Moment of Inertia for a solid circular disc

[Elmowgli]

hey kinda new to this and I know the rules say I am not allowed to be told how to do this but I am totally stumped and its to be handed in tomorrow. I've looked through everything and cannot find out how to do it anywhere I am 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

Homework Equations

I=0.5mr^2

The Attempt at a Solution

the only thing I have found that's in any way similar is the MOI of a sphere which equals 2/5MR^2, I think if there's some way of multiplying the two fifths by the regular equ then it would work, but i can't find anyway of doing that.

Any help is appreciated :)

 

[berkeman]

hey kinda new to this and I know the rules say I am not allowed to be told how to do this but I am totally stumped and its to be handed in tomorrow. I've looked through everything and cannot find out how to do it anywhere I am 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

Homework Equations

I=0.5mr^2

The Attempt at a Solution

the only thing I have found that's in any way similar is the MOI of a sphere which equals 2/5MR^2, I think if there's some way of multiplying the two fifths by the regular equ then it would work, but i can't find anyway of doing that.

Any help is appreciated :)

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

 

[Elmowgli]

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

 

[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.

 

[berkeman]

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.

Yes, good hint

 

[Elmowgli]

hi that's really helping, I just realized 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?

 

[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!

 

[TVP45]

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

The only dumb questions are the ones unasked.

 

[Elmowgli]

haha, too true! :)