Circular Cross-Section
For a circular cross-section of radius , Eq.
(B.11) tells us
that the squared radius of gyration about any line passing through the
center of the cross-section is given by

Using the elementrary trig identity
, we readily
derive






For a circular tube in which the mass of the cross-section lies
within a circular annulus having inner radius and outer
radius
, the radius of gyration is given by
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Striking the Rod in the Middle
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Rectangular Cross-Section