A steel disc 300 mm diameter of mass 29kg, is suspended from the end of a wire 2.5 mm diameter and 1.5 m long which is clamped into a central hole in the disc. The upper end of the wire being rigidly supported. When the disc is set in torsional vibration, it is found to make 10 complete oscillations in 78.2 seconds.
a) Find the modulus of rigidity of the wire,
b) Calculate the amplitude of the oscillation which may be allowed if the maximum permissible intensity of shearing stress in the wire is 140 MN/m2