Finding Force in Potential Energy vs. Interatomic Distance graph

[gunster]

Homework Statement

The figure below shows a graph of potential
energy versus interatomic distance for a particular molecule. Rank the magnitude of the
force at locations A, B, and C. (That is, which
is the greatest, which is smallest, and are any
of these equal to each other?)

http://img851.imageshack.us/img851/5526/captureyrd.png

Uploaded with ImageShack.us

Homework Equations

F = -dU/dx ?

The Attempt at a Solution

To be honest I really have no idea on this one :/. I thought I could find the force using the change of potential energy corresponding with change in interatomic distance but that does not give me the right answer.

It would seem more logical to calculate force based on R itself. Since it's interatomic, I assume force increased with decreasing R, but again that was wrong.Any ideas?

EDIT: Never mind. I realized my mistake and I did not read the "magnitude" word in the question. I figured out the right answer so this thread is no longer needed

 

[Spinnor]

You got the relevant equation right. You know that two atoms that might make up a molecule are attracted if they are far apart and if they get too close there is a repulsive force. You also know that that say the two atoms that make up a molecule will find some distance where they like to be separated where you could say the force goes to zero.

You have the points A, B, and C and you what to know the force at those points. From what I wrote and from what you probably know you should be able to come up with answers. What is the slope at points A, B, and C? F = -dU/dx

Good luck!