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**Another question on electric fields. :(**

I have another problem I don't understand on electric fields. I thought I understood these after my last post, but I guess not. I'm guessing it has something to do with the signs of the forces, but I am not sure.

**1. Homework Statement**

Particles of charge Q1 = +68 µC, Q2 = +49 µC, and Q3 = -80 µC are placed in a line (Fig. 16-37). The center one is 0.35 m from each of the others. Calculate the net force on each charge due to the other two.

Figure 16-37

**2. Homework Equations**

F = KQ_1Q_2/r^2

**3. The Attempt at a Solution**

F = K * Q1Q2/r^2

F_12 = (K)(6.8 X 10-5)(4.9 X 10-5)/.35^2

F_12 = 244.8 N (goes to the left, repels)

F_13 = K(Q1Q2)/r^2

F_13 = (K)(6.8 X 10-5)(4.9 X 10-5)/.7^2

F_13 ~= 100 N (goes to the right, attracts)

F_q1 = -244.8 + 100

F_q1 = -144.8 N (wrong)

However, the second part of the question asked which direction it went in, and based on my answer, I put "left" and that was correct. I'm confused.