Conservation of Angular Momentum

You recall what made regular old linear momentum such a useful quantity, right? Even in complicated interactions or collisions between objects, as long as there were no EXTERNAL forces acting -- so as long as the only forces were exerted by the objects themselves -- then the total linear momentum was conserved.

So, even in complicated interactions between humans on ice, we can use the initial properties of the skaters to deduce something about their final speeds.

It turns out that there is a similar and very useful connection between angular momentum and interacting objects. We know that

So, as long as there are no EXTERNAL torques affecting our objects, their overall angular momentum around any point must be constant.

Some examples involving Conservation of Angular Momentum
A "collision" involving angular momentum.
Some activities illustrating conservation of angular momentum
Some problems illustrating conservation of angular momentum