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Homework Help: Tension And Electric Charge

  1. Jul 6, 2006 #1
    Hey,

    I thought I did this correctly and my answer turned out incorrect. Here's the problem:

    http://synthdriven.com/images/deletable/help.jpg" [Broken]

    This is what I did:

    In order to use Coloumb's Law, I need r, the distance between the two charges.

    [tex]\arcsin{4}=\frac{x}{0.24m}[/tex]
    [tex]x=0.24\arcsin{4}=0.098m[/tex]
    [tex]r=2x=0.196m[/tex]

    So that's r.

    Then I need to pick one side of this thing apart and break it down into x and y-components.

    With the weight, I have three forces acting on this thing. The tension in the rope, the weight, and the electrical force repelling each sphere.

    My goal is to find the x-component of the electrical force so I can then plug it into Coulomb's law and find the charge.

    For the weight,
    Wx=0
    Wy=-mg=-0.98

    For the tension,
    Tx=-Tsin4
    Ty=Tcos4

    For the electrical force,
    Fx=Tsin4
    Fy=0

    To find T, all I did was:

    FyNET=Wy+Ty+Fy=Tcos4-mg
    (net force of y-component)

    T=mg/cos4
    Right??

    So for T, I get 0.098239

    Plugging that into Tx, I get 0.006853


    And then it goes into Coulomb's law. Which I have reformatted this way:

    [tex]F=\frac{kq^2}{r^2}[/tex]

    ...Because both charges are supposed to be equal...


    Moving that around, I get:
    [tex]q=\sqrt{\frac{Fr^2}{k}}[/tex]

    Plugging in values, I got 1.711e-7


    What did I do wrong?
     
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Jul 6, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    That first equation should have sin(4), not arcsin(4).
     
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