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 result of changes in velocity alone. If a constant force acts on a particle for a given time, the product of the force and the time interval, the impulse, is equal to the change in momentum. For any array of several objects, the total momentum is the sum of the individual momenta. See also angular momentum.

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