# Moment of Inertia for a solid circular disc

1. Jul 20, 2009

### Elmowgli

hey kinda new to this and I know the rules say im not allowed to be told how to do this but im totally stumped and its to be handed in tomorrow. I've looked through everything and cannot find out how to do it anywhere im starting to think there is a typo in the question paper :S

show that a disc rotating about an axis that passes through the edge of the disc and parallel to its diamter is I=1.25mr^2

2. Relevant equations

I=0.5mr^2

3. The attempt at a solution
the only thing I have found thats in any way similar is the MOI of a sphere which equals 2/5MR^2, I think if theres some way of multiplying the two fifths by the regular equ then it would work, but i cant find anyway of doing that.

Any help is appreciated :)
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 20, 2009

### Staff: Mentor

Welcome to the PF. Are you familiar with how to use integration to calculate the MOI?

3. Jul 20, 2009

### Elmowgli

thanks :) im vaguely familiar with it but im not exactly great at it.

4. Jul 20, 2009

### TVP45

Well, I don't really understand the question, i.e., the syntax seems a bit awkward, but never mind. I think it's OK to give you this suggestion: Look at the parallel axis theorem.

5. Jul 20, 2009

### Staff: Mentor

Yes, good hint

6. Jul 20, 2009

### Elmowgli

hi thats really helping, I just realised that it in my confused state I forgot to include that the question before this was to prove that,
a disc rotating about an axis that coincides with a diamter is I=.25mr^2.

and I was wondering would this give me the value that I would sub in for D in the equation?

7. Jul 21, 2009

### Elmowgli

I got it eventually, I was just looking at it the wrong way round,

ended up with

Io=Ic + md^2
= 0.25mr^2 + mr^2
= 1.25mr^2

feel very stupid now after seeing how easy it was!

8. Jul 21, 2009

### TVP45

All my problems have been easy once I saw how to do them.

The only dumb questions are the ones unasked.

9. Jul 21, 2009

### Elmowgli

haha, too true!! :)