A pendulum bob has a speed of $3\mathrm{~m~s}^{-1}$ at its lowest position. The pendulum is 50 cm long. The speed of bob, when the length makes an angle of $60^{\circ}$ to the vertical will be ($g = 10\mathrm{~m~s}^{-2}$) \_\_\_\_ $\mathrm{m~s}^{-1}$.