A railroad car of mass 3.4 x 104 kg moving at 4.00 m/s collides and couples with two coupled railroad cars, each of the same mass as the single car and moving in the same direction at 1.20 m/s.
(a) What is the speed of the three coupled cars after the collision?
_______m/s
(b) How much kinetic energy is lost in the collision?
______J