A rock attached to a string swings in a vertical circle. Which free body diagram could correctly describe the force(s) on the rock when it is at the lowest point?

Question

Quizplus · Accepted Answer

At the lowest point, the tension in the string is providing the centripetal force necessary to keep the rock moving in a circle. Gravity is also acting on the rock, but it is not providing the centripetal force at this point in the motion. Therefore, the free body diagram should show tension pointing towards the center of the circle and gravity pointing downwards. Choice B correctly shows these two forces.

A Rock Attached to a String Swings in a Vertical