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