Physics problem

Hi Guys,

I'm studying basic physics and having trouble with a question where I have to calculate the moment of inertia of a rotating disc.  The question:

"A disc of diameter 12 cm is spinning about a vertical axis through its center with an angular speed of 72 rpm. A bit of putty with mass 5 g drops on to the disc at a distance of 4 cm from the center. The angular speed drops to 60 rpm. Work out the moment of inertia of the disc. Assume that no external torques are applied to the system during this process."

I think that I have to use conservation of angular momentum in order to work this out, but I'm not sure where to start... any suggestions would be much appreciated :)

I_d \omega_0 = (I_d+I_p)\omega_1

where I_d is the inertia of the disc, I_p is the inertia of the putty, and the omega's are the rpm before and after

solve for I_d, you can compute the inertia of the putty

i think that works

The moment of inertia of the disk is I = mr^2/2, where r = 6cm and the mass is unknown.  The angular momentum is L = I*omega, where omega is the angular speed.  Now once you drop the putty on, you know the new angular speed, so you can calculate the angular momentum for the putty and the disk (in terms of the unknown disk mass).  You can then solve for the disk mass using the conservation of angular momentum; the angular momentum of the disk + putty at 60rpm has to be the same as the angular momentum of just the disk at 72 rpm.

1+1=2

Thanks for your help guys, I think I've got it now after reading your posts. 

I've worked out the moment of inertia as 0.015 kg m-2.  Does this look right to you guys? 

I'm only in the first year of my degree so I'm still not confident about my maths skills...