The disc of a torsional pendulum has a moment of inertia of 600 kg-cm2 and is immersed in a viscous fluid. The brass shaft attached to it is of 10 cm diameter and 40 cm long. When the pendulum is vibrating, the observed amplitudes on the same side of the rest position for successive cycles are 9”, 6” and 4”. Find (i) logarithmic decrement, (ii) damping torque at unit velocity and (iii) the periodic time of vibration.
Assume for brass shaft G = 4.4 ´ 1010N/mm2. What would be the frequency, if the disc is removed from viscous fluid?