4 Replies Latest reply on Apr 28, 2013 3:08 PM by Jerry Steiger

# Orientation of the center of mass

Hi, I have a question about the coordinate of the COM. I downloaded an assembly. The assembly is orientied the same as the global coordinate system. That is, facing the positive z-axis, top of the assembly is in the positive y direction, etc. When I checked the COM of the motor, I noticed that the coordinate of the COM (pink in the figure) is different in orientation as the one in the global coordinate system. My question is as followings:

SW gives x = -82.55, y = 43.50, z = 1.25. Which x,y,z is SW referring to? The same orientation as the global coordinate system or the same orientation as the pink coordinate of the COM? I am confused about which orientation SW is referring to.

• ###### Re: Orientation of the center of mass

Hajime

The pink triad is not a new coordinate system, it is the moments of inertia. Notice they are not x, y, z, but rather Ix, Iy, and Iz. The COM coordinates are given in the global coordinate system.

Dwight

• ###### Re: Orientation of the center of mass

Hi Dwight,

Thanks. What is the purpose of showing the moments of inertia? Is there any useful information or physical interpretation that I can obtain from the pink triad?

• ###### Re: Orientation of the center of mass

Hajime,

It's been too many years since I took Dynamics to answer your question properly. The moments of inertia are a measure of how difficult it is to change the rotation of a body around a particular axis. I don't remember what makes the principal axes important, but I did find the following by a google search:

Axes of Inertia, Principal

(or principal axes), three mutually perpendicular axes passing through any point of a rigid body that have the property that if these axes are taken as the coordinate axes, then the products of inertia of the body about these axes will be zero. If a rigid body that is fixed at a single point is rotated about an axis that is a principal axis at this point, then in the absence of external forces the body will continue to rotate about this axis as if the axis were fixed. The concept of principal axes plays an important role in the dynamics of rigid bodies.