Point charge formula and set-up

[rocomath]

I just have a general question. Coulomb's Law is ...

$$F=\frac{k\cdot |q_1||q_2|}{r^2}$$

Ok, solving this question ...

Three point charges are arranged along the x-axis. Charge $$q_1=+3.00\mu C$$ is at the origin, and charge $$q_2=-5.00\mu C$$ is at $$0.200 m$$. Charge $$q_3=-8.00\mu C$$. Where is $$q_3$$ located if the net force on $$q_1$$ is $$7.00 N$$ in the -x direction?

In the set-up, why is that in order to solve for the correct answer. I ignore the absolute value?

http://www.cramster.com/Answer-Board/Image/cramster-equation-2007129327456333276766594025007716.gif

 

[Gear300]

The charge sign (+ or -) has a direction in respect to the unit vector r. What you're using is a cartesian coordinate system. To deal with this, get the magnitude of Fe between each charge and simply put a + or - in front depending on what the direction of the force is.

 

[rl.bhat]

There is a force of attraction between q1, q2 and q1, q3. So the net force on q1 is the sum or difference of these forces depending on whether they are in the same side or in the opposite side of q1. You will get two answers.