Homework Help: Angular Velocity Q

1. Oct 28, 2004

Here's the problem as written:

A disk with moment of inertia I1 rotates with angular velocity Wi about a frictionless vertical axle. A second disk, with moment of inertia I2 initially not rotating, drops onto the first disk. Since the surfaces are rough, the two eventually reach the same angular speed.

I am suppost to find the final angular velocity, Wf, of the two spinning disks after the first disk falls onto the scond, spinning disk.

This is what I've got so far:
The problem is from the conservation of angular momentum section of my text book, so i figured I'd use:
I2*Wi = (I1+I2)Wf
after i solve for Wf(which is what i want to find) i get:
Wf = (I2*Wi)/(I1+I2)

Is this the correct way to go about solving this? I'm not getting the right numbers when I plug them in....

2. Oct 28, 2004

Staff: Mentor

You made an error in writing the conservation equation. It should be:
$$I_1 \omega_i = (I_1 + I_2) \omega_f$$