n.
Product of the mass of a particle and its velocity .
Newton's second law of motion states that the rate of change of momentum is proportional to the force acting on the particle. Albert Einstein showed that the mass of a particle increases as its velocity approaches the speed of light. At the speeds treated in classical mechanics , the effect of speed on the mass can be neglected, and changes in momentum are the results of changes in velocity alone. If a constant force acts on a particle for a given time, the product of force and the time interval, the impulse, is equal to the change in momentum. For a rigid body, the momentum is the sum of the momenta of each particle in the body. See also angular momentum .