# Centre of gravity of a 3D irregular object

## Homework Statement

A complex or irregular body with some mass, m. How to determine the 3 co-ordinates of centre of gravity?

## Homework Equations

Moment method with ∑M = 0.

## The Attempt at a Solution

Say I have a gearbox and I need to find the COG. I can mount the gearbox at two longitudinal ends and find one co-ordinate of COG through force acting in one end and entire weight of body acting at some distance in between, through moment method. I can then mount it upside down with the clutch part on the bottom and repeat the procedure. This way I can take the intersection of the two planes of probable COG and get a line at the intersection. But to find the 3rd co-ordinate is proving difficult. Is there any way to find the 3rd co-ordinate or any other totally different way to find all the 3 co-ordinates? Please help people. Thanks in advance!

P.S - I thought of mounting in another direction to get the third intersection too, but the mounting gets difficult. So for the gearbox or any other irregular body in general would be rather helpful. :)

mfb
Mentor
I thought of mounting in another direction to get the third intersection too, but the mounting gets difficult.
Well, that is an engineering issue. To measure it you'll need a third input in some way. Mounting it on a string (free to move in two dimensions) or balancing it on something (similar) would help as well.
You can also calculate it if you know the components well enough.

Well, that is an engineering issue. To measure it you'll need a third input in some way. Mounting it on a string (free to move in two dimensions) or balancing it on something (similar) would help as well.
You can also calculate it if you know the components well enough.
Hi, thanks for the suggestion. In case of an engine or a gearbox, there are lots of components so it's totally complex but balancing can be tried but, do you have any other method or suggestion to solve this problem?

CWatters
Homework Helper
Gold Member
Can you hang the gearbox from a wire?

Exactly my point. It's highly risky but it can be tried though it might not be possible in all directions. So I'm out of ideas.

mfb
Mentor
Hanging it from a string/wire and one additional measurement along a different axis would be sufficient.

You can also mount the whole device into a more sturdy frame (using any attachment points you like) if you know the mass and center of mass of this more sturdy frame. Ideally it has a small mass.

Stephen Tashi and Raghavendar Balaji
Hanging it from a string/wire and one additional measurement along a different axis would be sufficient.

You can also mount the whole device into a more sturdy frame (using any attachment points you like) if you know the mass and center of mass of this more sturdy frame. Ideally it has a small mass.

Oh thank you! I think that might work! Thanks for the suggestion!!