Question

What is radius while calculating moment of inertia of sphere, disc, and ring?

Answer 1

Answer:

The radius is assumed to be R while calculation of moment of inertia of sphere, disc and ring. They are independent of the moment of inertia formula used.

There is something known as Radius of Gyration . You can treat the collection of particles / continuous object as if it were a single point where the all the mass was concentrated at certain distance from the center. This distance is call Radius of Gyration, expressed as:  

R axis = I axis / mass