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http://www.askphysics.com/derive-an-expression-for-the-minimum-horizontal-velocity-to-be-given-to-a-ball-hanging-vertically-from-a-point-so-that-it-is-able-to-just-complete-a-vertical-circular-path/
Derive an expression for the minimum horizontal velocity to be given to a ball hanging vertically from a point so that it is able to just complete a vertical circular path