A 1.00 10-kg train car moving due east at 21.0 m/s collides with and couples to a 2.00 104-kg train car that is initially at rest. What is the common velocity of of the two-car train after the collision? What is the total kinetic energy K of the two train cars before the collision? What is the total kinetic energy Kf of the two train cars after the collision? Uf = m/s K J = J Kf =