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What is moment of inertia?

Answer:

Moment of inertia is the term used to measure or quantify the amount of mass located at an object's extremities. For example if all the mass of an object was located in a small compact size (like a lead ball) its moment of inertia would be small compared to the same amount of mass shaped into a dumbbell. Because a dumbbell has most of its mass located farther from its center. But there is a "qualification" here. Moment of inertia is calculated relative to a hypothetical spin axis. Once you choose the spin axis then you calculate the moment of inertia by multiplying the mass times its distance to the spin axis squared; I = MR^2

So in the example between the sphere and the dumbbell the moment of inertia of the dumbbell would be significantly larger relative to a spin axis perpendicular to the dumbbell length. If you instead choose your spin axis to lie thru the dumbbell parallel to its length then its moment of inertia, relative to this axis, would be much smaller because the mass would be located closer to that axis. And, in fact, it might even be smaller then the moment of inertia of a sphere about an axis thru its center.

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