# Coulumb's Law Question

1. Oct 6, 2006

### bjgawp

Question:
Two pith balls are free to float in space. If only gravitational and electrical forces are acting, and the balls have identical masses and charges (q = 4e), find the mass of the balls if they are moving at constant velocity.

I'm pretty sure that there is something I'm missing because when I try to solve for the mass, there are just too many variables to work with. Here's my work:

http://img291.imageshack.us/img291/2433/unsolvablemg8.png [Broken]

Known values:
k = 9.0 x 10^9 Nm²/C²
G = 6.67 x 10^-11 N /m²kg²
q = 4(1.60 x 10^-19 C)

Otherwise, there are 3 variables missing if we solve for m ...

Last edited by a moderator: May 2, 2017
2. Oct 6, 2006

### Staff: Mentor

You set the distance equal... that's not very meaningful! Instead, what must be true about the forces if the balls move at constant velocity?

3. Oct 6, 2006

### bjgawp

Ohh! Constant velocity indicate that Fnet = 0. But I don't understand how that applies here. Surely, Fg and Fe cannot equal to 0

4. Oct 6, 2006

### Staff: Mentor

They don't have to equal zero, but they have to add to zero. The net force on each ball must be zero.

5. Oct 6, 2006

### bjgawp

I can't believe I didn't realize this. Nonetheless, thank you very much :)