Magnitude and direction of net force acting on a charge.

[thatguythere]

Homework Statement

Four charges,
q1 = +195 µC, q2 = +60 µC, q3 = −145 µC, and q4 = +21 µC,
are fixed at the corners of a 4 m by 5 m rectangle, as illustrated in the figure below. What are the magnitude and the direction of the net force acting on q1? (Assume the x-axis extends from q1 to the right.)

q3--------------------q2
| |
| |
| |
q4--------------------q1

Homework Equations

F = k q1q2/r2

The Attempt at a Solution

I have followed the steps outlined here. http://dev.physicslab.org/Document....lectrostatics_AdvancedCoulombsLawProblems.xml

For Fq1q2 I get 26.30N
Fq1q4 I get 5.89N
and for Fq1q3 I get 24.82

I proceed to chart it as shown F X Y
q1q2 0 -26.30
q1q4 5.89 0
q1q3 24.82cos135°=-17.55 24.82sin135°=17.55
-11.66 8.75

Then F = sqr.rt(-11.66)^2+(8.75)^2
=14.58

Then a=tan-1|8.75 /-11.66|
=-36.89
θ=180-36.89
=143.11°

Neither of which are the proper answers apparently. What am I doing wrong? Any help would be greatly appreciated.

 

[szynkasz]

Rectangle is 4 by 5, so diagonal is not at 45^o angle

 

[ehild]

The problems are different. Do you understand what Coulomb's Law states? What is the magnitude and the direction of a force, one charge exerts on an other one?

ehild