2 identical small balls hang in a 100, 000 N/C electric field as shown. They have charges of 100 and -100 nC respectively. The string length is 50cm and the angle each makes with the vertical is 10 degrees. Find the mass of the balls. http://img514.imageshack.us/img514/633/phees.th.jpg [Broken] 2. Relevant equations F = Eq , F = k|q1||q2|/r^2, F = mg k = 9 x 10^9 Nm^2/C^2 3. The attempt at a solution For the left ball : Fx = Tsina - Eq - kq^2/(sina^2) = 0 T = (Eq + kq^2/sina^2)/ sina Fy = Tcosa - mg = 0 m = Tcosa / g m =(cosa/g) * (Eq + kq^2/sina^2) / sina but when i evaluate this with a = 10 , g = 9.8, E = 100 000, k = 9x10^9, q = 1 x 10^7 i get m = 7.5 grams the answer in the back of the book is 4.1 grams where am i going wrong?