When an object travel at a speed of in a circle of radius , the (centripetal) acceleration of that object would be .
In this question, the ball is travelling at in a circle of radius . The (centripetal) acceleration of this ball would be:
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By Newton's Laws of Motion, for an object of mass , if the acceleration of that object is , the net force on that object would be . Since the acceleration of this ball is , the net force on this ball would be: