Troubleshooting Force Calculation Errors

[r-swald]

Homework Statement

Three point charges lie along a straight line as shown in the figure below, where q1 = 6.60 µC, q2 = 1.42 µC, and q3 = -2.04 µC. The separation distances are d1 = 3.00 cm and d2 = 2.00 cm. Calculate the magnitude and direction of the net electric force on each of the charges. I attached the picture of it below.

Relevant Equations

Equation: F = k|q1||q2| / r^2

I attempted this a few times but keep getting the same wrong answer...
Converted the values:
q1 = 6.6E-6
q2 = 1.42E-6
q3 = -2.04E-6
d1 = .03m
d2 = .02m

Calculation for force on 1
F1 = - F12 + F13 = - (9E9*6.6E-6*1.42E-6 / .03^2) + (9E9*6.6E-6*2.04E-6 / .02^2)

(I used negative for F12 to account for the charges repelling.)
The answer I got for this one was 209 and it says it differs more than 100%!

I did likewise with the second and got 28.5 and that was off by more than 10%. I don't think I did anything different for these, but I'm not sure. Please help! And thanks in advance!

 

[berkeman]

Welcome to the PF.

Calculation for force on 1
F1 = - F12 + F13 = - (9E9*6.6E-6*1.42E-6 / .03^2) + (9E9*6.6E-6*2.04E-6 / .02^2)

Q1 is the left-hand charge, so the distance to Q3 is not just 2cm...

 

[r-swald]

Awesome! got it. I didn't even think about that, now I Just have to figure out what I did wrong with the other one

 

[r-swald]

Yeah so the second one I'm still getting wrong, it's probably something just as silly
F2 = F13 - F23 = (9E9*61.42E-6*6.6E-6 / .03^2) - (9E9*1.42E-6*2.04E-6 / .02^2)
I'm getting 28.54 for this and still saying I'm 10% off.

 

[berkeman]

F2 = F13 - F23 = (9E9*61.42E-6*6.6E-6 / .03^2) - (9E9*1.42E-6*2.04E-6 / .02^2)

I think you have a typo F13 should be F12, right? And I'd use sign convention where forces to the right on a charge are positive (positive x-axis), and forces to the left are negative. Maybe check the signs in that equation...?

 

[haruspex]

I think you have a typo F13 should be F12, right? And I'd use sign convention where forces to the right on a charge are positive (positive x-axis), and forces to the left are negative. Maybe check the signs in that equation...?

Also, there's an extra digit 6 in there (61.42).

 

[r-swald]

Oh yeah, it was a sign problem. Thank you!