A bob is suspended from a crane by a cable of lengtr $I=5 \mathrm{m}$. The crane and load are moving at a constant speed $v_0$. The crane is stopped by a bumper and the bob on the cable swings out an angle of $60°$. Find the initial speed $v_0·\left(g=9.8 \mathrm{m}/{\mathrm{s}}^2\right)$