An empty 2,500 kg train car is headed northbound at a velocity of 5 m/s. ahead of the first car, an empty 1,500 kg car is headed northbound on the same track at 1 m/s. the faster train bumps into the slower one, and they combine into one train. what is the velocity of the two-car train?

Let the mass of 2500 kg car be and it's velocity be and the mass of 1500 kg car be and it's velocity be .

After the bumping the mass be M and it's velocity be V.

     By law of conservation of momentum we have

                    2500 * 5 + 1500 * 1=4000 * V

                    V = 14000/4000 = 7/2 = 3.5 m/s

So the velocity of the two-car train = 3.5 m/s