Question

An empty rail car of mass 100kg is at rest on the railway track. A boy of mass 40kg running with a speed of 5m/s jumps on the rail car. Find the speed of the (car + boy) system.

Answer 1

we need to apply conservation of momentum to solve this problem.

momentum of the boy before jumping into rail car = 40×5 = 200 kg-m/s

momentum of the combined system of boy+rail-car after the boy jumping into rail car = 140×v
where v is speed of cobined system.

140×v = 200 or v = 1.43 m/s