Four railroad cars, each of mass 2.50 × 104 kg, are coupled together and coasting along horizontal tracks at speed vi toward the south. A very strong but foolish movie actor riding on the second car uncouples the front car and gives it a big push, increasing its speed to 4.00 m/s south. The remaining three cars continue moving south, now at 2.00 m/s.
(a) Find the initial speed of the cars.
(b) How much work did the actor do?