More force is needed to stop a train than to stop a car, even when both are moving at the same velocity. A bullet fired from a gun has more penetrating energy than a bullet thrown by hand, even if they have the same masses. The physical quantity that describes this aspect of the motion of an object is called momentum.

By definition, momentum is a product of the mass and velocity of the body. Momentum is a vector quantity, meaning it has magnitude and direction. Direction is usually the same as that of the object's motion. Momentum is represented by p. To calculate the momentum of the body the following equation is used: p = mv, where m is the mass of the body and v is its velocity. The unit for momentum is kilogram×meter/second or kg×m/s.

In the example of a car and a train, the greater mass of the train gives it more momentum than the car. Therefore, a greater change in momentum is involved in stopping a train than in stopping the car. In case of the bullets, the greater momentum of the fired bullet is due to its greater velocity; a large change in momentum takes place when the speeding bullet is stopped.

The law of conservation of momentum states: the momentum of any closed, isolated system does not change.