UCLA physics professor loses $10k bet about downwind kart


But I think one could argue, and if you were in it for 10k I would argue that the second mode where the windmill begins acting like a propeller driven by the wheels is actually not a “Directly Down Wind Faster Than The Wind” machine any more than a car or a bicycle is. It is not being driven by the wind at that stage, even if the wind got it going in the first place.

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I predicted this, didn’t I? Anyway, I worked out an analogy: I can build an electric oven that can heat up to a temperature higher than anywhere in the coal or gas plant supplying the electricity. That should be obvious, those temperatures are really two totally different things.

Same with this land yacht (or sailing ships tacking) : They take their energy from the wind by slowing down air and translate it to their own speed which can be faster than the wind speed as long as they can slow down enough air. As long the total kinetic energy taken from the moving air is more or equal to the kinetic energy gained by the vehicle (allowing for friction) there is no reason why the vehicle can’t accelerate.

The confusion about what happens if the Blackbird is at exactly the same speed (velocity and direction*) of the wind is understandable. Some very clever engineers did some good magic here after all :smiley:. But the solution is not that hard, just consider that the propellor is still slowing down the wind (seen from a spectator’s perspective) and so can still harvest kinetic energy.

(*And this is what a normal sailboat cannot do, a good one can tack at an angle and maintain an own velocity greater than the wind but it cannot turn fully into the wind.)
(Edit: I more or less confused more with less. Not sure how that happened)


Or a sailboat (though I see some mention of “rotary sails” in the comments: never seen them before). It’s well known that sailboats can sail against the wind faster than with it but that’s not using both media as in this case.

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I really don’t get what you’re trying to say. Are you suggesting that once this thing was up to a steady speed, with the wheels driving the propeller, that it could maintain that speed even if the wind stopped blowing? That really would make it a perpetual motion machine and is not how it works. That difference in speed between wind and ground is necessary not just when it’s starting out, but to keep it moving too.


Sailboats absolutely do use both media. If they couldn’t push sideways against the water with a keel they would just be blown in whatever direction the wind is blowing, at the same speed as the wind, like a balloon does. There’s a reason that Carl’s house in Up wouldn’t be steerable with sails the way it was in the movie.


I am not sure that you understand.

  1. You are saying that pushing the propeller into the wind by moving at a speed faster than the wind harvests energy from the wind and transmits it to the wheels.

  2. Others are saying the motion of the car rotates the wheels, which drives the transmission which spins the propellor which pushes the air to make thrust.

Which is it, because it is not both.

What I am saying is that 1 could be considered to be wind driven. 2 is clearly not, and should not be considered DDWFTTW.

Then the wheels are not in fact driving the propeller. The propeller being pushed through the air ( a windmill) is causing it to spin, and hence turn the wheels.

Are you saying that the windmill action on the propeller makes it spin fast enough to create thrust as well, and not just convert wind drag to transmission power? Because that sounds like perpetual motion. I don’t think that is what is happening.

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No, that’s incorrect. I’m really starting to feel like you didn’t watch the video all the way through because he went over a lot of the questions you’re asking. And the treadmill demonstrations show that it can’t be the wind turning the propeller once it’s up to speed because the air around the treadmills was still.


No, what you don’t understand is that I am not taking a position about how it’s working. I’m just saying that one scenario does not rely on the wind, and the other does. I’m just saying that I think the mode where it is not relying on the wind:

That this machine operating in this way is in fact not DDWFTTW, even if wind got it going, it could have as easily got going with a push from a car. Because its motion and power have nothing to do with the wind, if it is as you say it is.

And BTW you’ve contradicted your prior statement here:

Which is it? On the treadmill the air does not matter? But above you said it keeps it moving? Which do you think they said in the video? They did not say both?

Hmm, I was thinking more of an active turning force but I take your point.

Just because a sail boat can be pushed by a tug doesn’t mean its no longer a sailboat. Sure the kart could be pushed by something else, but the key point is that with no propulsion other than wind it can go in the same direction faster than the wind.


And they are right too. There are three valid ways of looking at this thanks to our good friend relativity. As always the frame of reference is essential. I described it from a spectator standing next to the track, most others here look at it seen from the car like it is shown in the video. I have also have seen a calculation from the reference of the wind.

All valid, all come to the same conclusion.

Remember that the wheels and the propellor are directly linked, one cannot go where the other does not.


Its certainly not a “sailboat” while its being pushed.

Look, this IS reliant on the direction of the wind, in order to create the differential mentioned above. I just think that pushing against the air - as the propeller does - is not the same as being driven by the wind - as does a sailboat.

Say instead of the wind, it relied the 2x4 moving on a conveyer next to the car moving at a slightly different rate - just like the example. That would propel the car in the same way - no wind. I’m just saying its not wind driven - its propeller driven. Its not DDWFTTW. Its pushing on the air, or a 2x4, it does not matter.

No, I said that a difference between wind speed and ground speed is necessary. In the case of the treadmill this difference is achieved by moving the ground instead of moving the air.

There is no scenario where this works if there’s no difference between ground speed and wind speed.


But if there was no wind, it could still go if you started it with another kind of push - so it is not relying on the wind, as much as it is relying on a PUSH of some manner to start it. Its not wind driven - it just uses air as its medium for thrust.

But there is - the scenario where the initial push does not come from the wind. AND thats why this is not DDWFTTW- because its not relying on the wind - its relying on the difference as you said. You create the difference with a push from another source, and it still works.

I’ll bet you $10,000 that won’t work (where there is no relative speed difference between the air and the ground).


Ha! Why?

There is no relative speed difference between the treadmill and the air, till you turn it on.

Same thing as pushing the windmill car. Read the post above about relativity - I just saved you 10k ––UR WELCOME

UCLA physics professor loses $10k bet about downwind kart

Because I have the mind of a 12 year old, I read that as “downwind fart” and was super-curious about the bet a physics professor would make on such a thing. Dispersal? Kinetic theory? Boyle’s Law?


It’s true that if you pushed it the propeller would spin and the wheels would roll, but the device would slow and stop fairly soon as its kinetic energy gave way to friction. For it to do otherwise would be perpetual motion.

Instead of the wind, imagine it was powered by gravity instead. So you disconnect the propeller from the wheels and push it down a very long shallow gradient. It will accelerate to a speed where drag is equal to the force of gravity and it will stop accelerating and continue on at that speed until it runs out of hill.

Now, conceptually reconnect the propeller and push it down the hill again. This time the propeller is powered by the wheels which are powered by gravity. The extra thrust from the propeller overcomes some of the drag of the chassis so the whole contraption travels slightly faster than in the first concept.

How does that seem?