Physics
posted by Aja .
A nonrotating 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?

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. 
1/2w