What do you mean by zero work? State the conditions for zero work. Give one example.

A bob of mass $m$ is suspended by a light string of length $l$. It moves with horizontal velocity $v$ at the lowest point $A$ such that it completes a semicircular trajectory in the vertical plane with the string becoming slack only on reaching the top-most point $B$. Show that minimum speed at $A$ should be $\sqrt{5gl}$.