A ball of mass 100 g is moving with a velocity of 3 m/s towards east. A cube of mass 200 gram is moving towards west with a velocity of 7 m/s. If the collision is perfectly elastic, what is the final velocity of the cube in m/s?

7

The initial momentum of the system = momentum of ball + momentum of cube = 1100 Kg-m/s eastwards. Since the momentum is conserved, the initial momentum = final momentum. Hence the final momentum is 1100 Kg-m/s eastwards. The collision is perfectly elastic, therefore, e = 1. Therefore, final relative velocity (of cube with respect to ball) = initial relative velocity (of cube with respect to ball). Difference in the final velocities of cube and ball = 4 m/s. From this information and the conservation of momentum, one can find the final velocity of the cube.