Center of Mass; Moment of Inertia: Feynman's Physics Explained
A thrown wrench tumbles in a complicated way, yet one special point — the center of mass — flies in a simple parabola, as if all the mass were concentrated there. That trick lets us separate an object's overall motion from its spin about that point. The moment of inertia describes how the mass is spread out around the axis of rotation. A figure skater spins faster by pulling her arms in — not by changing her mass, but by changing her moment of inertia.
The big idea
Every object has a balance point that moves simply, however it tumbles.
Think about it
Why does a skater speed up when she pulls her arms in and slow down when she stretches them out?
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