Three identical freight cars are sitting on the tracks, each separated by a small distance. A fourth car is coasting with velocity 4 m/s. The moving car collides and couples to the first stationary car. Those two then couple to the next car, and finally they couple to the last car. Assuming no friction,
A. How does the momentum of the four cars traveling together compare to the momentum of the first moving car?
B. How fast are the four cars moving?