How Does Reducing Distance Affect Force Between Charges?

[rcmango]

Homework Statement

Two charges attract each other with a force of 3.0 N. What will be the force if the distance between them is reduced to one-ninth of its original value?

Homework Equations

(rA/rB)^2

The Attempt at a Solution

not sure how to find this.

 

[Shooting Star]

What is rA and rB?

 

[rcmango]

sorry was a formula, i figured it out though,

(rA/rA9)^2 = 81

81* 3.0 N. = 243 N

Okay so i got the answer, maybe someone could explain what is going on if you will?

 

[Shooting Star]

Use Coulomb's law directly. Take the charges to be q1 and q2, and the initial dist between them to be r.