# 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.