# Momentum FAQ

Momentum is a physics concept that describes the way objects in motion behave. It explains why it is hard to stop a runaway train. The heavier something is and the faster it is going, the more force it will take to make it stop.

What is momentum?

In physics, momentum is mass times velocity. In symbols, it can be expressed by the formula p=mv. On the surface of the earth, mass approximately equals weight. Velocity is a measure of how something changes position with time. So for everyday purposes, momentum is how fast something with a certain weight is going somewhere.

In physics, velocity is a vector quantity. That is, something is not merely going; it is going in a particular direction. As a consequence, velocity has direction, too.

What is the difference between inertia and momentum?

Mass actually measures inertia. The more mass an object has, the more it resists being accelerated, therefore the more inertia it has. Also, the more mass it has, the more it resists changes to its motion once it is moving. That's Newton's First Law: An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an outside force. The law is about inertia, a property of a physical object.

Momentum increases as mass increases, but it also increases with velocity because momentum is the product of mass and velocity. So a slow train has a lot of momentum because of its mass. So does a speeding bullet, though in this case, because of its velocity.

What is conservation of momentum?

In a closed system, momentum is not gained or lost. An object can transfer its momentum to another object, the way the cue ball transfers its momentum to other balls in a pool game, but the momentum will not simply go away.

Objects on earth may appear to lose momentum all by themselves, but they do not. The motion of objects on earth is slowed by gravity, by air pressure and by friction. It is friction or gravity that brings objects to a halt, not an intrinsic loss of momentum. In an environment free of outside forces, such as a vacuum, an object set in motion would keep on going forever, never losing its momentum.

What is angular momentum?

Like linear momentum, angular momentum is a vector quantity. It has direction. Linear momentum is expressed as an amount of mass times the speed and direction it is traveling. Angular momentum can be expressed as an object's rotational inertia-determined by its mass-times its angular velocity, or the speed and direction of its rotation. Angular momentum is analogous to linear momentum, but it describes the motion of orbiting planets and rotating fans instead of that of billiard balls and sliding anvils.

What is the conservation of angular momentum?

Angular momentum is conserved in a system in which no outside torque, which is the twisting force, intrudes. If not for friction, particles set spinning would rotate forever, according to the tenets of classical mechanics.

The conservation of angular momentum explains how a figure skater increases the speed of a spin by drawing his or her arms closer to the body. It also explains why the planet Mercury, closest to the sun, travels faster than the distant planet Neptune.