A clutch consists of two disks with rotational inertias I1 = 4 kg∙m2 and I2 = 2 kg∙m2 which are mounted to rotate about a common axis. Disk 1 is initially rotating at 60 rpm and disk 2 is initially at rest. The two disks are then pressed together so that, after some slippage, they both rotate at the same angular speed.
What is their final angular speed?
(I already know this is 40 rpm.)
*How much kinetic energy, if any, was lost when the disks were engaged?*
(I know the answer is 263 J, but I don't know how to find this.)

Question

A clutch consists of two disks with rotational inertias I1 = 4 kg∙m2 and I2 = 2 kg∙m2 which are mounted to rotate about a common axis. Disk 1 is initially rotating at 60 rpm and disk 2 is initially at rest. The two disks are then pressed together so that, after some slippage, they both rotate at the same angular speed.
What is their final angular speed? 
(I already know this is 40 rpm.)
*How much kinetic energy, if any, was lost when the disks were engaged?* 
(I know the answer is 263 J, but I don't know how to find this.)