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Re: [Phys-L] Impossible vehicles.

In the Veritasium video they claim the vehicle (a sail car / land car with backwards facing propellor) is pushing back against the oncoming air. So if the air is moving, when pushing downwind if they can reverse the direction of the air approaching the sail, then they can extract more than just 1 unit of momentum. This’d be analogous to the old chestnut of bouncing a rubber ball rather than a clay ball off a door to close it, to get more than 1 mv (2mv if both balls have same mass and velocity) transfer to the door. So by BLOWING back the oncoming air at more than 1mv, they get to go downwind faster than the wind (neglecting other losses). I’m thinking the only surprise here (airboats exist and I’ve seen lots of Everglades boating footage; propellor driver aircraft also exist) is that the Veritasium cart uses gearing driven by the wheels to drive the fan blades so no additional external energy source except the wind.

On Jun 2, 2021, at 22:10, Bill Norwood via Phys-l <> wrote:

(In the one-dimensional case) the sailboat’s speed is said to max out at wind speed, but actually it must be less than wind speed, due to the water drag on the boat.

Bill Norwood
U of MD at College Park

Sent from my iPhone

On Jun 2, 2021, at 2:31 PM, Brian Whatcott <> wrote:

Here's the directly upwind version of a similar vehicle

On Wednesday, June 2, 2021, 01:09:42 PM CDT, Brian Whatcott <> wrote:

I know of two vehicles that give rise to invective from physics professors: The sand-yacht that rolls directly down wind at twice the speed of the wind

Couldn't find a video of a toy boat with a big air propeller driving a small water prop directly upwind.But they exist!
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