How can I represent electric forces on a system with 3 charged balls?

[jtm]

I have an illustration that look like this: - + + on top of a cart. The charges are balls and are very light, and have equal maginitude charge. The rods supporting and connecting them are made with an insulating material and are also light. The cart rests on a smooth table. How would I draw labeled arrows representing electric forces exerted on the WHOLE cart (a system with 3 charged balls)? I was able to do it for each ball.

Here's a better illustration:

(-) (+) (+)
|
------
o o
---------------------------------
The | should be in the middle of the middle ball.

The balls are equally spaced apart also.

 

[Chi Meson]

Three charges (call them A, B and C) are three pairs: AB, AC, BC. Each pair of charges produces a pair of equal forces. That' s a total of six different forces, so you need six arrows.

 

[jtm]

The questions before it made me map the attractions of each ball to the other 2 balls. Are you saying I make 1 x,y axis and just put all the arrows and mark them like q1 on q2 etc then the net force can determine the acceleration. Isn't that confusing? When the 2 force magnitudes are the same and in the same direction, does that mean its double the force? I always get confused with that. For example I have a ball on a rope hanging down and another ball is above it, both are the same charges, so the down force is Force of q1 on q2 and mg, would that mean the Tension = force of q1 on q2 + mg? or just mg since mg is bigger?

 

[Chi Meson]

Think Newton's 3rd Law. Each pair produces a pair of equal magnitude forces, but in opposite directions. If all 3 balls are attached to the cart, then the cart will experience all six forces, three in one way, three the opposite, and all cancelling out.

Think Law of conservation of momentum: these are all internal forces; can an internal force accelerate a system?

 

[jtm]

i don't know the correct answer to your question, I'm sorry, I don't follow, I don't remember laws by names, I just know how to do them. Can you explain more clearly, I'm having a real hard time here :( Thanks!