Comments by "EebstertheGreat" (@EebstertheGreat) on "A Physics Prof Bet Me $10,000 I'm Wrong" video.

Sailboats are pushed forward by the wind. The turbine actually actively pushes the wind backward like a fan. In many ways, it's more similar to a propeller than a sail. However, the propeller is powered only by the wheels, which makes the system pretty unusual. I don't think you can just compare it directly to something you are more familiar with.
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@FatedHandJonathon No, that's not the same thing. Sails get pushed directly by the wind. Sure, the wind may be at a sharp angle, and the centerboard may push sideways against the water to counteract the large horizontal component, but that doesn't fundamentally change the way it works. The force of the wind on its own pushes the sail forward. A sail is indeed like a wing, or like a windmill. It is not like a propeller, which operates in the other direction. The propeller is driven by an external force and pushes on the air. It is basically a turbine working in reverse; in other words, a fan. You don't need any wind at all to operate a propeller. For this craft, the propeller-like blades are chained directly to the wheels, and it is the force of friction that ultimately drives the blades. It's almost like if you had a hydroelectric generator on your boat which powered the screws. That's much closer to what's going on than tacking into the wind.
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@0Linerider0forever0 Anything can be used to overcome the initial rolling friction and get the cart moving. It could be the wind just pushing on the whole body of the cart like in these tests, or whatever. Once the cart is in motion, it is able to continue to extract more and more energy from the air. The increase in its kinetic energy plus the energy lost in the bearings, due to drag, and such should equal the kinetic energy lost by the air pushed backwards by the blades. The system does rely on the wind moving relative to the ground. If the wind stops blowing, the cart will slowly roll to a halt. However, for any fixed nonzero windspeed, there is in principle no limit to how fast the cart can go, as long as it is light enough relative to the air and resistance is sufficiently low.
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@FatedHandJonathon So . . . did you not read my post? I specifically talk about tacking, about the extreme angle the sail takes with the wind, and how that is not remotely the same thing happening here. The wind is indeed directly pushing on the sail. If you think it isn't, maybe you could tell me what really is pushing on it. See, in the cart, it's the bike chain that turns the blades. Is there some sort of bike chain in the sailboat that pulls the sail? No? Then it's not the same mechanism. Seriously, look at a high school physics diagram of a sailboat tacking into the wind. It can be easily modeled as a single force in the direction of the wind, a force of the wind deflecting off the sail, and a force of the water on the centerboard. There is no need for Bernoulli's principle in the simplest model, because there is no need to describe the fluid at all. It would work the same way if you just pushed on the sail with a stick. It is indeed a similar mechanism to the one by which airplanes with positive angles of attack generate lift. This does not involve Bernoulli's principle. If a sailboat could generate lift the same way a plane flying at negative angle of attack could, then you could turn in the opposite direction to which your sail was angled, which is clearly impossible with any ordinary sail. This also isn't a "modern" thing. Sailboats have always traveled faster upwind than downwind. It is not even possible to build a sailboat that doesn't have this property (unless you try really hard).
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@FatedHandJonathon The boat is moving into the wind. So the faster it goes, the harder the wind pushes. I don't see how that is remotely contradicting what I'm saying. You say you didn't read my whole post. Well go read it. You seem to be saying that modern sailboats can tack into the wind but ancient sailboats could not. That is totally false and doesn't even really make sense. Sailboats have been able to outpace the wind for millennia. Like, the blades on this cart are pushing the air backward. From the reference frame of the cart the wind is being accelerated in its direction of motion. This is always the opposite of what happens to a sailboat.
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Or to be even more explicit: this cart can pick up speed when there is no apparent wind. Obviously a sailboat cannot.
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@murph8411 The definition of power in classical physics is P = 𝛿W/dt, the definition of work is 𝛿W = F · d x , and the definition of velocity is v = d x / dt. If we assume no losses due to heat (i.e. Q = 0), then work is a state function, so the inexact differential 𝛿W becomes the exact differential dW. Thus, W = ∫ dW= ∫ F · d x = ∫ F · (d x /dt) dt = = ∫ F·v dt, where the ∫ represents a line integral. So P = dW/dt = d/dt ∫ F·v dt = F·v by the fundamental theorem of calculus. TL;DR: P = F·v whenever energy is conserved.
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@FatedHandJonathon Incidentally, the same could be said of how birds fly in still air. It's physically very different, but the energy considerations are the same.
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@FatedHandJonathon I misread "did read" as "didn't read." Which I suppose is ironic. A boat tacking into the wind is not pointing directly upwind. Suppose the wind is coming from the north and the boat is pointing 20 degrees NE. The sail is nearly parallel to the boat. The air deflects away from the sail in some southwestern direction. Subtracting these vectors gives a change in momentum that is east southeast. This does push back on the ship, but it mostly pushes it starboard. The keel, centerboard, or whatever then deflects water hitting it aft, which pushes the boat forward. If the keel is deep enough and the boat is massive and stable enough, it will hardly drift starboard at all, moving almost straight forward. It is more complicated for boats or ships with multiple sails, but the basic premise is the same. The wind pushes on the sails, causing the boat to push against the water, moving it forward. This idea has been used in Europe for over 2,000 years. It is not a modern invention. Bernoulli's principle should come into play, just like it does with anything moving through air. Wind moving past a sail loses speed, which should increase the static pressure. It's not clear to me how this would help it generate lift, but it might. But that is not the most important consideration. And it does not somehow prove that sailboats, which cannot sail directly downwind faster than the wind, are using fundamentally the same operating principle as the Blackbird, which can.
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