[hamdani yusuf (OP)]

Veritasium made a video showing that we can build a wind powered car that goes faster than the wind itself.
Alexander Kusenko, a UCLA Physics Prof. disagree.

https://twitter.com/veritasium/status/1403130178197278720

Big News! UCLA Physics Prof, @alexkusenko bet me $10,000 that I'm wrong about going downwind faster than the wind.

Our wager was witnessed by
@neiltyson
@BillNye
@seanmcarroll
If I win, to what charitable cause should I donate the funds?

Here is the response from Alexander Kusenko trying to debunk the claim in Veritasium's video.

Here is a set of 10 slides that (1) explain what is seen in the video (2) point out the errors theoretical arguments, and (3) provide a complete solution to the problem.

https://docs.google.com/presentation/d/1xuN-9C1Gs6MAVTJkhh1vMJw0atmSsTsKDkhEfN898K4/edit#slide=id.gdc0eb9892c_0_223

Replying to
@thephysicsgirl
 @alexkusenko
 and 3 others
Come on! If someone emails me saying I’m wrong, first I try to convince them that I’m right. Then if we continue to disagree I suggest we make it interesting. The key is to get to the truth. The best, scientifically accurate explanation should win.

[hamdani yusuf (OP)]

Let me make the first move. I'm on Veritasium's side. I'll explain my reasoning later.
Will someone give it a try to show their prowess in physics?

[hamdani yusuf (OP)]

Steve Mould already made a response video.

[hamdani yusuf (OP)]

And here is one slide found in Google Document of Alexander Kusenko (UCLA and Kavli IPMU) titled
Debunking the claim of propeller-assisted straight-downwind land sailing faster than the wind

Erroneous theoretical arguments

Some literature on the subject
Wikipedia page: https://en.wikipedia.org/wiki/Blackbird_(land_yacht)
[1]   Md. Sadak Ali Khan, et al, “Analysis of Down-Wind Propeller Vehicle”,
International Journal of Scientific and Research Publications, 3, 4. (2013)  http://www.ijsrp.org/research-paper-0413/ijsrp-p16135.pdf
[2]    M. Drela, "Dead-Downwind Faster Than The Wind (DFTTW) Analysis", unpublished
https://www.blueplanettimes.com/wp-content/uploads/2010/08/Drela-DDWFTTW-Analysis.pdf
[3]    Presentation by Rick Cavallaro (from 54:47 until 59:00) 

These references contain the same errors.  Let us discuss them.

And here is the conclusion slide

• The experiment in the Veritasium video is consistent with the car’s behavior expected in variable wind.  The experiment does not prove the possibility of a wind-powered sustained (v=const, a=0) straight-downwind land sailing faster than the  wind.
• Incorrect theoretical arguments are found in the literature.
• The correct description confirms the intuitive result that, in a steady state (v=const), the speed of the car is always smaller than the speed of the wind.
  If the car moves faster than the wind, the acceleration is always negative (v≠const, a<0), so that the car decelerates until the speed drops below the speed of the wind.

[alancalverd]

For the windmill to turn, there must be a net wind passing through the propellor disc..

If the wind vector over the ground is v and the car is moving downwind at v, there is no net wind passing through the disc.

[Halc]

I cannot read the legal document, the resolution of which is insufficient.
I did not read any of the links.

• A car can roll downhill on a nearly windless day.
  
• The propeller could be used to store energy while parked, and later use that energy to add thrust to the vehicle while going downwind.
  
• A normal sail-car (no propeller) could go sideways, tacking to a speed far higher than the wind (as illustrated in reply 2). Then just turn directly downwind and let momentum carry you.

I presume the document disallows such cheats, because the task would be pretty easy in all 3 cases.
If the vehicle is moving straight downwind, then in the inertial frame of the vehicle, the wind motion is directly from the front as is the ground motion, and it is hard to initially imagine generating positive net force from that.

Second thoughs:  Imagine an axle with 30 cm tires on a road with a trench between the wheels.  In the middle of the axle is a 50 cm spool of string wound up with the free end below the road facing the rear.  You take the end of the string and pull backwards, and the axle assembly moves forward away from the direction of pull.
It could work that way.  The car has like almost no mass, so the pressure from the front turns the propeller, and that energy is applied at the wheels to add more positive thrust at the wheels than the negative thrust against the propeller.

So despite my initial skepticism, I'd consider it.

Third thought:  In the optimal scenario, there would be no wind at all.  In the reference frame of the vehicle, it's the same air movement across the propeller, but maximum forward movement of the road helping the vehicle. That makes it a perpetual motion machine.  I vote no again.

[Bored chemist]

Imagine you are standing on the boat when it reaches the same speed as the wind.
You and the wind are both doing, for example, 20 miles per hour (WRT the ground).
So your speed with respect to the air around you is zero.
What is driving the windmill?

I hate to say it, but Alan is right.

Now, what I'm less sure about is whether you can sail into the wind faster than the wind.

[hamdani yusuf (OP)]

Here is the theory part of Wikipedia article.

Gaunaa, et al. describe the physics of rotor-powered vehicles. They describe two cases, one from the vantage point of the earth and the other from the vantage point of the air stream and come to the same conclusions from both frames of reference. They conclude that (apart from forces that resist forward motion):[6]

There is no theoretical upper limit to how fast a rotor-driven craft can go directly upwind.
Likewise, there is no theoretical upper limit to how fast a rotor-driven craft can go directly downwind.
These conclusions hold both for land and water craft.

Required for wind-powered vehicle (or water craft) motion are:[6]

Two masses moving with respect to each other, e.g. the air (as wind) and the earth (land or water).
The ability to change the velocity of either mass with a propeller or a wheel.

In the case of a rotor-powered vehicle, there is a drive linkage between the rotor and the wheels. Depending on one's frame of reference—the earth's surface or moving with the air mass—the description of how available kinetic energy powers the vehicle differs:[6]

As seen from the vantage point of the earth (e.g. by a spectator), the rotor (acting like a wind turbine) decelerates the air and drives the wheels against the earth, which it accelerates imperceptibly.
As seen from the vantage point of the air stream (e.g. by a balloonist), the wheels impede the vehicle—decelerating the earth imperceptibly—and drive the rotor (acting like a propeller), which accelerates the air and propels the vehicle.
The connection between the wheels and the rotor causes the rotor to rotate faster with increasing vehicle speed, thereby allowing the rotor blades to continue to obtain forward lift from the wind (as seen from the ground) or to propel the vehicle (as seen from the air stream).[6]

And the achievement

On July 2, 2010, Blackbird set the world's first certified record for going directly downwind, faster than the wind, using only power from the available wind during its run on El Mirage Dry Lake. The yacht achieved a dead downwind speed of about 2.8 times the speed of the wind.[1][16]

On June 16, 2012, Blackbird set the world's first certified record for going directly upwind, without tacking, using only power from the wind. The yacht achieved a dead upwind speed of about 2.1 times the speed of the wind.[1]

[hamdani yusuf (OP)]

So your speed with respect to the air around you is zero.
What is driving the windmill?

As mentioned in the article I quoted above, it's the earth. So if the vehicle is running on a treadmill that has the same velocity as the wind, it can't accelerate.

[hamdani yusuf (OP)]

Now, what I'm less sure about is whether you can sail into the wind faster than the wind.

Yes, it can. The record is 2.1 times the speed of the wind.

[hamdani yusuf (OP)]

For the windmill to turn, there must be a net wind passing through the propellor disc..

If the wind vector over the ground is v and the car is moving downwind at v, there is no net wind passing through the disc.

What's needed is difference of velocity between the wind and the earth. Without the difference, it can't harvest the kinetic energy.
It should be more convincing if the video shows the case of running against the wind. Unfortunately it doesn't.

[hamdani yusuf (OP)]

IMO, people who are sceptic about it think that it would enable perpetual motion and overunity energy generators. But that's not the case.
Someone who is not familiar with buck converter may think that an electronic device can't produce higher dc voltage than the battery that powers it. It's a  similar case here.

[alancalverd]

No. The buck converter, electromechanical inverters, etc., generate less power than they consume, but at a different voltage. 

Sailboats and land yachts are interesting. On a beam reach you can certainly sail faster than the wind, but essentially perpendicular to the wind vector. Running downwind there is no net flow of wind over the sail so you can't exceed the wind speed.

A "crosswind" rotor, like a sail on beam reach, can generate as much power as you want by just making it bigger, so physics says your speed perpendicular to the wind depends only on the ratio of drag to rotor size, and the practical limit is set by engineering considerations. But a "downwind" rotor still requires an actual mass flow from one side of the disc to the other, and that will be zero if the disc is moving at the same speed as the wind. 

[hamdani yusuf (OP)]

No. The buck converter, electromechanical inverters, etc., generate less power than they consume, but at a different voltage.

I'm pretty sure that the Blackbird also produces less power than they consume, even when it's running faster than the wind. What makes you think otherwise?

think that an electronic device can't produce higher dc voltage than the battery that powers it.

Replacing the word for word,
think that a land yacht can't achieve higher speed than the wind that powers it.

[Bored chemist]

As mentioned in the article I quoted above, it's the earth.

The Earth isn't touching the windmill.
The only thing in contact with the windmill is the air.

[hamdani yusuf (OP)]

As mentioned in the article I quoted above, it's the earth.

The Earth isn't touching the windmill.

Not directly. They are connected by the wheels and through a gear mechanism.

As seen from the vantage point of the air stream (e.g. by a balloonist), the wheels impede the vehicle—decelerating the earth imperceptibly—and drive the rotor (acting like a propeller), which accelerates the air and propels the vehicle.
The connection between the wheels and the rotor causes the rotor to rotate faster with increasing vehicle speed, thereby allowing the rotor blades to continue to obtain forward lift from the wind (as seen from the ground) or to propel the vehicle (as seen from the air stream).[6]

[hamdani yusuf (OP)]

Another analogy I can think of. This time is using thermoelectric effect.
Two reservoirs are given. The first is cool, it's 0°C. The second is hot, it's 100°C. They are placed in a large fully hermetic room that can't exchange energy to the outside world.

Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?

It's easy to produce temperature between 0°C and 100°C, by simply connecting those reservoirs with a thermal conductor. There would be temperature gradient in the conductor, spanning from 0°C near the cool reservoir to 100°C near the hot reservoir. At the center of the conductor, it's around 50°C.

[hamdani yusuf (OP)]

Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?

https://en.wikipedia.org/wiki/Thermoelectric_effect
A device combining Seebeck effect and Peltier effect can produce temperature higher than the hot reservoir, as well as lower than the cool reservoir.



The device can't work if both reservoirs have the same temperature. Similar to land yacht that can't work when the wind has the same velocity as the earth surface. I hope this analogy makes sense to you, Halc.

Third thought:  In the optimal scenario, there would be no wind at all.  In the reference frame of the vehicle, it's the same air movement across the propeller, but maximum forward movement of the road helping the vehicle. That makes it a perpetual motion machine.  I vote no again.

[Bored chemist]

Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?

Yes*.
So what?

* for example, you could run a "steam" engine from the temperature difference using alcohol as the working fluid.
That engine could turn a generator and run an electric fridge or furnace.

[hamdani yusuf (OP)]

Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?

Yes*.
So what?

* for example, you could run a "steam" engine from the temperature difference using alcohol as the working fluid.
That engine could turn a generator and run an electric fridge or furnace.

So, we can build a machine that runs faster than the wind without violating known physical laws.
From the perspective of a space observer, the earth surface moves at v1, while the wind moves at v2. Energy can be harvested as long as v1≠v2.