Sorry that I don't have LaTeX, I'll do my best without it: A train with known mass collides with a stationary train and the two link together. If 27% of the initial kinetic energy is dissipated, what is the mass of the stationary train? We are given no velocities. M1 = mass of moving train (known) M2 = mass of stationary train Vi = initial velocity of moving train Vf = final velocity of the 2-train system Ki = initial kinetic energy Kf = final kinetic energy Wnc = work done by non-conservative forces Pi = initial momentum Pf = final momentum Using conservation of momentum: Pi = Pf (M1)(Vi) = (M1+M2)(Vf) Using conservation of energy: Ki + Wnc = Kf (1/2)(M1)(Vi)^2 - (.27)(1/2)(M1)(Vi)^2 = (1/2)(M1+M2)(Vf)^2 (.73)(M1)(Vi)^2 = (M1+M2)(Vf)^2 ...2 equations, 3 unknowns. What am I forgetting?