First slide

Special cases

Question

A ball of mass m is moving normally towards a wall of mass M(>> m) with a velocity 4 m/s. The wall is also moving. After striking elastically with the wall, velocity of ball becomes 8 m/s. Find the speed of the wall :

Moderate

A

4 m/s
B

2 m/s
C

$2 \sqrt{2} m / s$
D

$4 \sqrt{2} m / s$

Solution

Let V be the velocity of ball. Relative velocity of separation = relative velocity of approach.
$v_{2} - v_{1} = u_{1} - u_{2}; v_{2} + u_{2} = u_{1} + v_{1}$ as the velocity of the heavier body does not change after collision
2u₂=4-8=-4 and u₂ =-2m/s
velocity of the wall= -2 m/s i.e. towards the ball

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