A group of 4 rail cars with a total mass of 50,000kg moves at 3m/s at a stationary locomotives that has a mass of 10,000kg. What is the speed of the whole train after the cars connect with the locomotive?

Question

F=ma
50,000kg*3m/s= 150,000kg m/s
150,000kg m/s divided by 60,000kg
a= 2.5m/s

Answer 1

a is usually used for acceleration, which is m/s^2.

You ought to have used v, which in fact was entirely omitted:

50000*3 = 60000v
v = 2.5 m/s

Your answer was correct, but the "a" was misleading.