# Langevin Equation unusal form

1. Nov 9, 2009

### jbunten

I'm trying to understand a paper (Magnasco, 1993) which discusses "Forced thermal ratchets". It gives a Langevin equation as:

$$\stackrel{.}{x} = \epsilon(t) + f(x) + F(t)$$

where x (a cyclic coordinate) describes the state of the ratchet, f(x) is a force field, F is a driving force and $$\epsilon$$ is gaussian noise.

I can't understand the $$\stackrel{.}{x}$$ term. If x is a coordinate then the term is velocity, however the RHS is all force, so logically x could only be momentum.

I would very much appreciate some help with this as I'm rather confused.

Thanks