Problem 4.80

A disc $A$ of radius $R$ suspended by an elastic thread between two stationary planes (Fig. 4.24 ) performs torsional oscillations about its axis $O O^{\prime} .$ The moment of inertia of the disc relative to that axis is equal to $I,$ the clearance between the disc and each of the planes is equal to $h,$ with $h \ll R .$ Find the viscosity of the gas surrounding the disc $A$ if the oscillation period of the disc equals $T$ and the logarithmic damping decrement, $\lambda$.