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Like energy, what stays in a closed system is momentum. Momentum is a force multiplied by velocity and mass. How much effort you put into slowing down an object depends on the momentum. It depends on the mass and the velocity, which is interesting. For example, a toy car driving at 60 miles per hour does not kill you, but a truck does. Momentum is a vector of velocity. Because it has both direction and volume. Take, for example, a car traveling 60 miles an hour eastward. 60 miles per hour is the magnitude of the velocity, which is the east direction. Momentum is just like energy. It is exchanged both in quantity and in direction. For example, when two billiards balls collide with red and green, red exchanges the green’s direction and magnitude, and green do the same thing.