The ebook Elementary Calculus is based on material originally written by H.J. Keisler. For more information please read the copyright pages.


Moment

An object in space has a moment about each coordinate plane.

DEFINITION

If an object in space fills a region E and has continuous density ρ(x, y, z) its moments about the coordinate planes are

12_multiple_integrals-353.gif

The center of mass of the object is the point (x, y, z), where m is mass and

12_multiple_integrals-354.gif

JUSTIFICATION

A point mass m has moment Mxy = mz about the (x, y) plane (Figure 12.6.9). In an element of volume ΔE, the object has moment

ΔMxy ≈ z Δm ≈ zρ(x, y, z) ΔV (compared to ΔV).

12_multiple_integrals-355.gif

Figure 12.6.9

By the Infinite Sum Theorem,

12_multiple_integrals-356.gif


Last Update: 2010-11-25