This example is from Chapter 17, but it is a practice problem (Example 17.3, Add

Question

This example is from Chapter 17, but it is a practice problem (Example 17.3, Adding forces)

I understand that force fields point in the direction away from a positive and toward a negative, but when I need to figure out which direction F net is in when there are 3 or more charges I get lost.

Here's an example:

A)Two point charges are located on the positive x axis of a coordinate system. Charge q_1= 3.0 nC is 2.0 cm from the origin, and charge q_2= -7.0 nC is 4.0 cm from the origin. What is the total force (magnitude and direction) exerted by these two charges on a third point charge q_3= 5.0 nC located at the origin?


B)What is the total force (magnitude and direction) exerted on q_1 by q_2 and q_3?
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I understand all of the math, and have gotten the right answers (besides when I have to figure out whether to add or subtract the two forces depending on the direction)

So here is what I have:
A)

F_1on3= 3.37*10^-4 N
F_2on3= 1.97*10^-4 N

Then this is where I get confused, how do you know that the F_1on3 is in the - x direction and the F_2on3 is the +.

Same question for the next part
B)

F_3on1= 3.37*10^-4 N
F_2on1= 4.72*10^-4 N

This part your supposed to add the two together, since they are both pointing the + x direction, but I don't get this.

I just don't get the direction

Explanation / Answer

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