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 1 m/s towards the ball. The speed of the ball after an elastic collision with the wall is

• A.
  9 m/s away from the wall
• B.
  6 m/s away from the wall
• C.
  5 m/s away from the wall
• D.
  3 m/s away from the wall

Answer 1

The correct option is B

6 m/s away from the wall
Let v be the velocity of ball after collision
Collision is elastic:
$\therefore e=1$ or
relative velocity of separation = relative velocity of approach
$v-1=4+1$
$\Rightarrow v=6m/s$ (Away from the wall)
Final answer: (d)