Physics electrostatic force problem.?

Question

The point charges in the figure below have the following values: q1 = +2.8 µC, q2 = +5.3 µC, q3 = -0.89 µC. (Let the +x axis point to the right.)

http://www.webassign.net/walker/19-32.gif

(a) Given that the distance d in the figure is 4.35 cm, find the magnitude and direction of the net electrostatic force exerted on the point charge q1,

so i tried finding the magnitude of the components and got F12=70.5 and F13=11.04

then i found the magnitude of their x and y components (f12x=35.25 f12y=61.058 f13x=11.04 f13y=0)

then i found the magnitude ot be 76.6 N but it was wrong. what did i do wrong? anything helps....

Answer 1

Assuming q1 is free to move:

find the magnitude and direction of the net electrostatic

force exerted on the point charge q1,

q1=2.8 uc

q2=5.3 uc

q3=-0.89 uc

let k=9*10^-3

F=k*q^2/r^2

vector Force on q1 from q2: "third quadrant"

F=9*10^-3*(2.8)*(5.3)/

(4.35/100)^2 = 70.58 N

Componets of F: acts @ 240 degrees from x-axis

Fi=70.58* Cos(240)=-35.29i  N <---

Fj= 70.58* Sin(240)=-61.12j N

Force on q1 from q3

Fi=9*10^-3*(2.8)*(0.89)/

(4.35/100)^2 = 11.85 N --->

Sum x=

-35.29i+11.85i = -23.44 N

Sum y=-61.12j N

x= -23.44

y= -61.12

*************************************

The resultant Force on q1

= sqr(x^2+y^2) = 65.461 N

angle with positive x-axis is:

atan (y/x) =249.02 degrees