Ratchets, power strokes, and molecular motors

We present a method for determining the effective driving potential for a molecular motor from measurements of its stochastic position versus time. In developing the method we can make precise the previously vague notions of ‘Brownian ratchet’ and ‘power stroke’, and suggest means to experimentally distinguish between the two. In particular, we distinguish between two kinds of ratchets: ratchets that rectify large fluctuations and ratchets that bias small fluctuations.