# momentum

momentum
/moh men"teuhm/, n., pl. momenta /-teuh/, momentums.
1. force or speed of movement; impetus, as of a physical object or course of events: The car gained momentum going downhill. Her career lost momentum after two unsuccessful films.
2. Mech. a quantity expressing the motion of a body or system, equal to the product of the mass of a body and its velocity, and for a system equal to the vector sum of the products of mass and velocity of each particle in the system.
3. Philos. moment (def. 7).
[1690-1700; < L momentum; see MOMENT]

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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.

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product of the mass of a particle and its velocity. Momentum is a vector quantity; i.e., it has both magnitude and direction. Isaac Newton (Newton, Sir Isaac)'s second law of motion states that the time rate of change of momentum is equal to the force acting on the particle. See Newton's laws of motion.

From Newton's second law it follows that, 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 the momentum. Conversely, the momentum of a particle is a measure of the time required for a constant force to bring it to rest.

The momentum of any collection of particles is equal to the vector sum of the individual momenta. According to Newton's third law, the particles exert equal and opposite forces on one another, so any change in the momentum of one particle is exactly balanced by an equal and opposite change of the momentum of another particle. Thus, in the absence of a net external force acting on a collection of particles, their total momentum never changes; this is the meaning of the law of conservation of momentum (momentum, conservation of). See also conservation law; angular momentum.

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Universalium. 2010.

Synonyms:

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