A uniform horizontal disc fixed at its centre to an elastic
vertical rod performs forced torsional oscillations due to the moment
of forces N, = N,„ cos wt. The oscillations obey the law cp =
cpn, cos (cot — cc). Find:
(a) the work performed by friction forces acting on the disc during
one oscillation period;
(b) the quality factor of the given oscillator if the moment of
inertia of the disc relative to the axis is equal to I.