$[$Ch$2$ : $2]$ A disk of mass moment of inertia $J=0\mathrm{.}02kg*m^2$ is attached at the free end of a stepped circular

shaft $($with negligible mass and the shear modulus of $G=5{10}^8).$ Determine the natural frequency of

torsional vibration. Assume that $d_1=1,d_2=0\mathrm{.}5,l_1=15,l_2=10($all in cm$).$