A circular disc 50 cm outside diameter has a central hole and rotates at a uniform speed about an axis through its centre. The diameter of the hole is such that the maximum stress due to rotation is 85% of that in thin ring whose mean diameter is also 50 cm. If both are of the same speed, determine the diameter of the central hole and speed of the disc for the data given below.
Allowable stress = 900 kg/cm2
Specific weight = 7.8 gm/cm2
Poisson’s ratio = 0.3