# Coulombs law

1. Sep 4, 2006

### czaitz

I'm working with Coulomb's law, and finding the force from known charges is easy. But I don't know how to find mass from Coulomb's law, and I guess I don't know an equation that will help.

Two equally charged particles, held 3.8 x 10-3 m apart, are released from rest. The initial acceleration of the first particle is observed to be 5.8 m/s2 and that of the second to be 10 m/s2. If the mass of the first particle is 8.1 x 10-7 kg, what are (a) the mass of the second particle and (b) the magnitude of the charge of each particle?

A nudge in the right direction would be helpful...I have the mass of one particle, and acceleration. And distance between two particles, so I can find r2. But I am stumped how mass comes into things. All I can think of is F=ma.

2. Sep 4, 2006

### Andrew Mason

That is all you need. m = F/a where $F = kq^2/r^2$. If you know m, r and a you can find q. In order to find the mass of the other charge use Newton's third law - the centre of mass does not move.

AM

Last edited: Sep 4, 2006
3. Sep 4, 2006

### czaitz

Ah, yes, thank you so much!