Need help finishing Physics Problem, almost have answer.?

1. Dec 18, 2007

Frogger Man

Two Q = +5.85 mC charges are placed at opposite corners of a square d = 1.2 m on each side, and -Q = -5.85 mC charges are placed at the remaining corners, as shown in the figure.

I labeled the square A, B, C, D from top left to bottem left.

Fba = k • QA • QB / dBA2
Fca = k • QA • QC / dCA2
Fda = k • QA • QD / dDA2

Fba= -.21365
Fca= .10683
Fda= -.21365

I used the pythagorean therum to find Fba+Fda, and got .302, and then i subracted .10683 from that, but that was not the correct answer.

Any suggestions?

2. Dec 18, 2007

Staff: Mentor

What are you asked to find?

3. Dec 18, 2007

Frogger Man

Yea, probably should have put that in,

Determine the magnitude and direction of the force on each charge.

4. Dec 18, 2007

Staff: Mentor

What units are these in?

5. Dec 18, 2007

Frogger Man

They should be in Newtons

6. Dec 18, 2007

Staff: Mentor

Redo the calculations. (You are off by orders of magnitude.)

7. Dec 18, 2007

Frogger Man

Fba = k • QA • QB / dBA2
Fba = (8.99*10^9 • (5.85)*10^-6 • (-5.85)*10^-6) / 1.2^2
Fba = (-.30766) / (1.44)
Fba = -.21365

Are my numbers off, or just my sign, or both?

8. Dec 18, 2007

Staff: Mentor

Ah... you're using mC to mean microCoulombs. (Better to use $\mu C$.) In that case, the numbers look OK to me.

9. Dec 18, 2007

Frogger Man

I think you might be right, it should be milli. I will try that.

10. Dec 18, 2007

Frogger Man

I got it, 195872.2698.