Momentum

Momentum, also linear momentum, in physics, fundamental quantity characterizing the motion of any object. It is the product of the mass of a moving particle multiplied by its linear velocity. Momentum is a vector quantity, which means that it has both magnitude and direction. The total momentum of a system made up of a collection of objects is the vector sum of all the individual objects' momenta. For an isolated system, total momentum remains unchanged over time; this is called conservation of momentum.

Example: when a batter hits a baseball, the momentum of the bat just before it strikes the ball plus the momentum of the pitched baseball is equal to the momentum of the bat after it strikes the ball plus the momentum of the hit baseball. As another example, imagine a beaver jumping off a stationary log that is floating on water. Before the beaver jumps, the log and the beaver are not moving, so the total momentum is zero. Upon jumping, the beaver acquires forward momentum, and at the same time the log moves in the other direction with an equal and opposite momentum; the total momentum of the beaver plus the log remains at zero.

Conservation of momentum is one of the most important and universal of the conservation laws of physics; it holds true even in situations where modern theories of physics apply. In particular, conservation of momentum is valid in quantum mechanics, which describes atomic and nuclear phenomena, and in relativistic mechanics, which must be used when systems move with velocities that approach the speed of light.

According to Newton's second law of motion—named after the English astronomer, mathematician, and physicist Sir Isaac Newton — the force acting on a body in motion must be equal to its time rate of change of momentum. Another way of stating Newton's second law is that the impulse—that is, the product of the force multiplied by the time over which it acts on a body—equals the change of momentum of the body.

"Momentum," Microsoft® Encarta® Online Encyclopedia 2005