Question

A ball of mass $m$ approaches a wall of mass $M(>>m)$ with speed $4m/s$ along the normal to the wall. The speed $4m/s$ 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. $5m/s$ away from the wall
• B. $3m/s$ away from the wall
• C. $9m/s$ away from the wall
• D. $6m/s$ away from the wall

Answer 1

The correct option is D $6m/s$ away from the wall

After collision velocity of ball

$v_b=\frac{m-M}{m+M}\times 4+\frac{M\times 1(1+1)}{M+m}$

$=\frac{4m-4M}{m+M}+\frac{2M}{M+m}$

$=\frac{4m-4M+2M}{m+M}$

$=\frac{4m-2M}{m+M}$

Place $0$ in place of $m$

$=\frac{4\times 0-2M}{0+M}$

$=\frac{-2M}{M}=2m/s$