A small ball of mass $$m$$ is attached to one end of a massless un-stretchable string of length $$\ell$$ and is held at the point $$P$$. The other end of the string is fixed to a support at $$O$$ such that $$OP$$ is horizontal. The minimum downward speed $$u$$ that should be imparted to the ball at the point $$P$$ so that the ball can complete the vertical circle without any slack in the string is