Final distance between two charges, identical particles approaching each other

[Alfaraj]

Final distance between two charged, identical particles approaching each other

Homework Statement

Two identical particles, each with a mass of 4.5 mg and a charge of 30 nC, are moving directly toward each other with equal speeds of 4.0 m/s at an instant when the distance separating the two is equal to 65 cm. How far apart will they be when closest to one another? (Answer in cm.)

Homework Equations

Ki + Ui = Kf + Uf (Conservation of Energy)
U= (q1 * q2)/r12 (Potential energy of a system of two charges)

The Attempt at a Solution

I set the final kinetic energy of the system to be zero. In order to come up with an equation of the final distance (rf), I substituted the second equation into the first, and solved for rf. Unfortunately, I am still getting the wrong answer. Something is really missing in my approach to the problem.

 

[betel]

Your idea seems to be correct. Did you use the correct units?
Usually in SI-Units there will be an additional factor of 4Pi and the electrical permittivity in the electric potential.