Angular Momentum

Mostly refer up to :45....
I chose this video because it explains the idea of angular momentum with just demonstrations. When the guy in the beginning of the video is on a spinning chair, he starts out holding weights and his arms are out along with his legs. When he pulls his arms and legs in, he gets noticeably faster. This happens because when an object is closer to its axis of rotation, it has a smaller inertia and a faster velocity opposed to when it is farther away from the axis of rotation.
The formula for angular momentum is
(ROTATIONAL INERTIA)(rotational velocity) = (rotational inertia)(ROTATIONAL VELOCITY)
Inertia is measured in mass so when an object has more inertia, it has more mass, which makes it harder to move. When something is farther away from the axis of rotation it has a higher rotational inertia and a lower velocity. When something is closer to the axis of rotation, the inertia is smaller and has a higher velocity.
This video brilliantly shows angular momentum.