Electric force multiple choice

1. Jul 6, 2016

Clara Chung

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. Jul 6, 2016

cnh1995

I think any value of q (+ve) will do since the distances are fixed.

Last edited: Jul 6, 2016
3. Jul 6, 2016

Delta²

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?

4. Jul 6, 2016

cnh1995

Yes. I was about to edit..

5. Jul 6, 2016

Clara Chung

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.