Ashaft undergoing torsional vibrations is held fixed at one end, and it has attached at the other end a disk with a rotary inertia of 4 kg m2about the rotational axis. the rotary inertia of the shaft is negligible compared to that of the disk, and the torsional stiffness of the shaft is 8 nm /rad. the disk is placed in an oil housing, and the associated dissipation is modeled by using an equivalent torsional viscous damper with damping coefficient ct. what should the value of ctbe so that the system is critically damped? .