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Question: 

Particles of charge $+65$, $+48$, and $-95 \; \mu \textrm{C}$ and are placed in a line (Fig. 16–52). The center one is 0.35 m from each of the others. Calculate the net force on each charge due to the other two.

Problem 11.

Figure 16-52.

Giancoli, Douglas C., Physics: Principles with Applications, 7th Ed., ©2014. Reprinted by permission of Pearson Education Inc., New York.
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Quick Answer: 

$F_1 = -1.2 \times 10^2 \textrm{ N, } $$F_2 = 5.6 \times 10^2 \textrm{ N, } $$F_3 = -4.5 \times 10^2 \textrm{ N}$

Giancoli 7th Edition, Chapter 16, Problem 11

(4:47)

Comments

when calculating the Force of 2, why isn't the force from 2 to 1 negative?

Hi moopen, thanks for the question. When calculating the net force on charge 2 we're interested only in forces exerted on charge 2. The charge 1 is exerting a force to the right on charge 2, so that force is taken as positive. It's true that charge 2 exerts a force to the left on charge 1 (this is the Newton's 3rd Law counterpart to the force exerted on charge 2 by charge 1), but this force isn't relevant since it isn't exerted on charge 2.

Hope that helps,
Mr. Dychko

It seems more intuitive to mark pushes as positive because of how the book defines attractive force as negative and positive force as repelling. Is there some other thing behind this seeming contradictory technique- for instance, does it prepare us for something later on doing it this way? Or, am I misunderstanding something?