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Homework Help: Electric force, tension, charge

  1. Sep 4, 2011 #1
    1. The problem statement, all variables and given/known data

    Two small plastic balls hang from threads of negligible mass. Each ball has a mass of 0.65 g and a charge of magnitude q. The balls are attracted to each other, and the threads attached to the balls make an angle of 20.0° with the vertical, as shown in the figure below.


    (a) Find the magnitude of the electric force acting on each ball.

    (b) Find the tension in each of the threads.

    (c) Find the magnitude of the charge on the balls.

    3. The attempt at a solution

    I found part (a) 0.00232 N and part (b) 0.00679 N, but I can't seem to figure out part (c)
    I used q = sqrt[ (r^2*Ft*sin 20 )/ k ] = 1.93e-8
    That answer is not right. Is my formula incorrect? Please help
  2. jcsd
  3. Sep 4, 2011 #2


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    I think the equation looks right. I got a completely different answer using it though. I suspect you have made an arithmetic error. Check your calculations.
    Last edited: Sep 4, 2011
  4. Sep 4, 2011 #3
    I was using the electric force instead of the tension! Silly mistake. Thank you
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