train car with mass m1 = 521 kg is moving to the right with a speed of v1 = 7.9 m/s and collides with a second train car. The two cars latch together during the collision and then move off to the right at vf = 5.1 m/s.

Question

train car with mass m1 = 521 kg is moving to the right with a speed of v1 = 7.9 m/s and collides with a second train car. The two cars latch together during the collision and then move off to the right at vf = 5.1 m/s. 
1)What is the initial momentum of the first train car? I got 4115.9 kg m/s as the correct answer.
2)What is the mass of the second train car? 
I don't know how to do the second question please help.

drwls · Answer

1) 521*79 = 4115.9 kg*m/s 2) 4115.9 = (M1 + M2)*5.1 M1 + M2 = 807.0 kg M2 = 807 - 521 = 286 kg ..not very heavy for a railroad car.