# Homework Help: Potential energy equation to force equation?

1. Oct 17, 2016

### MattNguyen

1. The problem statement, all variables and given/known data
The binding potential energy in a diatomic molecule (N2 for example) is often approximated by the Morse potential energy which has the form U(r) = De (1-e^[-a(r-re)])2 where De is a constant that reflects the well depth (dissociation energy), a is a constant that reflects the width of the well, r is the distance between the atoms, and re is the equilibrium distance. Input an expression for the force as a function of the separation distance (r).

2. Relevant equations
F=-du/dx

3. The attempt at a solution
F=2a*De*[e^(-2a(r-re))-e^-(a(r-re))]
I'm just unsure about the re part. I feel like I'm almost correct.

2. Oct 17, 2016

### haruspex

Looks ok except for the sign.

3. Oct 18, 2016

### MattNguyen

Which sign? I entered it into my online homework and it was wrong so that's why I'm asking.

4. Oct 18, 2016

### LemmeThink

Seems to be correct.

5. Oct 18, 2016

### MattNguyen

Now I have this
2 a D
e ( Exp ( - 2 a ( r - re ) ) - Exp ( - a ( r -| re ) ) )
and I inputted it and it says there's a sign error. I just have no idea where,