Outline · [ Standard ] · Linear+ Moments Of Inertia Debate

[tlehnhaeuser]

Ok, theres a debate going on here which is out of my league since I am not a PE. Maybe the Class A guys can help resolve this.

I have some colleagues that feel the units of measurement on the Moments of Inertia Compute Dialog Box are not right. There is question that some of these units should not be in lb/cu.in.. Does this sound right to you?

Again, I cannot answer due to lack of schooling

Any feedback is welcomed.
Thanks
Tom


This post has been edited by tlehnhaeuser: Mar 9 2006, 08:43 AM

[Cary OConnor]

Well there are two different ways to get values. MOI in a 3D and MOI of 2D Plane(area moment of inertia). If you do these in IronCAD, the units will adjust accordingly.

3D -- lb/in^2
2D -- in^4

Make sure they are asking for the correct information. Look at the link below for more informtation. Hope that helps.
Cary


http://en.wikipedia.org/wiki/Moment_of_inertia -- For some information.

[EricFoy]

Well there are two different ways to get values. MOI in a 3D and MOI of 2D Plane(area moment of inertia). If you do these in IronCAD, the units will adjust accordingly.

3D -- lb/in^2
2D -- in^4

Make sure they are asking for the correct information. Look at the link below for more informtation. Hope that helps.
Cary
http://en.wikipedia.org/wiki/Moment_of_inertia  -- For some information.

Just a little bit of a correction:
The moment of inertia of a mass taken about a given axis of rotation is equal to its total mass multiplied by the square of the orthogonal distance from said axis to the center of gravity (geometric centroid, in the case of a homgeneous mass) of said mass.

I=mr^2
-not-
I=m/r^2

Therefore, the units are

mass X length^2
-not-
mass / length^2

Therefore, the units are
lb in^2
-not-
lb/in^2

In many engineering problems (such as the analysis of beam cross-sections), a different kind of moment of inertia is used. This is proerly called the "area moment of inertia". This is a 2D object, and therefore has no mass. The area moment has units

length^4

If you select a surface shape and analyze it in IronCAD, it will compute the area moments for you, providing the proper units in the dialog.

Looking back at your post, Cary, It is now clear to me that you said all that.

[Cary OConnor]

oops...yeah a little slip of the keyborad with that "/"...
Cary