Eli5: Angular momentum, how does it work?
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All momentum is an offshoot of inertia, which is an object’s tendency to keep doing what it’s doing, whether that’s being still or moving. Angular momentum just refers to spinning instead of linear movement.
Think of a spinning top; even though you only held it and accelerated it for a bit, it still keeps spinning for a while after you’ve let it go. Of course, friction and drag eventually slow and stop it.
The other aspect of angular momentum is where the mass is located on the object. For linear movement/momentum, the entire object is moving at the same linear speed; one part of a falling apple can’t be moving faster than another. For a spinning object, however, that’s not true.
A point near the center of the object (or the axis of rotation) is moving slower than a point at the edge; this should be evident by the formula for an object’s circumference, where the circumference of a circle (which is the distance covered in one rotation) is dependent on the diameter of the circle. Larger diameter, larger circumference, longer distance over the same time (one rotation), faster movement.
So when you are spinning on an office chair with your legs out, there is a lot of mass that is far from the center of the chair, moving fast. When you bring your legs in, the mass moves closer to the center of the chair and moves more slowly. This changes the momentum of the spin, which can’t happen unless you put energy into the system to accelerate/decelerate it. The only solution is that the speed of rotation goes *up* so that the momentum (mass/*speed) is still the same.
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