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## Homework Statement

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.

## Homework Equations

F(r) = -(dU/dr)

## 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.