How Do You Calculate Forces on Charges Using Coulomb's Law?

[poohead]

Homework Statement

three point charges are placed at the following points on the x-axis +2 uC at x=0, -3uC at x=40 cm, -5 uC at x=120 cm. find the force (a) on the -3 uC charge, (b) on the -5 uC charge

ans. (a) -0.55N ; (b) 0.15 N


uC=nano coulomb

Homework Equations

Fm=qvB ?

Fe=|E|(q)?

Fe=kq1q2/r^2

The Attempt at a Solution

I cannot seem to solve such a simple problem, i need sufficient help. I have the idea that it has something to do with minusing each Fe charge by each other but I am uncertain please need help with a solution

 

[ravioli]

I cannot seem to solve such a simple problem, i need sufficient help. I have the idea that it has something to do with minusing each Fe charge by each other but I am uncertain please need help with a solution

You'll use the 3rd equation, and you will sum the forces. For instance, for the -3 nC charge, you will have to figure out the force created by the +2 nC Charge, then for the -5 nC charge. Once you get those two numbers, just add them together.

 

[poohead]

but wait, try it for yourself, apparently you don't get the given answer of -0.55 N, what is the answer u thus get?

 

[ravioli]

Well, I didn't get -0.55 N exactly, but I did get -0.5479 N. Which is close enough. I came to this solution saying the uC is actually micro-coulombs. Usually nano-coulombs is denoted nC, and if you don't want to go into LaTex, microcoulombs is uC. This is because u is close enough to the Greek Letter mu.

A few more hints on this problem... Draw a free-body diagram. Then think about what "should" be happening, like which direction a force should go.

 

[poohead]

got it
now I am trying part b
thanks man