A bullet of mass m is fired with a velocity of $50{\rm{m}}{{\rm{s}}^{ - 1}}$ at an angle $\theta $ with the horizontal. At the highest point of its trajectory, it collides had on with a bob of massless string of length $l = 10/3m$ and gets embedded in the bob. After the collision, the string moves to an angle of ${120^0}$. What is the angle $\theta $?.