ABSTRACT
The first moment of area about a fixed axis of a lamina of area A, perpendicular distance y from the centroid of the lamina is defined as Ay cubic units. The second moment of area of the same lamina as above is given by Ay2, i.e. the perpendicular distance from the centroid of the area to the fixed axis is squared. Second moments of areas are usually denoted by I and have units of mm4, cm4, and so on.
Several areas, a1, a2, a3, . . . at distances y1, y2, y3, . . . from a fixed axis, may be replaced by a single area A, where A D a1 C a2 C a3 C Ð Ð Ð at distance k from the axis, such that Ak2 D ∑ ay2. k is called the radius of gyration of area A about the given axis. Since Ak2 D ∑ ay2 D I then the radius of gyration, k =
