A light rod with the balls $A$ and $B$ is clamped at the centre in such a way that it can rotate freely about a horizontal axis through the clamp. The system is kept at rest in the horizontal position. A particle $P$ of the same mass $m$ is dropped from a height h on the ball $B$. The particle collide with $B$ and sticks to it. Find the minimum value of h for which the rod will make a complete revolution after the collision.