# Charges in a sqaure!

stuti911

## Homework Statement

IP One of the - charges in the figure (sqaure, with one charge at each corner) is given an outward "kick" that sends it off with an initial speed while the other three charges are held at rest. If the moving charge has a mass , what is its speed when it is infinitely far from the other charges?
Express your answer in terms of the variables (Q (charge), m, v and a (a is the length of the side of one sqaure)) and appropriate constants.

Im not sure how to start. Can anyone help?

Homework Helper
First draw a picture and show us. Then try to apply conservation of energy as the Coulomb force is conservative.

ehild

Gold Member
also use the fact that potential energy is 0 at infinity. tell us what you get.

toosha88
I am working on the same problem and have attached a link for the diagram:
http://session.masteringphysics.com/problemAsset/1126168/2/Walker4e.ch20.Pr100.jpg

I took the advice already given on this board and used conservation of energy and started with:
1/2mvo^2+K-Q+Q/a=1/2mvf^2+0
(0 because as stated above PE at infinity is 0) so we are left with:

1/2mvo^2+K-Q+Q/a=1/2mvf^2

Im not quite sure where to go from here though.

Homework Helper
I do not get your equation. It is certainly wrong. What is the initial potential energy of the selected charge?

ehild

toosha88
Would it not be the equation kq/r?

I used K(-Q)(+Q)/a (because the the charges are Q with positive or negative and the length between them is a..hope that makes a bit more sense.

Homework Helper
It is better now. But that selected charge has got potential energy from all the other three charges.

ehild