Question

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

• A. $5 \, ms^{- 1}$ away from the wall
• B. $9 \, ms^{- 1}$ away from the wall
• C. $3 \, ms^{- 1}$ away from the wall
• D. $6 \, m \, s^{- 1}$ away from the wall

Answer 1

Answer: (D)

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