A brass bob is suspended by an inextensible weightless string of length `l` from a peg fixed on a vertical wall. A nail is fixed in the wall at a distance `d` vertically below the point of suspension of the string. The bob is then drawn to a position where the string becomes horizontal and it is released from there. Show that d must be at least `0.6 l` if the bob is to swing completely around in a circle centred on the nail.