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.