The moment of inertia or MOI for any solid sphere with a mass M and radius R is given by:

I = 2/5 MR2 ………….. (1)

If K implies the radius of a solid sphere, then

I = MK2…………………. (2)

On combining both the equations 1 and a couple of, the equation also can be written as

MK2 = 2/5 MR2

Canceling M and taking root on both the edges, our equation now becomes:

K = root of 2/5 R