Q. Show Constant Moment Arm?

The simplest case is that of a point mass, and the moment of inertia can be immediately written as                                  

I = mR²,

Where I be the moment of inertia m is the mass of the object and R is the distance from the axis to the object. It is worth noting that this relation as well holds for a thin hoop or ring with the axis perpendicular to the object and through its center. By extension it as well applies to a thin cylindrical shell with the axis of rotation coincident with the axis of the cylinder. The entire these shapes have essentially the same moment arm.