# Question of electricity net force

1. Mar 29, 2016

### toesockshoe

1. The problem statement, all variables and given/known data

Three point charges are located on the positive x-axis of a coordinate system. Charge q1 = 1.0 nC is 2.0 cm from the origin, charge q2 = -3.5 nC is 4.0 cm from the origin and charge q3 = 6.0 nC located at the origin. What is the net force ((a)magnitude and (b) direction) on charge q1 = 1.0 nCexerted by the other two charges?

The online homework grader is marknig my answer for part a wrong. Can someone tell me whats wrong with my solution for part a? My work is in the attachment.

2. Relevant equations

see attachment

3. The attempt at a solution

see the attachment for my solution. The online homework grader is saying my answer is wrong.

#### Attached Files:

• ###### image3.jpg
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Last edited by a moderator: Mar 29, 2016
2. Mar 29, 2016

### Staff: Mentor

Check the locations of the individual charges. q3 is at the origin according the problem description. Your image has it off to the right.

3. Mar 29, 2016

### Staff: Mentor

It looks like you maybe placed the charges in the wrong spots? q3 is at the origin, q1 is at 2cm and q2 is at 4cm. So there is a repulsive force to the right from q3 on q1, and an attractive force to the right from q2 on q1...

EDIT: Rats, rats, rats, rats, rats!

4. Mar 29, 2016

### toesockshoe

if the pic isnt clear enough, here is a close up.
oh i fixed that. $$F_{q3q1}$$ should be 1.35x10^-4. The answer though is sitll wrong. I subtract the two forces because one pair is attractive while the other force is repulsive. The answer I get is 5.625x10^-5. The answer from homework grader gave me is 2.1x10^-4. This is what you would get if you add the two forces. Why would you add them?

5. Mar 29, 2016

### Staff: Mentor

What directions do the two force vectors point? Make a sketch.

6. Mar 29, 2016

### toesockshoe

oh yeah. i see the problem. thank you.