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Homework Help: Electrostatic forces helpp

  1. Jan 14, 2009 #1
    Electrostatic forces helpp!!

    1. The problem statement, all variables and given/known data
    Two spheres, each having a mass of 50.0 mg, are suspended from a common point by massless threads 50.0 cm long. One of the spheres have been given a charge twice that of the other. The strings make an angle of 6.00 degrees.
    What are the charges on each of the spheres?

    2. Relevant equations
    F = k ((q1)(q2))/r^2

    3. The attempt at a solution

    Since the lengths of the strings on both sides are 50 cm, I drew an imaginary line between the two spheres. I found out that the other two angles had to be 87 degrees because both of the sides were equal. Then, I used law of cosines to find that the length between the spheres is 27.4 cm, which would be r in the equation. I also know that either q1 = 2q2 or q2 = 2q1, but I do not understand how I am supposed to use the masses of the spheres to find out the charge. Is there an equation that I'm missing here??
  2. jcsd
  3. Jan 14, 2009 #2


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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi krnaudio ! Welcome to PF! :smile:

    There are three forces on each sphere … tension, gravity, and electric …

    so make the vector sum zero (or draw a vector triangle ) :wink:
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