#### 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

Similarly, the radius of gyration about a vertical axis passing through the center of the cross-section is . 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.