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. How much kinetic energy, if any, was lost when the disks were engaged?

Answer 1

Are you told the final rpm of both disks?

Answer 2

no

Answer 3

This is a classic problem, and not easy because neither angular momentum nor KE are conserved.

Answer 4

angular momentum is conserved as there is no external force which has tendecy to produce torque.