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Electromagnetic force and some trig

  1. Jan 14, 2012 #1
    1. The problem statement, all variables and given/known data

    The diagram below shows two pith balls, equally charged and each with a mass of 1.5 g. While one ball is suspended by a thread, the other is brought close to it and a state of equi- librium is reached. In that situation, the two balls are separated by 2.6 cm and the thread attached to the suspended ball makes an angle of 20 ̊ with the vertical. Calculate the charge on each of the pith balls.

    2. Relevant equations

    N = (k(C1C2)/r2

    where k = 9 * 109

    3. The attempt at a solution

    My problem here is that I'm dealing with unknowns on both sides of the equation. I have to find out N and x.

    N = (9*10^9(x^2))/1.3^2

    I don't know why they give us the information that the angle is 20 degrees. In any case I figured out the dimensions of the triangle which are

    7.2, 2.6, 7.65

    but I can't figure out how that helps me.
  2. jcsd
  3. Jan 14, 2012 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Force is a vector quantity.

    There are three forces involved.

    Break the forces down into horizontal and vertical components. (That's why you need to know the angle.)
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