The total amount of momentum in a closed system is constant. Some examples of closed systems include: marbles colliding or two bumper cars bouncing off each other. Momentum is the mass of an object multiplied by its speed. If two cars collide, the momentum of the system (car one plus car two) is the same after collision as it was before collision. Each car might have a different speed after colliding, but the total momentum is the same, it does not increase or decrease. This is known as conservation of momentum.