Difference between Rotation Inertia of Rod V.S Cylinder

[apples 429042]

A rod and Cylinder with a same axis of rotation relative to its body have different rotational inertia equation. WHY?

Thanks in advanced.

 

[zabini]

The rod is assumed to have negligible radius. It is assumed to be a straight line having a fixed mass. In case of a cylinder, it has a radius hence its moment of inertia is different from that of a rod.