\(U=\) work done
We know
Work done \(=\frac{\pi S r^4 \phi^2}{4 L}\) \(\left\{\begin{array}{l}\text { Where, } \\ \phi=\text { Angle of twist }=\alpha \\ S=\text { Modulus of rigidity }=4\end{array}\right.\)
Substituting values
\(U=\frac{\pi \eta r^4 \alpha^2}{4 L}\)