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Coulombs Law Please Help

  1. Sep 16, 2009 #1
    Coulombs Law !! Please Help

    1. The problem statement, all variables and given/known data


    Two metal balls A and B of negligible radius are floating at rest on Space Station Freedom between two metal bulkheads, connected by a taut nonconducting thread of length 1.30 m. Ball A carries charge q, and ball B carries charge 2q. Each ball is 1.66 m away from a bulkhead.
    (a) If the tension in the string is 4.00 N, what is the magnitude of q?
    (b) What happens as time passes ?

    2. Relevant equations

    F= Ke x q1 q2/r2

    3. The attempt at a solution

    I know the constant for Ke = 8.9875 x 10 9 N*m2/c2

    And know the distance between the two objects is held together by a string with a certain tension of 4 N.

    Quite honestly, I dont even know where to begin. She says this is a simple problem, but I dont feel the same way :)

    Any help or suggestions as to where I should start would be greatly appreciated !
     
  2. jcsd
  3. Sep 16, 2009 #2

    gabbagabbahey

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    Re: Coulombs Law !! Please Help

    Hi jayz618, welcome to PF!:smile:

    Hint: Why not start by using the equation you posted under "relevant equations" to caluclate the electrostatic repulsion between the balls (Don't worry about the fact that you aren't given a numerical value for 'q', just leave it written as 'q')....What does Newton's third law tell you about the relationship between the tension in the string and the electrostatic repulsion?

    P.S. This level of problem should really be posted in the introductory physics help forum in the future.
     
  4. Sep 16, 2009 #3
    Re: Coulombs Law !! Please Help

    So 8987500000 x q x 2q/1.30 m ?

    Im really lost, I cant get a response from my professor for any help.

    Is that the proper set up ?

    And in the future i will post these over there :)
     
  5. Sep 16, 2009 #4

    gabbagabbahey

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    Re: Coulombs Law !! Please Help

    Yes, that's the electrostatic force between the balls.....Now ask yourself, "what force(s) cause the tension in the thread?":wink:

    EDIT: You mean 8987500000 x q x 2q/(1.30m)^2, right?
     
  6. Sep 16, 2009 #5
    Re: Coulombs Law !! Please Help

    So for the force between the two balls, I have come up with 1.518 x 1010=q x 2q <--- should be the right number now :)

    The tension in the thread is caused by both balls, A having half as much force as b ?

    Am I going in the right direction ?
     
  7. Sep 16, 2009 #6
    Re: Coulombs Law !! Please Help

    Oops, you are absolutely correct.

    I forgot to square the 1.30m.
     
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