In the top row labeled Before Collision, the left train car approaches the right car with a grey arrow pointing to the left car 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 = 0 meters per second. In the bottom row labeled After Collision, two train cars move away linked together with a grey arrow pointing toward the right of the car on the right. Consider a system to be one train car moving toward another train car at rest. When the train cars collide, the two cars stick together.

What is the total momentum of the system