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Homework Statement
Two point charges are placed on the x-axis as follows: one positive charge, q1, is located to the right of the origin at x= x1, and a second positive charge, q2, is located to the left of the origin at x= x2.
What is the total force (magnitude and direction) exerted by these two charges on a negative point charge, q3, that is placed at the origin? Use [tex]\epsilon_{0}[/tex] for the permittivity of free space. Take positive forces to be along the positive x-axis. Do not use unit vectors.
Homework Equations
Coulomb's Law: F = [tex]\frac{1}{4\pi\epsilon_{0}}\frac{\left|q_{1}q_{2}\right|}{r^{2}}[/tex]
The Attempt at a Solution
http://img5.imageshack.us/img5/3753/equationu.jpg
MasteringPhysics keeps giving me a "Check your signs error". Yet, as far as I can tell, I should be subtracting the force which is going left / the negative direction (ie, q2) from the force going to the right (ie, q1).
Any hints?
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