1. The problem statement, all variables and given/known data Here is the drawing which shows the situation: http://img507.imageshack.us/my.php?image=drawingae3.jpg The mass of each ball is equal to m. A rod which connects the balls is weightless. A vertical wall affects a ball which is below with force F. I need to find this force when the angle between the rod and ground is alfa (the rod is vertical at the beginning). 3. The attempt at a solution Stress in the rod T. Potential energy of the right ball is PE = mgR sin(α) Kinetic energy of the right ball is KE = 1/2 m v²(α) Conservation of energy: KE + PE = Eo 1/2 m v²(α) + mgR sin(α) = mgR v²(α) = 2 gR (1 - sin(α)) Centripetal accleration: a(α) = v²(α)/R = 2g(1 - sin(α)) Balance of force along the rod: ma(α) = mg sin(α) - T(α) 2mg(1 - sin(α)) = mg sin(α) - T(α) mg(2 - 3sin(α)) = -T(α) T(α) = mg(3sin(α) - 2) Answer: F(α) = T(α) cos (α) = mg(3sin(α) - 2)cos(α) But the last equation means that if α<arcsin(2/3) = 41.8 deg, the left ball will no longer touch the wall. Is it possible? Maybe there are some mistakes in the solution? Thanks in advance.