Two 2.5 g pith balls hang in air on cotton threads 48 cm long from a common point of support. The balls are then equally charged and they spring apart, each making an angle of 10° with the vertical. Find the magnitude of the charge on each ball. F = (k q1 q2) / r^2 Q1 and Q2 are the same so F = (k q^2) / r^2 q = sqrt[ (F r^2) / k ] where r is the distance between the balls (taken as point charges) Using cosine rule, a^2 = b^2 + c^2 - 2 b c cosA where a = r b and c = 0.48m A = 10 degrees a = sqrt( 2 * 0.48^2 - 2 * 0.48^2 * cos 10) r = 0.0083669513m F = force on each ball F = mass * acceleration due to gravity * sin (5 degrees) F = 0.0025kg * 9.81 * sin(5) F = 2.14 * 10^-3 k = electrostatic constant = 9 * 10^9 N m^2 C^-2 so q = sqrt[ (F r^2) / k ] q = sqrt[ (2.14 * 10^-3 x 0.0083669513^2) / 9 * 10^9] q = sqrt( 1.664584116E-17) q = 4.1E-9 C I have tried almost everything, but I keep getting the wrong answer. Where am I going wrong? Am I even on the right track? Any help would be greatly appreciated.