Momentum is the product of the mass (m) and linear velocity (v) of a moving object, usually denoted by the letter p, so that p = mv. Like velocity, momentum is a vector quantity. In effect, momentum indicates the effort needed to move an object or to stop or change the direction of its motion. The linear momentum of a system of interacting particles is the sum of the momenta of its particles, and is constant if no external forces act. The latter fact, known as the law of conservation of momentum, can be stated as follows: when two or more objects interact (such as when a cannon fires a shell or a rocket shoots out a fast jet of hot gas) the total vector sum of their momenta is unchanged.
In rotational motion, the analogous concept is angular momentum.