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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 ĚŠ 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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