A uniform horizontal disc fixed at its centre to an elastic vertical rod performs forced torsional oscillations due to the moment of forces Nz = Nm cos cot. The oscillations obey the law φ = φm cos (wt – a). 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.