Problem

A 2.4 kg metal disk is horizontal and is spinning at 30 rpm. A second disk with the same radius, but with a mass of 5.2 kg is initially not spinning and is lowered gently onto the spinning disk. The system of the two disks is then isolated, such that there is no external torque on it. The disks scrape against each other until they are moving with the same speed. What is the final rotational speed of the disks, in rpm?

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