Four railroad cars, each of mass `2.50 xx 10^4` kg, are coupled together and coasting along horizontal tracks at speed `v_(I)`towards the south. A very strong 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` southward. 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?