# Center of Mass and Inertia Tensor Experimentation

1. Sep 6, 2011

### landong

Hello All,

I am trying to create an experiment to determine the center of mass and inertia tensor of an aribtrary object. This object is small (softball size), and non symmetrical on all axis. I have some thoughts concerning gyroscopes and accelorameters, but am curious to see what thoughts others might have on equipment/methods to determine these characteristics experimentally.

Any insight/suggestions you would be willing to provide would be greatly appreciated.

Thanks.

2. Sep 7, 2011

### Studiot

When balancing rotating objects the entries in the tensor corresponding to moments of inertia can be balanced by static balancing. Those coresponding to product of inertia can only be balanced by dynamic balancing.

see this thread for an explanation, in particular posts 7 and 8

So your experiment should involve static and dynamic balancing.

go well

3. Sep 7, 2011

### landong

I have created a static balancing experiment to determine the center of mass of this object.

However, I am having trouble creating a "dynamic balancing" project to find the Inertia Tensor. I believe this is most likely due to my lack of understanding of the theory behind what these properties really mean. I reviewed the other thread and found your diagram helpful in differentiating between principle moments of inertia (Ixx) and products of inertia (Ixy). I was hoping you might be able to give me some examples as to how I could complete a dynamic balancing scenario and what data would need to be monitored to calculate the inertia tensor.

Thanks again.

4. Sep 7, 2011

### Andy Resnick

Finding the center of mass is easy- hang the object sequentially from three points, drawing a line from where you hang the object straight down. The lines intersect at the center of mass.

Getting the principal axis of the moment of inertia can be found here:

https://www.physicsforums.com/attachment.php?attachmentid=13128&d=1205872956

(Shamelessly stolen from Fred Garvin)

http://www.space-electronics.com/Products/moment_of_inertia_instruments.php
http://www.idicb.com/moimeas.htm [Broken]

Last edited by a moderator: May 5, 2017
5. Sep 7, 2011

### landong

When considering the experiment in:

https://www.physicsforums.com/attachm...8&d=1205872956 [Broken]

Would the stiffness of the wires used not come into play?

Also, this would give you the moments of inertia about the principle axis: Ixx, Iyy, Izz

How could the remaining factors of the inertia tensor be found (Ixy=Iyx, Ixz=Izx, & Iyz=Izy)?

Thanks for your help.

Last edited by a moderator: May 5, 2017
6. Sep 8, 2011