Consequently, the radius of gyration is given by

The physical interpretation of the radius of gyration is that it is the radius of a uniform thin hoop (or ring), having the same moment of inertia (about an axis passing through its geometric center – shown below), as the given body about the specified axis.

The moment of inertia of the thin hoop about the blue axis passing through its geometric center is:

The radius of gyration can be useful for listing in a table. If you want to know the moment of inertia of a complex shaped body about a given axis you simply look up its radius of gyration, and then (knowing its mass) apply the above formula to find the moment of inertia.

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