Physics: Locomotive Pulling a Freight Car?

Question

If, theoretically, a locomotive of 3000 kg is pulling a freight car of 5000 kg, with a coefficient of static friction of 0.8 and an effective coefficient of friction of 0.2 and the entire train (locomotive + car) is accelerating to the right at 0.5 m/s^2, What is the magnitude of the force creating this acceleration? What is the object exerting this force on the train?

Answer 1

Newton's 2nd says that the net force on the train is (m1+m2)a, where m1 is the mass of the engine, m2 is the mass of the freight car, and a is the acceleration--all given. I'm assuming that "effective friction" is the overall coefficient of rolling friction for the train in motion. (The static coefficient does not matter other than to place a limit on the maximum thrust the engine can apply before the wheels slip. This isn't happening in the problem.)

The engine's wheels push horizontally backward on the track and, by Newton's 3rd, the track pushes back with the same force on the train. It is this reaction force that moves the train. It must be, because that is the only force that is pushing forward. It is reduced by the overall friction [0.2*(m1+m2)g, if my guess about "overall" is correct], and the difference is the net force mentioned above.

Hope this helps.