# The iron ball shown is being swung in a vertical circle at theend of a 0.7-m string. How slowly, in m/s, can the ball go throughits top position without having the string go slack?

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by rajniattlee

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by rajniattlee

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force acting downward = weight of ball(mg) and tension

force acting upward = centrifugal force (mv²/r)

given that r = 0.7m

The string will slack just after the tension becomes 0.

So when tension becomes 0, equilibrium equation at top point becomes

mg+T = mv²/r

⇒ mg + 0 = mv²/0.7

⇒ g = v²/0.7

⇒ v² = 0.7g

⇒ v = √0.7g = √(0.7×9.81) = 2.62 m/s

So minimum velocity at the top should be