In the top row labeled Before Collision, two train cars approach one another with a grey arrow pointing to each car. The left car has below it m subscript 1 = 600 kilograms v subscript 1 = 4 meters per second. The right car has below it m subscript 2 = 400 kilograms v subscript 2 = -2 meters per second. In the bottom row labeled After Collision, two train cars move away from one another with a grey arrow pointing away from each car. A Consider a system to be two train cars traveling toward each other. What is the total momentum of the system before the train cars collide? kg • m/s What must the total momentum of the system be after the train cars collide? kg • m/s