I have no idea why I can't do it.. I must have done it a million times before: Two cork balls of mass .2g hang from the same support point by massless insulating threads of length 20cm. A total positive charge of 3.0x10^-8 C is added to the system. Half of this charge is taken up by each balls, and the balls spread apart to a new equilibrium position. (There is a value of theta in the diagram, which looks roughly like: /\ (each line holds a ball with a charge). Theta is between one of those lines and a line down the middle, so between /| or |\ Find the tension in the threads. Now I get Fx = Tsin(theta) - Fq = 0 Fy = Tcos(theta) - mg = 0 I don't know how to solve it from there - to find Fq I need r, which is hard to find since it involves theta (I think). And I can't solve out for T because there is a sin and cosine function. There must be an easier way to solve this - what am I missing?