1. The problem statement, all variables and given/known data A ball of mass m is attached to a string of length L and released from rest at point A. Show that the tension in the string when the ball reaches point B is 3mg, independent of the length l. (there is an image in attachment ) 2. Relevant equations K = mv2/2 U = mgh Fcp = mv2/r 3. The attempt at a solution The mechanical energy at point A must equal the mechanical energy at point B. h is the vertical distance between A and B. So mv2/2 - mgh = 0 and v2 = 2gh The net force acting on the ball at point B is the centripetal force, which is mv2/L and is equal with T - mg. mv2/L = T - mg. So T = mv2/L + mg. T = 2mgh/L + mg θ is the angle between the string and the vertical when the ball is at point A. If I write h = L - Lcosθ I get to the equation T = mg(2 - 2cosθ + 1), which is 3mg only when θ is 90°. Something must be wrong.