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 0.61 if the bob is to swing completely around in a circle centred on the nail.

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