#### Rectangular Cross-Section

For a rectangular cross-section of height and width , area , the area moment of inertia about the horizontal midline is given by

The radius of gyration can be thought of as the ``effective radius'' of the mass distribution with respect to its inertial response to rotation (``gyration'') about the chosen axis.

Most cross-sectional shapes (*e.g.*, rectangular), have at least two
radii of gyration. A circular cross-section has only one, and its
radius of gyration is equal to half its radius, as shown in the next
section.

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Circular Cross-Section

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Circular Disk Rotating About Its Diameter