If M.I. about an axis of a system of mass M is I, then we may write, `I = MK^2` where K is called radius of gyration of the body about that axis.

`K=sqrt(I/M)`

In other words M.I. about an axis of system is same as M.I. of a particle of same mass placed at the distance equal to K from the axis of a uniform disc of radius R, radius of gyration about an axis passing through its centre & is to its plane is

`K=sqrt(((MR^2)/2)/M)=R/sqrt2` i.e. the disc will same rotational inertia as that of a particle of same mass placed at distance `R/sqrt2` from the axis.

`text(Note :)` In case of system of combination of various bodies `K=sqrt((I_text(total))/(M_text(total))`