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Homework Help: Columb force Question

  1. May 11, 2010 #1
    1. The problem statement, all variables and given/known data

    The figure shows two 5.0 g spheres suspended from 1.0-m-long threads. The spheres repel each other after being charged to +31 nC. What is the angle θ ?

    2. Relevant equations


    3. The attempt at a solution

    i tried it by writing equation in the x and y component for one of the ball and then i would have two unknowns tension and the radius.

    now for sin (theta) = r/1
    cos (theta) = sqrt(1-r^2)

    so for the columbs force the radius would be 2r.

    now i solved for r = 4.4108*10^(-5)

    this doesn't make sense, i would get the angle to be 2.53*10^(-3)

    but the choice for the angles are : k1ey61.jpg

    Thanks for helping
  2. jcsd
  3. May 12, 2010 #2


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    Homework Helper

    It T is the tension in the string, then
    Tsinθ = F

    Tcosθ = mg

    tanθ = 9*10^9*(31*10^-9)^2/r^2*5*10^-3*9.8

    Since θ is very small, tanθ is nearly equal to θ = r

    So r^3 = (θ)^3 = 9*10^9*(31*10^-9)^2/5*10^-3*9.8

    Now solve to find θ.
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