Magnitude and Direction of a charge interaction

[themonk]

Homework Statement

What is the direction of the force \vec{F} on the -10 nC charge in the figure? Give your answer as an angle measured cw from the +x-axis. Problem 26.39 in Physics for Scientists and Engineers Second Edition by Knight
Magnitude of the force is 4.3E-3 N

Homework Equations

As far as I know, this is the only relevant equations:

\vec{F}=k*q_{1}*q_{2}/d^{2}

The Attempt at a Solution

I already found the magnitude as listed above. But I need to find \theta in the clockwise direction.

The force of A (being the -5 nC charge) I figured was direction -4.3E-4 \hat{j} and -1.28E-3 \hat{i} and the other charge had a force of -4.5E-3 I double checked with an answer from someone else (MasteringPhysics also said it was correct). I figured I would use a trigonometric function for the final part, ie the angle measured in the clockwise direction. In lab we used tangent, but was confused as to why (tangent is opposite over adjacent):

tan(\theta)=4.07E-3/1.28E-3 ==> \theta=tan^{-1}(3.179)

which is 72.54 degrees. I added 180 to it to get 252.54, but apparently that is not correct (I don't completely understand the cw and ccw part).

Is there any line of thought that I should proceed with?

(sorry about my English)

 

[ehild]

Show the coordinate axes and the forces in your figure, please.

ehild

 

[themonk]

I can't edit the problem, but the x-axis (\hat{i}) is left-right and the y-axis (\hat{j}) would be up-down, if I am not mistaken. The origin would then be at the -10nC.

 

[ehild]

In this case the force on the -10 nC charge from the -5 nC one has only y component, and that form the 15 nC charge has both x and y components. Check your calculation and text.

ehild