posted by Freud .
A rock of mass m = 1.8 kg is tied to a string of length L = 2.4 m and is twirled in a vertical circle as shown in the figure below. The speed v of the rock is constant; that is, it is the same at the top and the bottom of the circle. If the tension in the string is zero when the rock is at its highest point (so that the string just barely goes slack), what is the tension when the rock is at the bottom?
2 M g
V^2/R must equal M*g in order for string tension to go to zero at the top. At the bottom, the same centripetal force gets added to the weight (M*g) when computing string tension.