# Homework Help: Ratio of inertia

1. Nov 13, 2008

### plus1

1. The problem statement, all variables and given/known data

Two disks have the same radius R and the same mass M, but one of the disks is twice as thick as the other. Disk A has thickness t, and disk B has thickness 2t. How does the moment-of-inertia of disk A about an axis through the center compare to that of disk B? Specifically, what is the ratio I_A / I_B ?

2. Relevant equations

above

3. The attempt at a solution

The moment of inertia of a disk is I=1/2MR^2 but I don't understand where the thickness would be. If the thickness has no significance in the equation then would ratio be 1:1

2. Nov 13, 2008

### Staff: Mentor

What's the moment of inertia of a cylinder compared to that of a disk?

3. Nov 13, 2008

### nasu

There are three axes through the center (actually an infinite number).
The moment of inertia for the axis perpendicular to the base(s) of the cylinder (disc) does not depend on the thickness.The other ones do.
But I suppose the problem assumes the simplest case, right?

4. Nov 13, 2008

### plus1

isn't the moment of inertia of a solid cylinder the same as that of a disk? i looked it up in my physics book and it gave the moment of inertia as 1/2MR^2 for a disk.
since the moment of inertia doesn't depend on thickness, the ratio of the two disks would be 1:1, right? that was my first guess but wasn't sure.

5. Nov 14, 2008

### Staff: Mentor

Exactly, assuming--as nasu points out--that we are talking about an axis perpendicular to the center of the disk. (Which is the simplest case.)

My reason for bringing up the cylinder was so that you didn't have to guess.