Understanding Forces and Electric Potential

[Shaunzio]

Homework Statement

So here is the problem:
http://i.imgur.com/IlklC.png"

I already know the answers but I can't figure out why it is the case. How do you know when the forces cancel out?

Homework Equations

The Attempt at a Solution

The answers should be:
a) 1,2
b)None
c) No
d) yes

 

[tiny-tim]

Hi Shaunzio!

How do you know when the forces cancel out?

draw a forces diagram …

assume there's a positive charge at the point, and draw two arrows to show the direction of the forces on it …

if the arrows are in the same direction, then they can't cancel! ​