Question

A ball of mass m approaches a wall of mass M (>> m) with speed 4 m/s along the normal to the wall. The speed of wall is 1m/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

Answer 1

Answer: (D)

6 m/s away from the wall

Answer 2

Let v be the velocity of ball after collision, collision is elastic

\ e =1

Or relative velocity of separation = relative velocity of approach

\ v – 1 = 4 + 1

or v = 6 m/s (away from the wall)