Circular Motion

Question 22 (SL) and Question 14 (HL) in the May 2016 IB Physics Paper 1  involved a mass on the end of a rod moving in a vertical circle in such a manner that the speed of the mass was constant. This question received the lowest percentage of correct responses in both papers and so it is worthwhile to discuss the Physics involved in this question. The question asks about the force exerted by the rod on the mass. The forces acting on the mass are its weight due to the Earth's gravitational field and the contact force of the rod pulling on the mass. Since the mass is moving at a constant speed in uniform circular motion the resultant force acting on the mass must always be directed towards the centre of the circle and must be constant in magnitude. As the mass moves around the circle the weight force is always directed downwards and so the direction of the contact force must change so that the vector sum of the weight force and the contact force is always directed towards the centre of the circle. At the top of the circle both forces point towards the centre of the circle and so the contact force has its minimum value at this position. Therefore the answer is D.