Finding center of gravity

[AlexDring]

I have a 3D model in Autocad Mechanical 6.0 and I need to find the center of gravity. It is an assembly drawing, meaning different solids used in one drawing that I created. If I unioned all the parts to make one solid, could I find by using the mass properties? Is the centroid the same as the center of gravity? I have other seats in CAD software, if someone knows a to figure it. The seats include: Mechanical Desktop 5.0, Autocad 2002, Solid Works 2001+, Solid Edge, TurboCad 9.0 Pro and CoCreate's One Space Designer 11.0 and 11.6

Any help would be nice. Thanks.

 

[EnglishMuffin]

In Solidworks, you just call up "mass properties" (under "tools" and it gives you the center of mass for any homogeneous solid that you happen to have created, or an assembly of any number of parts, provided you have correctly specified the density.
I probably shouldn't be answering this 'cause I only know Solidworks (of the ones you've listed), but I can't believe that AutoCAD mechanical, or any of the others, can't do this.
We recently had a long rambling discussion on a thread about centroid v center of gravity, but it got wiped out, I think because it was homework related. But for all intents and purposes, for a homogeneous solid, geometrical centroid = mass centroid = center of mass = center of gravity. For a non homogeneous solid, mass centroid = center of mass = center of gravity

 

[AlexDring]

EnglishMuffin-

Mechanical does do it, but I was trying to verify I had the concept correct. I also wanted to use the other programs I have to verify and validate each others solution.

 

[EnglishMuffin]

You may want to check this : thread507-68577 for some other info.

 

[hanibal]

holllooo
if i undestand your question her is a simple way to do it!!

export your geometry file to Ansys or better to LSDYNA3D it will do it for you if it is an AutoCad file it should be exportable..., in general the center of gravity it is not coincident with the centroid it is only the case for regular shapes a ball for examle.

Hanibal

 

[EnglishMuffin]

hanibal : your statement is not correct. Center of gravity and geometrical centroid are equivalent, regardless of the shape, provided the density is uniform.

 

[jerzy]

Uniform density AND uniform gravitational acceleration make center of gravity and centroid coincident. Uniform density but non-uniform gravitational field make CENTER OF MASS and centroid coincide, but not center of gravity.

 

[EnglishMuffin]

Jerzy : There was a long discussion about the relationship between center of mass and center of gravity in a thread which has unfortunately been removed, and I made exactly the point you are making. However, there is no measureable difference between center of gravity and center of mass in any practical situation that you are likely to encounter, and many textbooks actually make no distinction, so I was accused of being a pedant. I actually did try and compute the difference for a specific case, and the program failed because of a lack of quad precision.

 

[jerzy]

Yes, I agree it is almost nitpicking. Conceptually they do differ, but practically the distinction can almost always be ignored.

 

[hanibal]

EnglishMuFFIN:
you are right, do read my words carefully, IN GENERAL....
if i had the time i can prove that....
thanks
Hanibal

 

[EnglishMuffin]

Jerzy & Hanibal : I think the problem with this whole business is that you can argue forever about definitions - as we frequently do. If enough people define a term a certain way, even if it's technically questionable to begin with, that's still eventually going to become the definition, and of course this happens with language in general as well. Many respected textbooks actually say "center of gravity is the same as center of mass" (for example, Mathematics Dictionary, James & James, or Principles of Mechanics, Synge & Griffith). If ninety percent of reliable sources define it that way, how can you argue ? On the other hand, if you make the statement that the force of gravitational attraction between two bodies is identical to that between two equivalent point masses at the centers of mass of the original bodies, this is unarguably incorrect unless the bodies happen to be spheres. Another example of incorrect usage eventually becoming accepted is the term "moment of inertia" - which in the US has come to be universally used to refer also to second moment of area, which actually has nothing to do with inertia. Of course, that's not to say that you can't get a lot of fun and insight out of these endless discussions.