two identical freight cars roll without friction towards each other on a level track. one car rolls 2 m/s the other 1 m/s. after the cars collide they couple and roll together with a speed of
If they have identical weights the momentum is 2*m for one and -1*m for the other one. The total momentum is m, while the total mass is 2*m, giving a final speed of 0.5 m/s :|