Find the fundamental frequency for torsional vibration of a shaft of length \(2 \mathrm{~m}\) and diameter \(50 \mathrm{~mm}\) when both the ends are fixed. The density of the material is \(7800 \mathrm{~kg} / \mathrm{m}^{3}\) and the modulus of rigidity is \(0.8 \times 10^{11} \mathrm{~N} / \mathrm{m}^{2}\).