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Electric force! cant firgure this out.

  1. Sep 1, 2009 #1
    Two small plastic balls hang from threads of negligible mass. Each ball has a mass of 0.200 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.


    so the question is to Find the tension in each of the threads.

    okey, so i tried to figure this out. i was pretty sure i was getting this right, but no...

    3. The attempt at a solution

    so i set each x,y component equal to 0.

    y=tcos20-mg =0

    so from the y i tried to get the tension which would be mg/cos20..and i am getting .0021.but its not the right answer. does anybody know what i am doing wrong...help would be greatly appreciated.
  2. jcsd
  3. Sep 1, 2009 #2
    I think your should incorporate the equation for the electric force in your net force equations since the mass of the balls have charge; Are you familiar with Coulomb's law ?
  4. Sep 1, 2009 #3
    yea i am but how would i put that in there since they did not give my any charges....
  5. Sep 1, 2009 #4
    The problem only specified that the ball had a charged q but did not give a value for q?
  6. Sep 1, 2009 #5
    nope but i actually found the mistake for the tension, it was in my calculations. but the third part of the question ask to find the chrage on the balls. since i found the electric force acting on the balls after i found tension...do i use the Coulombs law?
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