Momentum (physics)

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Momentum is the "quantity of motion" an object possesses. In classical physics, the linear form of momentum is defined as the product of mass and velocity:

Hence, the faster an object goes, or the more mass it posesses, the more momentum it has. Momentum is a vector quantity, and therefore has both a magnitude and direction. It is important to physicists because it is a conserved quantity, making it useful for solving problems.

In common usage, the words "momentum" and "inertia" are sometimes used interchangeably. Inertia is the tendency for a body to resist changes in its motion until and unless a force acts on it.

The motion of an object will continue until something makes it change its motion. A railroad car, once it gets going, will continue its motion for a long time, until the tiny forces of friction cause it to slow down and stop. This can take miles. Even putting on the brakes can take up to mile, because there is so much momentum.

A force in the same direction as the body is moving will increase its speed. A force in the opposite direction will slow it down.

A force coming from the side will cause a deviation from straight-line motion.

An interesting case of a sideways force is a weight on the end of a string (like the Biblical slingshot used by David against Goliath). When you twirl the weight around above your head, the string is pulling the weight toward you - but it never gets any closer! This kind of force is called a centripetal, or center seeking, force.


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