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Muller’s bet reminded me of the Mysthbusters episode where they put a fan in a boat that blows on it’s own sail and moves the boat forward, which I’m pretty sur

by dahart 5y ago

Muller’s bet reminded me of the Mysthbusters episode where they put a fan in a boat that blows on it’s own sail and moves the boat forward, which I’m pretty sure I thought wasn’t possible before it aired. https://mythresults.com/blow-your-own-sail https://mythresults.com/blow-your-own-sail It also reminds me of the Brayton Cycle, and turbofan/turbojet engines where the output thrust of the blades is applied to subsequent blades that turns the shaft, causing the first fan to spin even faster. To me this feels similar to blowing on your own sails, and to Veritasium’s Blackbird that blows air against the wind to go faster than the wind. These all feel counterintuitive, but they work.

tootie 5y ago
Whether it's an electric fan or your own lungs, it's pulling energy from storage (a battery or your own chemical energy storage) and converting it to kinetic energy. It's no different than if you put the fan or your face in the water facing backwards to propel the ship.
- dahart 5y ago
  That seems too dismissive of the part that’s counter-intuitive, and I think this is legitimately counter-intuitive, which is why physics professors are losing bets. The fan on the boat is blowing air forward from behind the sail, and by it’s very presence on the boat, it applies a force to the boat that is trying to move the boat in reverse. Without the sail, the boat does move in reverse. It’s surprising that one can seemingly recover more force to push the boat forward than was already used to push the boat in reverse. IIRC Myth Busters confirmed the result but didn’t come to a detailed conclusion as to why, which could involve the shape of the hull, directions of the hull & sail relative to the fan direction, or some aerodynamics of the air blocked by the sail. In any case, I think it’s quite different from putting the fan in the water facing backwards to propel forward.

by dahart 5y ago

tootie 5y ago

Whether it's an electric fan or your own lungs, it's pulling energy from storage (a battery or your own chemical energy storage) and converting it to kinetic energy. It's no different than if you put the fan or your face in the water facing backwards to propel the ship.

dahart 5y ago
That seems too dismissive of the part that’s counter-intuitive, and I think this is legitimately counter-intuitive, which is why physics professors are losing bets. The fan on the boat is blowing air forward from behind the sail, and by it’s very presence on the boat, it applies a force to the boat that is trying to move the boat in reverse. Without the sail, the boat does move in reverse. It’s surprising that one can seemingly recover more force to push the boat forward than was already used to push the boat in reverse. IIRC Myth Busters confirmed the result but didn’t come to a detailed conclusion as to why, which could involve the shape of the hull, directions of the hull & sail relative to the fan direction, or some aerodynamics of the air blocked by the sail. In any case, I think it’s quite different from putting the fan in the water facing backwards to propel forward.