Equilibrium separation

by Ginny Mac
Tags: equilibrium, separation
 P: 17 Oh boy...this problem has stumped me. I am not even sure I know where to start: The potential energy of a diatomic molecule (a two-atom system like H2 or O2) is given by U= (A/r^12) - (B/r^6) where r is the separation of the two atoms of the molecule and A and B are positive constants. This potential energy is associated with the force that binds the two atoms together. (a) Find the equilibrium separation - that is, the distance between the atoms at which the force on each atom is zero. Is the force repulsive (atoms are pushed apart) or attractive (atoms are pulled together) if their separation is (b) smaller and (c) larger than the equilibrium separation? Okay. I started out thinking that because we are looking for the force, we would need to use F(x) = - (d/dx)(U(x)), and after differentiation F(x) = (12A/r^13) + (6B/r^7) So here is the big "now what?" Please help. Thank you - any help is greatly appreciated. ~Gin