Answer to Question #59895 in Physics for Raven

Question

four railroad cars, each of mass 2.50x10^4 kg, are coupled together and coasting along horizontal tracks at speed vi toward the south. A very strong but foolish actor, riding on the second car, uncouples the front car and gives it a big push, increasing its speed to 4 m/s southward. The remaining three cars continue moving south, now at 2 m/s.
a) find the initial speed of the four cars.
b) by how much did the potential energy within the body of the actor change