1. The problem statement, all variables and given/known data A 2000-kg car moving with a speed of 20 m/s collides with and sticks to a 1500-kg car at rest. Show that because momentum is conserved in the rest frame, momentum is also conserved in a reference frame moving with a speed of 10 m/s in the direction of the moving car. 2. Relevant equations Not sure 3. The attempt at a solution Let's have the larger (2000-kg) car be mass M, and the smaller (1500-kg) car to be mass m. Car M is traveling at speed v. After the collision, the two cars become one mass (M+m) and its velocity we will call v'. To an observer on the ground... mv + 0 = (M+m) v' v' = MV/(M+m) To an observer in a moving frame... M is moving at speed V-v (towards the smaller car, m) and m is moving at speed -v (towards the larger vehicle, M). After the collision, (M+m) is moving at speed v'-v. M(V-v) - mv = (M+m)(v'-v) MV - Mv - mv + Mv + mv = (M+m)v' v' = MV/(M+m) These two equations are the same, meaning the final speed of the indecent is v' from any observer. Does this mean momentum is also conserved in a reference frame? If I'm on the right track, what good would it do plugging in numbers?