MOMENTUM

product of the mass of a particle and its velocity. Isaac Newton's second law of motion states that the time 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 only. 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 a rigid body is the sum of the momenta of each particle in the body. Being proportional to velocity, momentum has direction; consequently, when a body in plane motion rotates, the momentum of each particle has a moment about any point in the plane. The sum of these moments of momenta is called the angular momentum of the body about the point and is equal to the product of the moment of inertia of the body about the point and the angular velocity of the body. The time rate of change of the angular momentum of a body about a point is equal to the moment of the applied forces about the point. Applied to elementary particles such as electrons, angular momentum is called spin.