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 magnitude of the final momentum of the system in Kg-m/s?

1100

The initial momentum of the system = momentum of ball + momentum of cube = 1100 Kg-m/s. Since the momentum is conserved, the initial momentum = final momentum. Hence the final momentum is 1100 Kg-m/s.