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 :

Let V be the velocity of ball. Relative velocity of separation = relative velocity of approach. v2-v1=u1-u2 ; v2+u2=u1+v1    as the velocity of the heavier body does  not change after collision2u2=4-8=-4  and u2 =-2m/svelocity of the wall= -2 m/s i.e. towards  the ball