Thursday, January 8, 2009

DDWFTTW: empirical observation v. intuition

DDWFTTW stands for "directly downwind faster than the wind," and it has been the subject of several heated debates over the internet in the last month or so. The question is this: can one travel downwind, faster than the wind, powered only by the wind, at a steady state? For such an esoteric question, this has spurred quite a bit of outrage among many on the internet, claiming that such a proposition would violate the second law of thermodynamics and basically amount to perpetual motion. The problem with this objection is that—it actually works.

For those of you so inclined, here is a place you can research on this phenomenon further, but the simple answer is that going downwind faster than the wind is pretty uncontroversial. See, for example, this:

A sailboat goes faster than the wind because it isn't getting pushed by the wind—the sail acts like a wing on an airplane and it is propelled by the air pressure differential. Likewise, the propeller acts as a wing and the air pressure pushes it along. Counterintuitive, but well within the laws of physics.

As a science question, this isn't incredibly useful. Due to friction, the practical applications for wind-powered carts is fairly limited, and this isn't exactly new technology. However it can teach us a good lesson about the limits of relying on our intuition. Too often we end up trusting instinct rather than our own eyes.

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