The ratio of the radii of gyration of a hollow cone and a hollow cylinder of the same mass and radius about a tangential axis parallel to their central axis is:
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For a thin hollow cylinder about its own axis:
\[
I = MR^2
\]
For a hollow cone about its own axis:
\[
I = \frac{1}{2}MR^2
\]
Always apply the Parallel Axis Theorem carefully when shifting to a tangential axis.