A car of mass 1ton is moving with a velocity of 25 m/s towards left. A truck of mass 5 ton is moving towards left with a velocity of 3 m/s. If the collision is perfectly elastic, what is the final velocity of the truck in m/s?

4

The initial momentum of the system = momentum of car + momentum of truck = 40 Kg-m/s leftwards. Since the momentum is conserved, the initial momentum = final momentum. Hence the final momentum is 40 Kg-m/s leftwards. The collision is perfectly elastic, therefore, e = 1. Therefore, final relative velocity (of truck with respect to car) = initial relative velocity (of truck with respect to car). Sum of the final velocities of the car and the truck = 4 m/s. From this information and the conservation of momentum, one can find the final velocity of the truck.