question_answer

A ball of mass m approaches a wall of mass \[M\text{ }\left( >>m \right)\] with speed 4 m/s along the normal to the wall. The speed of wall is 1 m/s towards the ball. The speed of the ball after an elastic collision with the wall is -

A) 5 m/s away from the wall

B)                    9 m/s away from the wall

C)                    3 m/s away from the wall

D)                    6 m/s away from the wall

Correct Answer: D

Solution :

Let v be the velocity of ball after collision, collision is elastic \[\therefore \,\,\,e=1\] or relative velocity of separation = relative velocity of approach \[\therefore \,\,\text{ }v-1=4+1\] or \[v\text{ }=\text{ }6\] m/s (away from the wall)