Question

A pendulum string has a length l = 4.0 ft. When the pendulum ball is released at 90 degrees to the vertical, it will swing down. A nail is located distance d below the point of suspension of the pendulum. Show that d must be at least 0.6l if the ball is to swing completely around in a circle centered on the nail.

...So far, all I ve been able to guess is that this has something to do with centrifugal force or something. But I can t find the height...and I have no idea how in the slightest.

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