A cart (m1 = 110 kg) is moving to the right along a track at v1i = 17 m/s when it hits a stationary cart (m2 = 390 kg) and rebounds with a speed of v1f = 7 m/s in the opposite direction. a) With what speed does the 390 kg cart move after the collision? I used conservation of momentum, m1v1=m2v2, because there are no net external forces acting on the system. However, my answer of 1.97 is incorrect. Is there more to this problem that I'm not seeing?