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Homework Help: Point charges hanging on a string

  1. Feb 23, 2009 #1
    1. The problem statement, all variables and given/known data
    What would be the angle theta as shown in the picture, if the 5 gram balls hangin off 1m strings have been charges to 100nC each

    http://img178.imageshack.us/img178/5103/mooche.th.jpg [Broken]

    3. The attempt at a solution

    I'm not really sure how to solve such a question, can i get a hint?
    i know the force each one exerts on the other is kq1q2 / r^2 and i can find that the distance between them is 2sin(theta) so the force is kq1q2/(2sin(theta))^2
    So for , say, the left ball, we have
    Fx = Tsintheta - kq1q2/(2costheta)^2 = 0
    Fy = tcostheta - mg = 0

    but what can i do with those eqations really :S
    I definitely am stuck and have no idea how to proceed for such a q
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Feb 23, 2009 #2


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    Homework Helper

    Can't see the diagram - too small. Clicking on it doesn't work.
    Check the Fx equation - looks like you copied the 2sin(theta) incorrectly.
    Two equations, two unknowns (theta and T) - you should be all set. Solve the Fy equation for T and sub into the Fx one to get . . . a nasty bit of trigonometry. Well you could always solve it numerically - just graph the trig function on your calculator and trace to the value you want it to be, then read off the angle.
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