A thin uniform disc of mass m and radius R suspended by an elastic thread in the horizontal plane performs torsional oscillations in a liquid. The moment of elastic forces emerging in the thread is equal to N = aφ, where a is a constant and φ is the angle of rotation from the equilibrium position. The resistance force acting on a unit area of the disc is equal to F1 = ηv, where η is a constant and v is the velocity of the given element of the disc relative to the liquid. Find the frequency of small oscillation.