Question

A ball of mass m moves towards a moving wall of infinite mass with a speed $‘v’$ along the normal to the wall. The speed of the wall is $‘u’$ toward the ball. What will be the speed of the ball after ‘elastic’ collision with wall ?

• A. $u+v$ away from the wall
• B. $2u+v$ away from the
• C. $\left|u - v\right|$ away from the wall
• D. $\left|v - 2 u\right|$ away from the wall

Answer 1

Answer: (B)

From the frame of wall, the ball moves towards the wall with speed $u+v \, $ and after collision returns with velocity $u+v$ as shown in Figure 1. Then from ground frame, the velocity of ball after collision with wall is $2u+v$