Moments of Inertia of area: Parallel axis theorem
In many cases, the moment of inertia about an axis, particularly an axis passing through the centroid of a common shape, is known (or relatively easier to calculate) and the moment of inertial of the area about a second axis parallel to the first axis is needed. The parallel axis theorem relates these two moments of inertia.
To derive the theorem, an area as shown in Fig. 10.9 is considered. The centroid of the area is denoted as , the axis is an axis crossing the centroid (a centroidal axis), and the axis is an arbitrary axis parallel to .
If is a differential element of the area, its (perpendicular) distance to the axis can be written as where is the distance between the two parallel axes shown in Fig. 10.9. Therefore,
The term equals zero because and (measured from the axis) because passes through the centroid. Consequently,
With and , then
which reads the moment of inertia about an axis is equal to the moment of inertia about a parallel axis that crosses the centroid of , plus the product of area and the square distance between and .
Equation 10.7 can be written for any two parallel axes with one crossing the centroid of the area. If is an axis crossing , and a parallel axis to as shown in Fig. 10.10a, we can write,
where is the distance between the two parallel axes.
The parallel axis theorem also hold for the polar moment of inertia. If is a point in the plane of an area and distant from the centroid of the area as shown in Fig. 10.10b, the polar moments of inertia about and are related as,
Determine the moment of inertia of a rectangular area about the x and y axes shown. Use the moment of inertia about the centroidal axes parallel to its sides.
Let and be two axes crossing the centroid of the area as shown. According to Example 10.1.1,
Therefore, by Eq. 10.8, we can write,
If for the depicted triangular area, determine the moment of inertia about the axis crossing the centroid of the area and parallel to the x axis.
Find the distance from the x axis to the centroid of the area, and use Eq. 10.8.
Figure 10.11 demonstrates some basic areas and the formulations of their moments of inertia (see this document for some other areas).