# Coulomb's law use

1. The problem statement, all variables given/known data

A particle of charge 6 µC is held fixed while another particle of charge 8 µC is released
from rest at a distance of 1.4 m from the first particle. If the mass of the second particle is
3x10-6 kg, what is its speed when it is very far away from the first particle?

F=ma
F= kQ1Q2 / r2

## The Attempt at a Solution

I pretty much worked through most the problem. In the end, I used coulomb's law in combination with newtons 2nd law to get a= 73469 m/s2. However what's bugging me is the distance that the speed is "very far away". I'm under the assumption that means at a point where the force from the first particle no longer affects the second. However, when I back-tracked using the answer(454m/s) the distance ends up being 1.4( the original distance) with the velocity equations.

Related Introductory Physics Homework Help News on Phys.org
Apologies, I searched google and found a similar problem: rocket is launched straight up from the earth's surface at a speed of 1.60×10^4 m/s.
What is its speed when it is very far away from the earth?

One of the helpers suggested conservation of energy, and I applied this to this problem it worked! Sorry!

gneill
Mentor
You do realize that the acceleration is not constant? It decreases as the distance between the particles grows. So your V2 = Vo + 2ad formula is not valid over the trajectory of the second particle.

Why not try a conservation of energy approach?