Today Derek Muller posted a video about a [wind-powered vehicle][1] that can go downwind faster than that downwind itself.

The vehicle is custom made for that case only: the case of going *straight* downwind.

The vehicle has three wheels, I estimate about 5 meters from front wheel to back wheels, and and a two-bladed propellor is mounted about 5 meters up.

The mechanical connection between the propellor and the wheels is such that when the vehicle is rolling forward the propellor is moving air from the front of the vehicle to the rear of the vehicle.

The vehicle is as lightweight as possible for its size, so it's quite flimsy. This is definitely not a practical device, it is a proof-of-concept device.

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In the video Derek himself indicated that he was not quite confident that he understood the physics of it. I anticipate that questions will start coming in on physics SE, so I present this case as a self-answer.

How can this vehicle, when going *straight* downwind, go faster than the wind itself?


Previous question about that vehicle:  
[Details about mechanics of directly-downwind-faster-than-wind vehicle][3]


  [1]: https://www.youtube.com/watch?v=jyQwgBAaBag
  [2]: https://en.wikipedia.org/wiki/Ratchet_(device)
  [3]: https://physics.stackexchange.com/q/521352/