Wind powered vehicle that goes downwind faster than the wind?

[spork]

Not that I'm a big fan of winning a debate by quoting an authority, but you can't do much better than Mark Drela if you're looking for an authority on aerodynamics.

Much more convincing than Mark Drela's say so, are the demonstrations and the straightforward proofs. How about ynot's turntable videos or his circular wind-tunnel videos?

[Harold Bricer]

Originally Posted by F X

"hate to be on the negative side, but no"

Damn, that is some fine writing there. Let's see, what do we know about him?

No shit? And yet he takes time to get involved in stupid internet shit, watch videos, and reply in bad English with no Caps, to a complete stranger online.

Oh yea, that is so believable.

FX.; You obviously have never seen an email from Dr. Kammen. I can confirm that he does indeed write with no caps, and he does not take particular care with his grammar. The email that spork posted is very obviously from Dan Kammen. I have received one from him recently and the style is identical. Since the email is private communication, I will not be posting it here. He will not be participating in this debate for several reasons, which are personal. He has not changed his mind about ddwfttw. He signs his name dan with no caps.

[Eric A. Blair]

I sure am glad I had time to catch up here today. So the notorious humber is back. I didn’t think hed show up here with so many pretty smart people quickly agreeing with spork. I heard from my cousin Logan today. She is stilled pretty pissed off at me for posting about her email from Dr. Kammen as shes still hoping for a more definitive response. She did hear from him again recently, and apparently he did read some links she sent him but said he just didnt have time to get involved in the debate. I was actually surprised she got any reply from him, but as it was a Berkeley grad asking I guess he was just trying to help out. As his words suggest Logan clearly stated she believed spork’s claims and simply asked about spork speaking at his Berkeley lab and what he thought about ddwfttw. As to getting the wrong email address, if you click this link http://www.nuc.berkeley.edu/People/Daniel_Kammen and most others related to him you will see the older address which I guess they forgot about has “Socrates” in it. Below is a copy of the email she forwarded to me, and youll see at the bottom Socrates was removed. And lots of busy people dont bother to use caps, and I doubt he was concerned about impressing her with his secretarial skills.

Logan hasnt given up on doing a video of spork speaking at Berkley. She is persuing other contacts, and is trying to set up a phone call where spork can respond directly to any questions he has. Id say it was long shot with Kammen, but she is determined and Ill be surprised if she doesnt make something happen. In the meantime Im interested in what humber has to say about Ynots treadwheel and wind tunnel. I recall he had some reasons why those tests werent valid on Dawkins but forget what they were.

Heres the copy of the email.

hi logan,
i've looked the videos over.

hate to be on the negative side, but no. direct downwind (no side to side, which can get
you faster than the wind) faster than the wind is not possible due to conservation of
energy. note that vehicles can do better than wind speed if they use tacking back and forth,
but that is not the parameters here.

dan
- Show quoted text -

Class of 1935 Distinguished Professor of Energy
Energy and Resources Group
Goldman School of Public Policy
Department of Nuclear Engineering
Co-Director, Berkeley Institute of the Environment
Founding Director, Renewable and Appropriate Energy Laboratory
310 Barrows Hall #3050
University of California
Berkeley, CA 94720-3050
Tel: 510-642-1640 Fax: 510-642-1085
http://kammen.berkeley.edu
http://rael.berkeley.edu

Harold, I just saw your reply. Can you at least tell us why Kammen said he doesnt want to comment on sporks claims? His opinion certainly seems to be in the minority of those who have carefully considered ddwfttw from what Ive read.

[Harold Bricer]

Originally Posted by Eric A. Blair

Harold, I just saw your reply. Can you at least tell us why Kammen said he doesnt want to comment on sporks claims? His opinion certainly seems to be in the minority of those who have carefully considered ddwfttw from what Ive read.

His opinion may be in the minority if you simply weigh it numerically against the number of those who believe in this. But when you consider his expertise, Dr. Kammen’s opinion outweighs the total of all those others!

I really do not feel I should post any details of his reply without his permission. I have not asked for or received such permission. My interpretation of his reply is that he feels that some people’s belief in this is so strong that when faced with a logical refutation, even from Dan Kammen, they will continue to believe because they want to. He does not want to waste his time in that manner.

I may add that I can well understand that position!

[Eric A. Blair]

Well Harold, I can appreciate hes a busy guy. but he does work for a tax supported institution and as Logan mentioned to him in her last email if so many have been misled by spork why wouldn't he want to correct their thinking? In the name of good science and public education if nothing else! Just saying the cart dont comply with conservation of energy doesn't begin to address all the arguments Ive seen by spork, seebs, Brother Daniel and others that explain why it isnt. Also, why couldnt he just get one of hid grad students to reply as Logan has suggested to him? Personally I thought this matter had been pretty well settled but having someone of Kammens status say no has me confused. And what about Mac Guanna? I'll bet if he doesnt have his doc yet hes close and unlike Kammen propellers and wind powered vehicles are his specialty. So let me ask you. If Mac gets back to us and the Denmark engineering society raised to objections to his telling them about sporks carts and experiments, would that create any doubt in your mind?

[semper]

Originally Posted by humber

To the ether, Semper, the ether.

Well Hi Humber! Its been so long, its good to see they ether let you out, or at least gave you back internet privileges. So much has happened, sure it got a little quiet, but since everything was proven and demonstrated so conclusively I suppose there wasn't really that much to talk about. Of course there are still some poor souls lurking in tormented doubt. Don't lets get started about Harold, for a while we really thought he was you. I mean he had similar traits to the old days when you just couldn't accept observable, qualifiable and quantifiable reality from any number of physical experiments and mathematical proofs! Perhaps you will be able to help him, after all you know what he is going through, surely there can be a healthy resolution if everyone is simply honest.

[Schneibster]

This is really simple. At the point where the cart is moving the same speed as the wind, the force of the wind is zero. At that speed, the ground moving past the wheels makes the propeller spin; this provides thrust. So when the cart is moving the same speed as the wind, there is an unbalanced force pushing it forward. It will therefore accelerate until the unbalanced force is balanced by friction; and if it was moving the same speed as the wind and accelerated, obviously it's moving faster than the wind. Nothing more to it than that.

[cold one]

Originally Posted by Schneibster

This is really simple. At the point where the cart is moving the same speed as the wind, the force of the wind is zero. At that speed, the ground moving past the wheels makes the propeller spin; this provides thrust. So when the cart is moving the same speed as the wind, there is an unbalanced force pushing it forward. It will therefore accelerate until the unbalanced force is balanced by friction; and if it was moving the same speed as the wind and accelerated, obviously it's moving faster than the wind. Nothing more to it than that.

There's a problem with that argument, which is that there is friction between the wheels and the ground, and in the wheel bearings, that provides a braking force. You have to show the thrust beats that.

You might reply "I can make the friction very small with good wheels etc.". But suppose there was no wind but the cart is moving. The situation is the same except now there's wind resistance and the prop sees a headwind. At the same level of argument you could say you're going to make wind resistance small and the prop very efficient so the cart will accelerate, but that's impossible.

I'm not saying it doesn't work. I'm just saying your argument is too quick and not very convincing.

[recursive prophet]

Have to tell you semper, I really doubt anyone ever thought Harold was humber. Christoph? Well, you know how cynical I am. Certainly I’ve considered it, the viewpoints and style being so similar. Then there’s the fact Harold posted his first reply here and quickly became a central character as the only counter-point available. Bottom line is it doesn’t really matter. What does is the fact that if Kammen actually watched spork’s video he must also have also rejected the equivalency of the treadmill. And now we get confirmation from-of all people-Harold verifying Kammen’s email style. No nonsense, terse, to the point, and fuck the caps, punctuation, and proof reading for grammar slips. For me that went down almost as well as the fact he’d take the time to reply in the first place. Asking Sherlock to help track a parking violator comes to mind. This guy is trying to save the world and still takes the time to indulge some soft-science grad asking about toy carts exceeding wind speed!!??

Now I know his phone number maybe I’ll give dan a call and ask if he thinks an asymptote is a maths model of Zeno’s paradox. And what’s with the angels on pins? Yeah I’m being sarcastic. I too wish we could get a few known physicists to express an opinion and at least to a limited degree defend it. Hard to understand why a high flyer like Kammen or Drela wouldn’t have someone they could assign to take a look at the arguments made on this subject and respond after a quick vetting with the boss.

And semper, I really don’t think either Harold or humber are in any way being dishonest. As Harold mentioned, we have to weigh Kammen’s conclusion heavily, however superficial. I would hope we can learn more about his thoughts, but if not it’s time to tap some established expertise elsewhere. This is not to say I haven’t seen an abundance of informed input, and still believe the cart can exceed wind speed. I’m just again wondering about some of humber’s arguments that seemed so compelling. I’m off to look for some now, and look forward to seeing the responses.

@Cold One: You make an interesting point that I have seen presented quite often in various ways by Christoph, Harold, and humber. So unless there is a "land tide" how is the ground enabling the wheels to turn the prop enough to overcome the headwind created once you exceed wind speed? I understand the explanation; I'm just not yet 98% sure, given all the variables, that it is accurate. Hope you'll stick around and respond to spork's coming explanation. If I come across one-he's covered this before-I'll post a link in my next reply.

[spork]

Originally Posted by Eric A. Blair

Well Harold, I can appreciate hes a busy guy. but he does work for a tax supported institution and as Logan mentioned to him in her last email if so many have been misled by spork why wouldn't he want to correct their thinking?

Yes, his purported claim that us "believers" will believe no matter what simply doesn't hold water. Not after Mark Drela weighed in. Surely he doesn't think that little of Drela. Not after I offered a $100K bet at 10:1. Surely Berkeley could use a $100K donation even if Kammen doesn't need the cash. The bottom line is this: I've made my arguments, done the analysis, built the carts, taken and posted video, pointed to the most highly regarded aerodynamicists that back up my claims, put my money where my mouth is, and shared every detail of the experiments with the world as is demanded by science. Kammen on the other hand has done nothing more than purportedly claim "the believers simply want to believe". I think on the balance, I win.

Just saying the cart dont comply with conservation of energy doesn't begin to address all the arguments Ive seen by spork, seebs, Brother Daniel and others...

In fact, it isn't even a complete statement. He makes no claim at all as to what makes him think such laws are violated. We all know we can extract energy from wind. I might as well argue that heavier than air aircraft violate such physical laws. No need to explain this after all. No worries that physical evidence shows otherwise. It's just plain silly.

Personally I thought this matter had been pretty well settled but having someone of Kammens status say no has me confused.

You need not be confused. When Kammen puts up his $10K against my $100K - then you should start wondering. But you won't have to wonder for long. The party will be at my place. And best of all, we'll do the full-scale experiment right in his back yard for him to witness personally, along with the press, the clergy, and whomever else he'd like to have present.

And what about Mac Guanna? I'll bet if he doesnt have his doc yet hes close and unlike Kammen propellers and wind powered vehicles are his specialty.

Interestingly, Mac has only raced turbine powered carts to date (i.e. upwind), but I expect that to change soon. I've sent him the parts to make three carts, and he plans to present his results to the race committee. I hope and expect we'll see at least a downwind leg in this race soon.

Originally Posted by recursive prophet

if Kammen actually watched spork’s video he must also have also rejected the equivalency of the treadmill.

In fact we have no idea what Kammen thinks about the treadmill - or just about anything else. He might think we're pulling it with a string or committing any other fakery. Frankly, I don't care. If he cared, he could see a working demonstration in his office within 24 hours.

[spork]

Originally Posted by F X

I just got an email from Dan, and he is annoyed at people making things up about him. He writes that he never received any email or request from anyone in regards to this. Not about spork, the cart, or speaking at Berkeley. He has not watched any video, and had no idea what I was asking him about. Much less any comments about over unity.

Originally Posted by Harold Bricer

I have received one from him recently... He will not be participating in this debate for several reasons, which are personal. He has not changed his mind about ddwfttw.

Interesting. So which are we to believe - that he's never heard a word about DDWFTTW and this debate, or that his mind hasn't changed regarding where he stands on DDWFTTW?

[cold one]

Originally Posted by recursive prophet

@Cold One: You make an interesting point that I have seen presented quite often in various ways by Christoph, Harold, and humber.

I'm deeply honored to be mentioned in the same breath with such fabled luminaries of science.

So unless there is a "land tide" how is the ground enabling the wheels to turn the prop enough to overcome the headwind created once you exceed wind speed? I understand the explanation; I'm just not yet 98% sure, given all the variables, that it is accurate. Hope you'll stick around and respond to spork's coming explanation. If I come across one-he's covered this before-I'll post a link in my next reply.

I'm not doubting whether it works, because it obviously does. I was just saying Schneibster's "really simple" explanation was a little too simple.

[F X]

Originally Posted by spork

So which are we to believe - that he's never heard a word about DDWFTTW and this debate, or that his mind hasn't changed regarding where he stands on DDWFTTW?

I may be able to help out there. I never sent or received Danny email, nor did I contact the White House, nor did they ask to see the cart. I know, it's hard to beleieve.

The cool thing is, based solely on the internet discussions, the videos, and using my powers of logic and scientific deduction and shit, I would back spork on any bet on this matter.

I think he (and thinairdesigns, and ynot, and all the others) are 100% correct, and the fact that they did spend time, money and energy on testing this, proving it, as well as taking videos, and sharing them, is way fucking cool.

And the lazy idiots who won't lift a finger but want to constantly try to tell them they are wrong, they can go fuck themselves for all I care.

Did a semi-famous person ever look at any of it and dismiss it? If they haven't, they will. Who gives a shit?

That is the beauty and the joy of this whole brain teaser, and sporks constant hammering and challenging anyone. It's entertaining.

[humber]

Originally Posted by cold one

Well, I guess we should get this show on the road.

So, uh, 'humber', do you still think the cart can't go dwfttw? 'Cause I saw some videos that showed it going against a treadmill, and that's the same as going dwfttw.

Or will friction with the ether wind slow it down too much?

There is no need for the quotation marks around my name, cold one. It has been suggested that I have been several posters, including Spork. The latter makes no sense (both interpretations), because to be effective, his puppet 'humber' would have put forward ideas against ddwfttw travel that were cogent and consistent enough to be accepted by 'real' skeptics.

So far, there has been more talk about the authority of others, and what they think of ddwfttw travel, and that is for the same reason that accusations of sock-puppeting are made; carteers can't support their own ideas.

If you carteers understand your creation, you would be able to support it, without simple resort to external authority, or to suggest that all arguments made to the contrary must be the work of a serpent in the garden, yet the arguments you use in support of ddwfttw, and the very idea itself, are those of the one poster who's only authority his is own.

The treadmill is not like "going ddwfttw", but a simple illusion based upon a false premise. Its failures are not just those of relative velocity or "energy balance", but that the concept is fundamentally flawed.


What you see is the result of Spork tweaking the cart ( by tuning the wheels and tyres so that the cart remains in place), so that it performed in a manner that was in accordance with his naive and misconstrued ideas of "frames of reference".

The whole idea is flawed, and demonstrates only that a belt can spin a propeller. The belt is also used to allow comparison with jogging machines, dynos and windtunnels, but they are actually quite different, and belong to different classes of simulators. These ideas of the treadmill, are combined with those applied to the real cart, yet they are not even compatible, though there are many from which to chose.

Is the propellor "tacking" like a sailboat? If that doesn't appeal, then how about the wheels driving the propellor, which drives the wheels?
No good? Then the propellor is like a corkscrew, and wheels push against that? Sill not convinced? Then it's like a 2:1 geared cart, or a tumbleweed....
add salt to taste.

That lack of consistency, and the woefully presented empirical evidence, should alert you that is is likely to be the work of a scientific wannabe.
(Not a hoaxer. Hoaxers are usually technically well informed, and so anticipate objections that may be made. They don't make simple errors that would soon expose them)

I can support any criticism of the treadmill. Would you like to forward a challenge, cold one?

[cold one]

Originally Posted by humber

The treadmill is not like "going ddwfttw", but a simple illusion based upon a false premise. Its failures are not just those of relative velocity or "energy balance", but that the concept is fundamentally flawed.

You mean because of the ether, 'humber'?

[humber]

Originally Posted by cold one

You mean because of the ether, 'humber'?

Choose either.

[cold one]

Originally Posted by humber

Choose either.

I choose neither.

[humber]

Originally Posted by spork

Yes, his purported claim that us "believers" will believe no matter what simply doesn't hold water. Not after Mark Drela weighed in. Surely he doesn't think that little of Drela. Not after I offered a $100K bet at 10:1. Surely Berkeley could use a $100K donation even if Kammen doesn't need the cash. The bottom line is this: I've made my arguments, done the analysis, built the carts, taken and posted video, pointed to the most highly regarded aerodynamicists that back up my claims, put my money where my mouth is, and shared every detail of the experiments with the world as is demanded by science. Kammen on the other hand has done nothing more than purportedly claim "the believers simply want to believe". I think on the balance, I win.

1) Drela has not even mentioned a bet with you. It's entirely your fantasy, as are any putative bets with Kammen. Why must you constantly ride on the tail coats of others?
Drela's paper is speculative, and contains many ideas that are quite different from, and contradictory to, yours. If he is right, you are wrong.

2)Your analysis contains several elementary mathematical errors, and these were pointed out to you, but you simply ignored them. Not posting until the bad men went away, as it were.

3) Your bets are worthless. An adult does not take seriously a bet from a five year old, that he can run the faster. The child may well be vocal and insistent, and truly believe that he could run the faster, but the bet would only end in tears.

But why not take the bet? Why not take the $100K candy from the hirsute baby?

Any rationalist would the speed measurements to be made in accordance with reliable and scientifically-established methods (and considering the money involved, be traceable to an established calibration facility). However, Spork would insist upon his colored balloons and streamers as being reliable and valid indicators of windspeed.

As it can be shown that instrumentation and telltales or balloons can produce contradictory outcomes, the bet would not proceed until one side adopted the methods of the other. The rationalist will not accept a bet that would potentially produce a false result, while Spork has never taken any formally acceptable measurements, so the bet would not get past the initial discussion. Spork then trumpets this failure of detente as a 'win', and further evidence that he is right.

Originally Posted by recursive prophet

Just saying the cart dont comply with conservation of energy doesn't begin to address all the arguments Ive seen by spork, seebs, Brother Daniel and others...

Originally Posted by spork

In fact, it isn't even a complete statement. He makes no claim at all as to what makes him think such laws are violated. We all know we can extract energy from wind. I might as well argue that heavier than air aircraft violate such physical laws. No need to explain this after all. No worries that physical evidence shows otherwise. It's just plain silly.

The conservation of energy is an established scientific precept that leads to the conclusion that the cart's claims are false.
There are many scientific precepts that support the validity of heavier-than- air aircraft. What is your point?

Originally Posted by spork

You need not be confused.

But it helps.

Originally Posted by spork

When Kammen puts up his $10K against my $100K - then you should start wondering. But you won't have to wonder for long. The party will be at my place. And best of all, we'll do the full-scale experiment right in his back yard for him to witness personally, along with the press, the clergy, and whomever else he'd like to have present.

I suggest inviting Micheal Jackson's doctor.

[uncool]

Originally Posted by cold one

There's a problem with that argument, which is that there is friction between the wheels and the ground, and in the wheel bearings, that provides a braking force. You have to show the thrust beats that.

You might reply "I can make the friction very small with good wheels etc.". But suppose there was no wind but the cart is moving. The situation is the same except now there's wind resistance and the prop sees a headwind. At the same level of argument you could say you're going to make wind resistance small and the prop very efficient so the cart will accelerate, but that's impossible.

I'm not saying it doesn't work. I'm just saying your argument is too quick and not very convincing.

The problem with your counterargument is that the air resistance already plays a role in moving the cart. As such, you cannot arbitrarily decrease it and say that the cart will move faster. However, the wheel friction (not including the friction that causes the wheels to turn, as that is not really a friction - it doesn't cause a loss in energy) is not included in the calculations anywhere until you decide to add it in - so you can arbitrarily decrease it.
=Uncool-

[Harold Bricer]

Originally Posted by Eric A. Blair

Well Harold, I can appreciate hes a busy guy. but he does work for a tax supported institution and as Logan mentioned to him in her last email if so many have been misled by spork why wouldn't he want to correct their thinking? In the name of good science and public education if nothing else! Just saying the cart dont comply with conservation of energy doesn't begin to address all the arguments Ive seen by spork, seebs, Brother Daniel and others that explain why it isnt. Also, why couldnt he just get one of hid grad students to reply as Logan has suggested to him? Personally I thought this matter had been pretty well settled but having someone of Kammens status say no has me confused. And what about Mac Guanna? I'll bet if he doesnt have his doc yet hes close and unlike Kammen propellers and wind powered vehicles are his specialty. So let me ask you. If Mac gets back to us and the Denmark engineering society raised to objections to his telling them about sporks carts and experiments, would that create any doubt in your mind?

Dr. Kammen is an extremely busy guy! Have you taken a look at his full CV, which mentions all his publications, and projects that he is involved in? I don’t know how he finds the time to do all that.

I was very pleased that I had the opportunity to exchange emails with him, but that in no way gives me license to speak for him on this forum or anywhere else. In my last email to him, I promised not to take up any more of his time with this so I that is the status quo.

As far as all the arguments go, just the fact that IF the cart worked, it would violate conservation of energy, is more than enough to invalidate the claim of ddwfttw. Violating the conservation of energy is hardly a small matter! There is no need to address any other points. That is how the scientific method works.

As for Mac Guanna, I feel certain that once he actually tests the cart “in the wind” and not on a treadmill, he will find it is nothing more than an upwind cart and not much different from his own designs. I maintain that any upwind cart can also advance on a treadmill, as the two cases are equivalent and nothing to do with ddwfttw.

I reentered this thread only to confirm that the email posted by spork, and by you, is in fact a legitimate email from Dr. Kammen and to refute the lie that FX is telling. I now see that FX has retracted his statement about receiving an email from Dr. Kammen, which saves me some trouble. Maybe he remembered something about email headers and some questions I could have asked him about which domain server Dr. Kammen uses?

As far as I can tell, this issue is already settled; there is no cart that can go directly down wind faster than the wind, powered by the wind, because it is impossible. However, like everyone else here, I do look forward to what the legendary Humber has to say about all of this.

[cold one]

Originally Posted by uncool

The problem with your counterargument is that the air resistance already plays a role in moving the cart.

"Air resistance" of the prop, maybe, but that's not what I was talking about. I was talking about air resistance of the body of the cart, its wheels etc. For the prop I think you're better off discussing its thrust or efficiency than talking about "air resistance".

To make Schneibster's argument correctly you need to demonstrate that the thrust generated by the prop in still air or a slower headwind can overcome the force from the ground on the wheels, but that the same thing can never happen when the prop sees a headwind at exactly ground speed.

It's a little weird, because there doesn't seem to be a theoretical upper limit on the speed the cart can move, which means that even if the headwind is slower than ground speed by a tiny amount the forces can still balance, and if it's FASTER they can balance too (then it's an upwind cart), but if it's the same they can't. But that's the way it goes sometimes, it's like saying if two large objects are at slightly different temperatures you can extract tons of useful work from the difference (no matter which is hotter), but if the temperatures are identical you can't get anything out.

[cold one]

Originally Posted by Harold Bricer

As far as all the arguments go, just the fact that IF the cart worked, it would violate conservation of energy, is more than enough to invalidate the claim of ddwfttw. Violating the conservation of energy is hardly a small matter! There is no need to address any other points. That is how the scientific method works.

I don't get this conservation of energy thing. There's tons of energy in the wind. Why can't the cart use a little of it? A bigger cart or sailboat uses much more energy than this little thing, going downwind or not, and those are obviously possible. Also we know sailboats and sailcars can go much faster than the wind, so that's not a problem either.

As for Mac Guanna, I feel certain that once he actually tests the cart “in the wind” and not on a treadmill, he will find it is nothing more than an upwind cart and not much different from his own designs. I maintain that any upwind cart can also advance on a treadmill, as the two cases are equivalent and nothing to do with ddwfttw.

That ought to be easy to test. Couldn't spork and co. just reverse the propeller gearing on one of their carts to make it an upwind cart and try it on a treadmill? They could use a fan to prove it's an upwind cart first.

[spork]

Originally Posted by Harold Bricer

As far as I can tell, this issue is already settled; there is no cart that can go directly down wind faster than the wind, powered by the wind, because it is impossible.

Wanna bet?

[uncool]

Originally Posted by cold one

"Air resistance" of the prop, maybe, but that's not what I was talking about. I was talking about air resistance of the body of the cart, its wheels etc. For the prop I think you're better off discussing its thrust or efficiency than talking about "air resistance".

To make Schneibster's argument correctly you need to demonstrate that the thrust generated by the prop in still air or a slower headwind can overcome the force from the ground on the wheels, but that the same thing can never happen when the prop sees a headwind at exactly ground speed.

It's a little weird, because there doesn't seem to be a theoretical upper limit on the speed the cart can move, which means that even if the headwind is slower than ground speed by a tiny amount the forces can still balance, and if it's FASTER they can balance too (then it's an upwind cart), but if it's the same they can't. But that's the way it goes sometimes, it's like saying if two large objects are at slightly different temperatures you can extract tons of useful work from the difference (no matter which is hotter), but if the temperatures are identical you can't get anything out.

You are entirely right - it is the same idea. The energy comes from the assumption that the wind is constant over an infinite length - that is, that no matter what happens to the wind hitting the cart, more will come "right behind it," such that the wind is completely unaffected by the cart. In real life, of course, this is not true - the cart would make vortices in the wind which would most likely slow it down eventually - but it does work out in the theoretical version.
=Uncool-

[spork]

Originally Posted by cold one

It's a little weird, because there doesn't seem to be a theoretical upper limit on the speed the cart can move, which means that even if the headwind is slower than ground speed by a tiny amount the forces can still balance, and if it's FASTER they can balance too (then it's an upwind cart), but if it's the same they can't.

As long as you're well within the subsonic range this is true. And certainly it's one of the surprising results. While it would require a ludicrously efficient prop and transmission, both an upwind cart and a downwind cart have no theoretical top speed relative to wind speed - not until you get into compressible flow limitations (i.e. transonic).

Originally Posted by cold one

I don't get this conservation of energy thing.

Neither does Kammen - and he's an advisor to Obama on energy. This worries me - quite literally.