A block of mass m = 1kg is placed on a frictionless horizontal surface. One end of a massless string of length 0.8 m is connected to the block. The string is straight and the other end of the string is moved vertically up with constant velocity $v_0$. When the string makes an angle $\theta ={30}^0$ with the horizontal surface the block leaves the surface. Find the speed $v_0$ (in m/s) (given $g=10 m/s^2$)