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,
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?