The rotational equivalent of linear momentum. Angular momentum is often approximately conserved in collisions, and is usually conserved when external torques sum to zero. The action of purely internal forces can sometimes create a change in the system moment of inertia while conserving the angular momentum, which leads to interesting effects.
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Motivation for Concept
Imagine a wheel spinning in space. In the absence of external influences, the wheel will continue to spin. To stop the spinning would require the application of force. The faster the wheel is spinning, the more work the force will have to perform to halt the wheel. These concepts are completely analogous to the linear ideas of force and work. As such, there must be some quantity analogous to the linear momentum which is conserved in the absence of external influences. Angular momentum is that quantity.
Conservation of angular momentum is a more complicated topic than conservation of linear momentum, however. It is very rare for an everyday system to experience a significant change in mass. Thus, in physics problems involving conservation of linear momentum, the center of mass velocity of the system is usually constant. It is not difficult, however, for a system to alter its moment of inertia. In a case where angular momentum is conserved, such an alteration must produce an inverse alteration in the rotational speed. This is easily seen in sporting events like figure skating and diving. When an athlete is rotating essentially free of external forces (such as a figure skater in a spin on slippery ice or a diver rotating in mid-air) they can still affect a dramatic change in their rotational rate simply by tucking in their arms and legs or, conversely, by extending their arms and legs.