How Do You Calculate the Net Electrostatic Force in a Three-Point Charge System?

[duki]

Problem:

A charge of +18 uC is on the y-axis at y = +3.0m. A charge of -12 uC is at the origin. Last, a charge of +45 uC is on the x-axis at x = +3.0m. Determine the magnitude and direction of the net electrostatic force on the charge at x = +3.0mm. Specify the direction relative to the -x axis.


So what I have drawn is:

O-> 18 x (10)^-6
|
|
| /\
| 3.0m
| \/
|
|
|
|
| Origin Charge: -12x10^-6
|O_ _ _ _ _ _ _ _ _ _ _ O -> 45 x (10)^-6
< 3.om >

So I did:

F1 = (8.99 x 10^9)(12 x 10^-6)(18 x 10^-6) / 9 = 1.942 N
F2 = (8.99 x 10^9)(12 x 10^-6)(45 x 10^-6) / 9 = 0.539 N

Can I use trig to determine the magnitude now? I tried it but I got this:

1.942cos90 = 0
1.942sin90 = 1

Surely I've messed up somewhere. Can someone point me in the right direction?

Thanks a bunch!

 

[duki]

or would I do:

.539 cos 45
.539 sin 45

 

[Tomsk]

If you're finding the force on the charge at x=3m, you need 18x45 and -12x45 (with appropriate powers of 10, of course), and use pythagoras to find the distance between the 18uC and 45uC charges.

To find the magnitude, find the resultant force in the x and y directions, then use pythagoras.

 

[duki]

Ok so I've done some of it right?

Then I do:

(3²)(3²) = 9² = 9??

i don't understand the second part

 

[duki]

*bump*...