How to graph potential energy for 2 atoms in diatomic molecule w/ 3 unknowns

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

The potential energy of two atoms in a diatomic molecule is approximated by

U(r)=a/r^12 - b/r^6,

where r is the spacing between the atoms and a and b are positive constants.

Find the force F(r) on one atom as a function of r.

Make two graphs one of U(r) versus r and one of F(r) versus r.

2. Relevant equations

F(r) = -(dU/dr)

3. The attempt at a solution

I know F(r) = -(dU/dr).

So, F(r) = (12a/r^13) - (6b/r^7)

And then I get stuck. I don't know how to make a plot of U(r) vs r and F(r) vs r when I don't know a and b.

I want to plug values of r=[0.1, 0.2, 0.3, ...] into the F(r) equation, but I have no idea what to do about a and b.

This question has been asked before on this forum and I have looked at the hints, but I am still completely lost.

Could someone please get me pointed in the right direction.

Thanks.

 

So I figured out that U(r) = a/r^12 - b/r^6 is a simplified version of the Lennard-Jones potential and I know what the graph looks like now, but I still don't understand how you get it. I am so lost. Could someone please explain. Thanks.