# Homework Help: Determining Coulomb's constant

1. Jul 18, 2011

### ulash

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

Determining Coulomb's constant through a force vs. separation distance graph.

2. Relevant equations

F = kqQ/r^2

3. The attempt at a solution

So, I'm a little confused about what I should do! We were shown a video of two charges moving towards each other. We were given force/distance data, and I used this to plot a graph. However, I need to make a straight line in the graph...which can be done by working with the equation u = 1/r^2. I've tried using the force values and the distance values but to no avail. I used the distance values as r and then sub those into the x-axis, which is what I did ... so should I simply try to change the scale of the graph ... or something else

2. Jul 18, 2011

### Delphi51

Welcome to PF, uflash.
So, you are trying to find the k, right?
The usual approach is to make a graph whose slope is the thing you want to find. You must make F = kqQ/r² look like y = mx so you would choose x to be qQ/r² and y to be F. Then the slope will be k. That is, graph F vs qQ/r² and find the slope.

3. Jul 19, 2011

### srivi

if u =1/r2

then F = ( KqQ ) u , which closely resembles the straight line equation y = mx +b.

where F represents the "y" values, kqQ is the slope, and u represents the "x" values. In this case, the "b" value (y-intercept) equals zero.

so , try substituting u= 1/r2 values into the x-axis to get the solpe .

4. Jul 19, 2011

### ulash

Thank you so much! That really cleared it up for me.