A non-rotating cylindrical disk of moment of inertia I is dropped onto an identical disk rotating at angular speed w. Assuming no external torques, what is the final common angular speed of the two disks?

Answer 1

You doubled moment of inertia. Angular momentum total must be the same.

If you keep angular momentum the same, then angular speed must be 1/2 the original.

Rotational energy is not conserved.

Answer 2

1/2w