\text{Separation speed}=\text{Approach speed}$
or $v-1=2+1$
$\therefore v=4\text{m/s}$
Since final direction of motion is opposite to initial one, hence $v=-4\text{m/s}$