You are whirling a rubber stopper of mass something attached to a string in a vertical circle at high constant speed. At the bottom of the circle, the net force that causes acceleration is my answer was Vertically upward and greater in magnitude than MG. but the answer in the book says vertically upward and lower in magnitude than mg. WHY? I dont get this. At the bottom of the circle, The Centripetal force is the difference of Force tension pointing upwards and force of gravity pointing downwards. So if the netforce is smaller than MG, then there can't be a centripetal force.... please help me out.