# Moment of inertia (disk with off center hole)

1. Mar 3, 2009

### Imperitor

I have a disk with some thickness to it and I need its moment of inertia.
So this is the formula with r1=0

Now there is a "circular hole of diameter 'd' at a distance of 'r' from the geometric center of the disk." So I'm thinking that I should subtract the MoI of the hole from the disk. Here is a picture if you need it.

http://img99.imageshack.us/img99/782/50817033.jpg [Broken]

I could use the same MoI equation as I did for the disk on the hole but that doesn't account for the hole being off centered. Any ideas??

I need to eventually equate this to a small mass rotating around a center point, so if there is a more direct approach please tell me. Thanks in advance for the help.

Last edited by a moderator: May 4, 2017
2. Mar 3, 2009

### tiny-tim

Welcome to PF!

Hi Imperitor ! Welcome to PF!
That's right!

And then use the parallel axis theorem to find the moment of inertia about an axis not through the centre of mass.

3. Mar 3, 2009

### Imperitor

Thanks. I really should have known that...

I'm really enjoying this forum. It's a great resource for an engineering student.

4. Mar 3, 2009

### Imperitor

Wait a minute... hit another snag. To use that theorem I need to know what the mass of the missing hole is. I only know the mass of the disk as is (with the hole in it). Also the first formula I gave could only be used if I knew what the mass of the disk was with the hole filled in. This is driving me nuts. I can't even start working on the question I'm doing without getting this MoI.

5. Mar 4, 2009

### tiny-tim

just got up… :zzz:
Easy-peasy …

divide the given mass by the area, to give you the density

then multiply by each area separately!