## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

We can find the initial momentum of the system consisting of the five train cars. Let $M$ be the mass of each train car. $p_0 = 3M(2.0~m/s)+M(4.0~m/s)+ M(0)$ $p_0 = (10M)~N~s$ We can use conservation of momentum to find the final speed $v_f$ of the five cars when they are moving together. $p_f = p_0$ $(5M)~v_f = (10M)~N~s$ $v_f = 2.0~m/s$ The speed of the five-car train is 2.0 m/s.