angular momentum is conserved
the momentum lost by the disc is transferred to the cylinder
A uniform disc of 2.98 kg is mounted so that it can rotate in the horizontal plane around a frictionless axis through its center of mass. The angular velocity of the disc at the start is 140 rpm. A hollow cylinder with thin walls and the same radius as the disc is released from rest just above the turntable, so that the axis of rotation also passes through its center of mass. Friction forces ensure that the two objects rotate at the same angular speed of 77.8 rpm after a short time. Determine the mass of the hollow cylinder.
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