b$)$ Show that a solid shaft of length $L,$ Radius $R,$ angle of twist $($radians$)$ subjected to Torsion T under shear stress $$ is given by $\frac{T}{J}=\frac{G}{L}=\frac{}{R}.$ Where G is the modulus if rigidity. Note: State all assumptions made.

$(7$ Marks$)$