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Homework Help: Electric force multiple choice

  1. Jul 6, 2016 #1
    1. The problem statement, all variables and given/known data
    PQRS is a square. Three point charges -Q, +Q and -3Q are fixed at P, Q and S respectively as shown.

    A point charge q is then placed at R and it experiences a net electric force towards S. Which of the following is equal to q?

    photo :
    http://s613.photobucket.com/user/Yan_Wa_Chung/media/01_zps421rebzc.png.html

    2. Relevant equations
    F=kq1q2/r^2

    3. The attempt at a solution
    in any kind of calculation, Q and q are cancelled out and undetermined ,ie : (k1) (q) (Q) / (r1)^2 = (k2) (q) (Q) / (r2)^2, (turn force PR to vertical and horizontal component cancel the vertical component with force QR) . I have no idea on the question or maybe the question itself has mistakes ?The right answer is q=1/9 Q.
     
  2. jcsd
  3. Jul 6, 2016 #2

    cnh1995

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    I think any value of q (+ve) will do since the distances are fixed.
     
    Last edited: Jul 6, 2016
  4. Jul 6, 2016 #3

    Delta²

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    But suppose vertical component of force PR is ##-cos(\pi/4)kQq/r_1^2## so since it is canceled by force QR we 'll have
    ##-cos(\pi/4)kQq/r_1^2+kQq/r_2^2=0##. Isnt the product Qq simplified by this last equation?
     
  5. Jul 6, 2016 #4

    cnh1995

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    Yes. I was about to edit..
     
  6. Jul 6, 2016 #5
    Thanks a lot for answering this question. I think the question may have mistakes itself. I hope I can get some help with the last stupid question that I've posted too. Thanks.
     
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