Moment Of Inertia Of A Semi-Circular Disk?

[Roomie]

Homework Statement

http://dl.dropbox.com/u/630750/Screen%20Shot%202012-04-26%20at%2010.03.36.png

Homework Equations

The Attempt at a Solution

http://dl.dropbox.com/u/630750/Photo%2026-04-2012%2010%2001%2034.jpg

I know this is wrong but I can't really see why, can anybody help?

Thanks!

 

[tiny-tim]

Hi Roomie!

(please type your answers in future)

Why 1/4 Mr² ?

 

[Filip Larsen]

Your approach is correct, but you've made two mistakes inserting the numbers (hint: you're missing a square and use wrong mass). Check your result again (actually, you should probably make a habit of doing this before posting in the first place)

 

[Roomie]

Ah yes I see that I missed a squared, and that makes my answer 0.111. But I don't see how I have used the wrong mass? The mass of the semi circular disk is 10kg?

 

[Filip Larsen]

Sorry, I misread the text as if the mass of the full disc was 10 kg.

Reading it correctly I can then only ask as tiny-tim did (that is, check your equation for the moment of inertia around O).