# Moment of Inertia and axis of rotation

1. Oct 31, 2007

### markjc

What I'm about to ask isn't a homework question, but it is a personal question relating to a home-project I'm working on.

My question is, what is the moment of inertia of a hollow square tube about an axis perpendicular to its length?

Here's a poor diagram: -----|----- (where | is the axis of rotation)

I've attempted to calcuate it a few times and I'm not sure any of my answers make sense. Then again, I'm pretty rusty on these things and there's probably some key concept I'm missing.

Can anyone offer any help and/or explanation?

Thanks

2. Oct 31, 2007

### Staff: Mentor

Assuming that you need to calculate it accurately (and thus you can't just treat it as a thin rod), model it as a set of four rectangular sheets. You'll probably need to use the parallel axis theorem. And you can look up the rotational inertia of a rectangular sheet, if you need to.

3. Oct 31, 2007

### FredGarvin

Take a look here and see if the size you need is in the tables. I have my chart at work that has all of my formulas on it. So this will hopefully suffice for tonight.

http://www.steeltubeinstitute.org/pdf/brochures/dimension_brochure.pdf [Broken]

EDIT: Ahh crud. I just realized that this doesn't include the data you're looking for. Whoops.

Last edited by a moderator: May 3, 2017