1. The problem statement, all variables and given/known data A freight car of mass 1800kg is timed at 4.2m/s just after it runs into a spring-loaded bumper at the end of the track. The bumper consists of an 800kg mass that the car runs into, and a pair of large springs. The car travels 2.4m before coming to rest. a)using an appropriate conservation law, find the speed of the freight car before it struck the bumper. 2. Relevant equations i'm not sure, conservation of momentum? or kinetic energy? 3. The attempt at a solution Before it struck the bumper, it's moving at a certain speed(unknown) at a given mass. hmmm...looking at what's given to find the initial speed we have to use conservation of momentum: m1v1 +m2v2 = 1800v1 + 800(4.2) Does that look right? Can someone guide me please?