# Homework Help: Angular momentum (two discs).

1. Jan 6, 2007

### MathematicalPhysicist

a disc with moment of inertia I1 turns around its axis which passes through its centre with an angular velocity of w0. we attach to it another disc with moment of inertia I2 (at first it doesnt spin), as a result of friction between the discs they achieve a combined angular velocity of w.
i need to find the combined angular velocity, w?
how much mechanical velocity was lost in the process.

basically i think i need to use here conversion of angular momentum, i.e the a.m before is I1w0, my problem is what the angular momentum after we add the second disc, i think it's I2w+I1w am i correct here?
if so then I1w0=(I1+I2)w.
and to calculate the loss of energy we have:
1/2I1w0^2-1/2(I1+I2)w^2, right?

Last edited: Jan 6, 2007
2. Jan 6, 2007

### Hootenanny

Staff Emeritus
Assuming they rotate about the same axis, yes you are correct.

3. Jan 7, 2007

### MathematicalPhysicist

yes they rotate about the same axis.