# Homework Help: Moment of inertia

1. Apr 17, 2007

### bidhati

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

A horizontal disc of diameter 12.0 cm is spinning freely about a vertical axis
through its centre at an angular speed of 72 revolutions per minute. A piece
of putty of mass 5.0 g drops on to and sticks to the disc a distance of 4.0 cm
from the centre. The angular speed reduces to 60 revolutions per minute.
Calculate the moment of inertia of the disc. You should assume that no
external torques are applied to the system during this process.

2. Relevant equations

conservation of momentum
I0w0=Ifwf

parallel axis theorim
Iw=Iw+mr^2

3. The attempt at a solution

combining two equations gives Iw=Iw +mr^2
but do I need to convert angular speed to radians per sec? and I assume the mass is the putty mass not the disc?

Last edited: Apr 17, 2007
2. Apr 17, 2007

### Mentz114

If you knew the mass of the disc, the question would have no point would it, because that's what you're asked to find ?

Yes, use radians/sec for angular velocity.