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# Lennard Jones Potential watch

1. Hi can anyone explain in the simplest terms why for the lennard jones potential (or just potential energy in general??), a positive force implies repulsion between particles and vice versa? where Force= -del(Potential)
2. (Original post by darren86)
Hi can anyone explain in the simplest terms why for the lennard jones potential (or just potential energy in general??), a positive force implies repulsion between particles and vice versa? where Force= -del(Potential)
It's purely convention.
Once you define potential energy as zero at infinity, then if the force is repulsive you have to do work on the object to move it towards the other particle. (Against the repulsion) If you do work on something, by convention, you give it energy. In this case potential energy. It's the same reasoning behind the idea of potential energy being stored in a spring when you compress it. You have to do work to do this. The work done on the spring is positive and the energy transferred to the spring is positive.
The reason Force is negative potential gradient is that the potential energy is a maximum nearest the particle and goes to zero at infinity. So the potential gets less as distance from the particle gets greater. This is a negative gradient. The force is repulsive so acts in the positive direction, away from the particle. So the force is positive and the gradient is negative for a repulsive force.

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Updated: October 14, 2013
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