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PhilGoetz comments on The Correct Contrarian Cluster - Less Wrong
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Assume a uniform wind of 20 km/h flowing in the direction from A to B and that B is 100 km from A. Fred is an expert sailor and has a top line sailboat. While Fred is stationary at A, he notices Joe floating past him in a hot air balloon going at the 20 km/h wind speed. Assuming no changes to the wind is it possible for Fred to catch up to Joe using only sailboat before Joe reaches B five hours later?
Has this actually happened? Does this prove anything if it did, given that winds at altitude and ground level are vastly different?
It is impossible for any existing sailboat to have a downwind component that is faster than the wind. If it were possible, you could sail the boat with no wind at all. (This argument does not apply to the ground vehicle under discussion.)
Will you at least agree that it is impossible to sail with the boat pointed directly downwind faster than the wind in a conventional sailboat (including racing sailboats)?
A sailboat can reach faster than the wind because the mass of the wind is greater than the mass of the sailboat, and the energy in the wind is transferred to the boat. Moving downwind faster than the wind is very different, and requires a different mechanism. And you cannot use apparent wind to explain moving downwind faster than the wind, because the apparent wind would be in the opposite direction.
The greater wind at higher elevation would only be an advantage to the hot air balloon.
No. If there is no wind at all, then if the boat moved forward in any direction, it would face an apparent direct headwind opposing its motion. But with some amount of wind, the boat can travel at some angle from dead down wind, so that the apparent wind will not be directly ahead, so that the perpendicular lift will have a forward component.
Yes.
Well, the total mass of all the air that was deflected by the sails as the boat accelerated past wind speed will be greater than the mass of the boat, but that does not really explain what is going on. The point is that the boat is able to continue to derive enough thrust from the apparent wind to overcome drag forces. And this continues to work even when a component of the boat's velocity is downwind.
It is not exactly the opposite direction, and even a small deviation can be significant.
There are potential ambiguities in the language used. Considering a specific example like this allows us to establish whether we are disagreeing about the physics itself or just using different words. I get the impression that we are disagreeing on the nature of physics itself. Fred can win.
I didn't want to dwell on technicalities and hoped 'uniform' was sufficient to convey my intended meaning.
Yes.
Hence the applicability of the 'sailboat' analogy to the vehicle in question.
Possible world too convenient!