A disc is recast into a hollow and thin cylinder of the same . . .

Question : A disc is recast into a hollow and thin cylinder of the same radius. Which will have larger M.I.? Explain.

Doubt by Manav

Solution : 

As we know, 
Moment of Inertia of a solid Disc along an axis which is passing through the centre and perpendicular to the plane is given by
I = ½MR²

Also, Moment of Inertia of a Hollow cylinder along an axis which is passing along its axis of rotation is given by 
I = MR²

Clearly, Moment of Inertia of hollow cylinder is double the Moment of Inertia of solid disc. So the hollow cylinder have larger Moment of Inertia than solid disc. 

Alternate Answer : 

Moment of Inertia of a hollow cylinder is more because most of its mass is placed on the outer surface of the hollow cylinder as compared to solid disc.