# Need help finishing Physics Problem, almost have answer.?

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?

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Doc Al
Mentor
What are you asked to find?

Yea, probably should have put that in,

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

Doc Al
Mentor
Fba= -.21365
Fca= .10683
Fda= -.21365
What units are these in?

They should be in Newtons

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

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?

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

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

I got it, 195872.2698.