\(U_{i}=-M B_{H}\)
Final potential energy of dipole,
\({U_{f}=-M B_{H} \cos 60^{\circ}=-\frac{M B_{H}}{2}}\)
\(W = {U_f} - {U_i} = - \frac{{M{B_H}}}{2} - \left( { - M{B_H}} \right) = \frac{{M{B_H}}}{2} \ldots ...(i)\)
Required torque, \(\tau=M B_{H} \sin 60^{\circ}\)
\(\tau=2 W \times \frac{\sqrt{3}}{2} \quad[\text { Using eqn. ( i ) }]\)
\(=\sqrt{3}\, W\)