A small charged ball of mass m is suspended on a light insulating string of length L. A large vertical neutral metal plate is placed 1.5 L away from the ball.

The ball is pulled directly away from the plate in the vertical plane until the string makes the angle of 30 degrees with the vertical, and then released from rest. During the subsequent oscillations, the ball periodically comes to a momentary stop just as the string becomes horizontal. Find the charge q on the ball.

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