# Electrostatic force-Coulombs law

## Homework Statement

Hi, Im working on the following question.
A 64 µC charge is located 30cm to the left of a 16 µC charge. What is the resultant force on a -12 µC charge positioned exactly 50mm below the 16 µC charge?

Coulombs law

## The Attempt at a Solution

I see a right triangle So I tried to solve using the vector components as follows
Fac=9.0e9(65x10e-6 * 12x10e-6)/(.3^2+.05^2) = 74.72C
Fbc = 9.0e9(16x10e-6 * 12x10e-6)/(.05^2) = 691.2C

For the rest, I'm not sure if I'm attacking this thing properly to solve for the vectors? I've tried at least 6 times... but no luck.

I could use a little direction...I attached a picture of the problem. My be helpful if it show's up.

#### Attachments

• Problem.bmp
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kuruman
Homework Helper
Gold Member
You have calculated the magnitudes of the vectors. Now you need to add them as vectors to find the resultant force. If you tried this without success, you need to show what you did so we can point out where you went wrong. By the way, force is measured in Newtons, abbreviation N, not C.

OK...heres what I've done. Flipped the triangle a little setting C and B on the x axis. I'm new to all this stuff so please forgive my ignorance.
Fbc = 691.2
Fac = 74.72

Fbcx = 691.2
Fbcy = 0

Facx = 74.72N cos(80) = 12.28
Facy = 74.72 sin(80) = 73.58

Fbcx + Fbcy
691.2 + 12.28 = 703.48
Fbcy + Facy
0 + 12.28 = 73.58
Finally
Fc = Sqrt(703.48 ^2 + 73.58^2) = 707.32N
Not sure where I’m going wrong…but I know I’m incorrect.

Doc Al
Mentor
I didn't check your arithmetic, but your basic method looks OK. How do you know it's incorrect?

The text book I have gave an answer. According to them the correct answer is 2650N, 113.3deg.

kuruman
Homework Helper
Gold Member
I get 707.3 for the resultant. Looks like you are OK here.

Doc Al
Mentor
The text book I have gave an answer. According to them the correct answer is 2650N, 113.3deg.
The textbook answer is way off. (What textbook?)

kuruman