What Does 'Moment About a Plane' Mean in the Context of Solids?

[Nikitin]

http://www.math24.net/physical-applications-of-triple-integrals.html

"The static moments of the solid about the coordinate planes Oxy, Oyz, Oxz are given by the formulas"

What exactly does that mean? I understand what a moment about an axis is, but about a plane? what? Do they mean moment about two axis at the same time, or something?

And how did they arrive at the following formulas? http://www.math24.net/images/7tri2.gif

 

[Nikitin]

Help?

 

[lavinia]

Help?

The integrals give you the average mass density weighted value of each coordinate.

One shows that these weighted coordinates determine the center of mass. For a rigid body, its kinematics can be reduced to the motion of its center of mass - I think.

 

[Nikitin]

If a solid has a moment about a plane, what does this mean? I know that you can find the centre of mass in a solid, and then use this point to balance the solid, but how does this relate to "moment about plane"?

Does the "moment about the xy-plane" simply mean the solid's moment about the x and y axis?