Figure 5.

Schematic illustration of how random energy from a hail storm can make it possible for a very small car to drive uphill given an appropriate brake design (adapted from ref. [8b]). There are two possible mechanisms shown, a) an energy ratchet and b) an information ratchet. The car is modeled as a small green sphere in each, where the fire hydrants act as fiduciary markers. a) In the energy ratchet, the car is equipped with a special brake modelled after a mechanical ratchet shown in the upper left hand corner. When the brake is on, the car is forced into the notch of the ratchet just to the rear of the fire hydrant. When the brake is released, the car tends to roll backward but because of the hail the car also jitters back and forth. Owing to the asymmetry of the ratchet teeth it is more likely for the car to initially move forward past the hydrant to its front than backward past the hydrant to its rear, although eventually the car will move downhill if the brake is kept off for a long time. Reapplying the brake, however, at intermediate times when the car is more likely to have moved the short distance forward past the hydrant to the front than the long distance backward past the hydrant to the rear, forces the car on average forward to the next notch to the front. This process can be repeated, resulting in net uphill motion of the car. Note that the energy comes not from the hail itself, but from the effort expended by the driver in applying the brake—that is, from a power stroke. This mechanism does not require the driver to observe the position of the car in determining whether to apply or release the brake, but only to make sure the brake is not kept off for too long. b) An alternate method involves the driver observing the position of the car relative to the hydrants. If the driver releases the brake only when the car is near the hydrant in front, and applies the brake whenever the car is near the hydrant to the rear, the car inexorably moves uphill, even with a very simple brake that simply prevents slippage and where no force needs be exerted when applying the brake. Here, the car moves uphill by virtue of the information obtained to determine when to apply and release the brake.