A circular disc of mass 10 kg is suspended by a wire attached to its centre. The wire is twisted by rotating the disc and released. The period of torsional oscillations is found to be 1.5 s. The radius of the disc is 15 cm. Determine the torsional spring constant of the wire. (Torsional spring constant %u03B1 is defined by the relation J = %u2013%u03B1 %u03B8, where J is the restoring couple and %u03B8 the angle of twist).