A torsional pendulum has a centroidal mass moment of inertia of 0.40 kg·m2 and when given an initial twist and released is found to have a frequency of oscillation of 200 rpm. Knowing that when this pendulum is immersed in oil and given the same initial condition it is found to have a frequency of oscillation of 180 rpm, determine the damping constant for the oil. The damping constant for the oil is