DDWFTTW Turntable Test: 5 Min Video - Is It Conclusive?

[schroder]

Here is a very simple example of what I am talking about, and it is relevant to the case of the cart on the TT and the cart in the wind:
Place a sailboat in a frame in which the water is calm but the wind is blowing. The sail is up, and the skiff is sailing downwind. Let’s say that the wind is from Left to Right at 10 mph. Let us also say the hull/water interface extracts energy which slows the craft by 50% or 5 mph. Inside the reference frame, the only thing that can be determined is that the craft is sailing downwind at 5 mph wrt the water. Now place the exact same sailboat in an equivalent reference frame, the air is calm, but the water has a current which is flowing in the opposite direction the wind was blowing in reference A and at 10 mph. It is flowing from Right to Left. This causes a relative wind to blow which is indistinguishable in frame B from the wind in frame A. The hull/water resistance is exactly the same, extracting a “cost” of 5 mph. The sailboat sails downwind exactly as in frame A, from the reference inside the frame nothing is indistinguishable. However, it is obvious to an independent observer, in his own frame of reference, that the first boat, wind driven is moving from Left to Right while the second boat, water driven is moving from Right to Left. The two frames are equivocal from the POV inside the frames, in fact a person inside the frames cannot detect any difference. But none the less, they are inverted from the POV of the external observer. I have established that two frames can be exactly equivocal, and at the same time inverted. Does everyone agree?
Now, if that is the way things stay, if there is no transition to another frame, then that is all there is to the story. BUT, if there is a transition to more frames, the inversion plays a very important role indeed!
Suppose that the two frames just described above are center frames. Each one has an additional frame on the left and the right. These frames are also exactly the same in both cases. On the Left is a lake with a nice continuous wind blowing. On the Right is a waterfall which is falling down from a greater height to end forcefully at the height of the water in the center frames. This is how the transition happens: The boat moving to the Left, the water driven boat moves into a steady state on the lake driven by a continuous wind and can continue in that state indefinitely. The wind driven boat move to the right and runs up against the force of the falling water, which pushes it back into the center frame. This action is repeated over and over.
This is what is happening with the wind driven cart; it cannot enter the steady state condition as can the cart on the TT! Yes, the TT shows that a steady state faster than TT is possible, and from that you are interpolating that a steady state faster than the wind is also possible. You are forgetting that the frames of reference, although equivocal, are NOT the same, they are inverted. It Does make a difference which force is the acting force and which is the stationary force. This is NOT a contradiction of Galilean reference frames at All! It is an extension of the idea to include transitions into additional frames. There are many mathematical solutions to DE which start off with equivalency, where one will lead to a steady state and the other will lead to an unstable oscillatory state, both starting out from equivalent transient states.

A little advice, never underestimate your opponent in a physics debate or assume he is in “error” or simply does not know his subject. Especially when you do not know exactly who it is you are talking to.

 

[vanesch]

Because you happen to disagree with me, does not give you the authority to say that I am making an error!

Eh, in your post, you asked where your error was...

Let me ask you this: Simply because two objects or events start off in equivocal frames of reference, does that mean that they will always be in equivocal frames of reference?

What are "equivocal" frames of reference ?

Objects are present in all reference frames at once of course, and there are as many reference frames as one is caring to imagine. Reference frames are conceptual entities from which one applies the laws of nature. Some reference frames have a particular property, which is that they are inertial (that means that Newton's laws hold in them, without having to add pseudoforces such as centripetal or centrifugal forces, or without having to add an artificial gravity field).
To objects, one can attach reference frames. One can attach reference frames which are *continuously* "attached" to the object, but one can also, at a given instant, consider the inertial reference frame in which the object is instantaneously at rest (has velocity 0) but nevertheless has an acceleration (a split second later, then that reference frame is not the instantaneous inertial reference frame of the object anymore, but that doesn't matter).

Now, our steady state situation is pretty simple, because we only need 2 reference frames, which are inertial: that is a frame attached to the ground, and a frame attached to the object (in the "real outdoor wind situation"). There's not much of a point considering one attached to the air, but you can, if you want.

The transient situation in the beginning needs the "instantaneous" reference frames which are inertial but which are only the attached reference frame for a split second.

With the turning table, we also need the "instantaneous" reference frame because of the rotation.

Are you saying that it is impossible for one frame to transition into a steady state while the other does not? Have you heard of reference frames which are equivocal, from the point of view of being inside the frame, but inverted from the point of view of an external observer?

I really don't know what these words mean (or how you mean them), technically. I don't know what it means for a frame to "transition to steady state". I don't know what it means for a frame to be "inside a frame", or what it means to be "inverted" (unless you simply mean the parity of the 3 axes, but I don't think so). I don't know what is meant with "external observer" in this context.

Reference frames are a set of an origin, a definition of 3 axes X, Y and Z as a function of time (in Galilean relativity which is all we need here, no need to switch to Einsteinian relativity for carts and wind blowing, we agree upon that). All objects and their motion can be described in all reference frames. No reference frame is "inside" any other, there is just a RULE OF TRANSFORMATION between two reference frames for:
- positions (if I give you position x1,y1,z1 in one frame, I know how to calculate x2,y2,z2 in the other frame)
- velocities (if I give you velocity vx1, vy1,vz1 in one frame, I know how to calculate velocity vx2,vy2,vz2 in the other frame)
- forces (if I give you force Fx1,Fy1,Fz1, i know how to calculate force Fx2,...)

Now, we can do our "mechanical reasoning" in one frame, or in another and the trick is that in inertial frames, the reasoning is the same and the overall outcome should be the same (that's the principle of Galilean relativity).

So if you have a given physical situation in which you have motions and forces and all that in a given frame, and you INVENT another frame from which you look upon things, you have now a different description of the same physical phenomenon.
If you look at another physical situation, and the description there is the same as the description of your looking upon things in the second frame of the first situation, then you KNOW how things will behave (namely, exactly as they behaved after having transformed your motion in the first frame in the second one).

Now, there are "tricks" in Newtonian mechanics which help us solve problems and which help us do "mechanical reasoning", and one of these "tricks" is conservation of energy. This trick works as well in one frame as in another (inertial). It means that if you calculate the contributions to the energy, you will find a number that doesn't change from time t1 to time t2. So you have a set of contributions of energy calculated from motions and forces in frame 1, and if you add them all, you will find a number that remains constant in time.
You also have a set of contributions of energy calculated from motions and forces in frame 2, and if you add them all, you will again find a number that remains constant in time.

However, nor that number, nor the different contributions are the same between frame 1 and frame 2. And that is the error in the presented reasoning.

And it is the inversion which accounts for the different steady state solutions. I can give examples, other than what I have already given regarding the cart on the TT and cart in the wind, but I want you to carefully consider what I have just said. Don’t dogmatically say I am in error, when in fact it may be you.

As the terms you have used have no technical meaning to me, you should first clarify that.
As to your advice, maybe you should apply it also to yourself As it is you who dogmatically tells us that both situations are NOT equivalent simply because of a vaguely expressed "there's no power available" without actually working out precisely what you mean.

 

[vanesch]

Here is a very simple example of what I am talking about, and it is relevant to the case of the cart on the TT and the cart in the wind:
Place a sailboat in a frame in which the water is calm but the wind is blowing. The sail is up, and the skiff is sailing downwind. Let’s say that the wind is from Left to Right at 10 mph. Let us also say the hull/water interface extracts energy which slows the craft by 50% or 5 mph. Inside the reference frame, the only thing that can be determined is that the craft is sailing downwind at 5 mph wrt the water. Now place the exact same sailboat in an equivalent reference frame, the air is calm, but the water has a current which is flowing in the opposite direction the wind was blowing in reference A and at 10 mph. It is flowing from Right to Left. This causes a relative wind to blow which is indistinguishable in frame B from the wind in frame A. The hull/water resistance is exactly the same, extracting a “cost” of 5 mph. The sailboat sails downwind exactly as in frame A, from the reference inside the frame nothing is indistinguishable. However, it is obvious to an independent observer, in his own frame of reference, that the first boat, wind driven is moving from Left to Right while the second boat, water driven is moving from Right to Left. The two frames are equivocal from the POV inside the frames, in fact a person inside the frames cannot detect any difference. But none the less, they are inverted from the POV of the external observer. I have established that two frames can be exactly equivocal, and at the same time inverted. Does everyone agree?

No, I don't know what you mean with "inverted but equivocal". In fact, you cannot even tell the difference between the two situations and you can have, in your first example, an "external observer" which has exactly the same findings as your external observer in the second case. There is no "independent" observer (which amounts to an "absolute reference frame" which is forbidden by Galilean relativity) which can distinguish between both situations. Place an observer in a balloon, and in the two cases, he will observe exactly the same thing.

It is only when you make a reference to yet another arbitrary object, which is not supposed to be of any mechanical importance here, that you can distinguish the two cases. For instance something that is attached to the ocean floor or so, or the stars, or a coastline.

Now, if that is the way things stay, if there is no transition to another frame, then that is all there is to the story. BUT, if there is a transition to more frames, the inversion plays a very important role indeed!

What role ?

Suppose that the two frames just described above are center frames. Each one has an additional frame on the left and the right. These frames are also exactly the same in both cases. On the Left is a lake with a nice continuous wind blowing. On the Right is a waterfall which is falling down from a greater height to end forcefully at the height of the water in the center frames. This is how the transition happens: The boat moving to the Left, the water driven boat moves into a steady state on the lake driven by a continuous wind and can continue in that state indefinitely. The wind driven boat move to the right and runs up against the force of the falling water, which pushes it back into the center frame. This action is repeated over and over.

Again, the difference comes from mechanically non-essential elements, such as the shore and bottom of the lake (which permit you to say that the water is not "flowing"), or the "waterfall" which permit you to say that it is the water that is flowing.

But when you consider that you are on the ocean, with no shoreline in sight, you cannot distinguish by any means an ocean current and a wind.

This is what is happening with the wind driven cart; it cannot enter the steady state condition as can the cart on the TT! Yes, the TT shows that a steady state faster than TT is possible, and from that you are interpolating that a steady state faster than the wind is also possible. You are forgetting that the frames of reference, although equivocal, are NOT the same, they are inverted.

Again, I don't understand the concept of "inverted" in the sense of reference frames. Galilean relativity tells us that a mechanical situation will behave identically under galilean transformations, in other words, that the laws of mechanics (and of nature in general) are independent of the frame in which we prefer to look upon them.

It Does make a difference which force is the acting force and which is the stationary force. This is NOT a contradiction of Galilean reference frames at All!

I'm sorry, but that is EXACTLY what Galilean relativity forbids you to do. There are no such things as "stationary forces" and "acting forces". It is exactly this kind of thing which is the core of Galilean relativity. You seem not to have grasped what Galilean relativity means.

It is an extension of the idea to include transitions into additional frames. There are many mathematical solutions to DE which start off with equivalency, where one will lead to a steady state and the other will lead to an unstable oscillatory state, both starting out from equivalent transient states.

Same equation, same initial conditions, and different solutions ? Not at all.
Same equations (same laws of nature) and same initial conditions = exactly the same solution.

A little advice, never underestimate your opponent in a physics debate or assume he is in “error” or simply does not know his subject. Especially when you do not know exactly who it is you are talking to.

Well, you have at least given a strong impression that you don't know what's the content of Galilean relativity by making a distinction between concepts which are indistinguishable. And your comment applies to yourself as well, but let us not get personal.

 

[schroder]

Eh, in your post, you asked where your error was...

You're right;I did ask that, but I didn't think anyone would actually do IT!

Thanks for the great discussion!

 

[A.T.]

http://en.wikipedia.org/wiki/Brennan_Torpedo

Directly down wire faster than the wire!

(Torpedo speed) < (- wire speed).

Compared to a DDWFTTW-cart the wire is not the wind, it is the ground:
air -> water
wheels driven by ground -> reels driven by wires

It is more clear if you fix the wires to the ground, and put the torpedo into a river heading downstream. It will go down the river faster than the river, without any motors. And the wires are just stationary extensions of the ground.

 

[zoobyshoe]

No, the relative energy of the wind to the ground is less after it passes through the propeller. The cart runs off of the difference in speed between the air and the ground. Try to look at it this way before the cart passes by all of the wind is moving at 10 mph with respect to the ground. After it passes by some of the air is now moving at let's say 8 mph. That air has less kinetic energy afterwords, where did the energy go? Into the cart propelling it faster than the wind. Please note this is not free energy, over unity or any other such nonsense. The cart is just extracting some of the energy of the wind in a unique fashion. If there is no wind there is no relative motion with the ground for the cart to work off of.

"No, the relative energy of the wind to the ground is less after it passes through the propeller."

But what has to happen to cause the propeller to pass wind? (Pun intended.) Let's examine it all from the cart's frame:

Previous to the transition point where tailwind becomes headwind, the cart saw the wind as a power source. The wind came from behind and applied force to its propeller giving energy to the cart. The cart took that energy and used it to turn the wheels to push the ground backward. The propeller received energy from the wind and did not apply energy to the wind.

Now the situation has changed, though. At the point where wind speed = 0 relative to the cart, the propeller must now switch from being a receiver of energy to a user of energy. To pass wind (pun intended), it must do work on the wind. It must apply force to the air. The air now represents a load.


Air speed is 0 to the propeller. To accelerate the air backward relative to itself the propeller has to both continue turning, and, increase it's energy to accommodate the new load. It needs to turn with increased torque at the same speed, or, alternately, increased speed. It requires more energy to make it turn, because it is now doing the work of moving the air.


The only energy available for this work is from the ground. The cart can extract energy from the ground to turn the propeller! It proceeds to do so! But it finds that, in extracting energy from the ground, the ground has suddenly lost energy! It is now slower!

The propeller has received energy from the difference in ground and airspeed, just as you say! But in doing so it altered the ground and airspeed relative to itself: the ground speed is slower and the wind is a tailwind!

Now, Flossie The Bear, sitting on the cart, cannot enjoy the excitement of saying "Yes, but the propeller now has more torque or speed to accelerate into the headwind!" because the cart is not in the same situation anymore. The prop has more energy, indeed, but the ground has less speed and the airspeed of the headwind it wants to accelerate into is no longer 0 it is less than zero! What a peculiar headwind: instead of coming toward you, it recedes from you! It is a negative headwind. The prop acquired more energy, didn't it? Yes, but Flossie is very frustrated because this didn't solve her problem at all. The headwind used to be standing still, but now it is running backward in front of her, away from her and her now more energetic propeller. In acquiring more torque or speed, the cart has only gotten itself into a situation where it now needs even more torque or speed than it needed before!

Flossie is in a pickle: The cart now needs yet more energy than it needed before!

To sum up: the propeller cannot acquire an energy increase without something else losing energy. In this case it is the ground that loses energy. The ground is now slower, representing a loss of E_k by the ground. Simultaneously and unavoidably the wind speed and direction also changes. It is now a tail wind. Slower ground = tailwind. Or we can call it a negative headwind. Anyway, the cart now needs even more energy than it needed before.
------
Now, let's consider that when the headwind becomes a tailwind that tailwind, in combination with the increased speed of the prop, creates denser, compressed air behind the prop which we feel may be used to accelerate the cart. The opposite catch 22 comes into play: as soon as the cart accelerates off this denser air, the denser air becomes more rarified losing it's ability to add energy to the cart. It is merely a spring, which can store the energy applied to it, not an energy source. The energy spike we might see, the cart bouncing forward off this cushion of compressed air, does not mean the cart can now bounce forward, jump over the 'windspeed = 0' mark, into the headwind and continue forward indefinitely running on the difference between the two headwinds. The problem remains: the energy difference between the headwinds is not enough for the cart to do what it wants to do.

The cart runs off of the difference in speed between the air and the ground.

The cart can extract energy from the difference in ground and air speed regardless of it's own speed, yes, agreed. But that is moot when the difference is not enough to do what the cart wants to do. Pointing out there is still a difference between ground speed and air speed does not address the problem, which is that this energy is not enough to do what the cart wants to do.

The ground contains very little energy here: an amount equal to the energy represented by the light and flimsy cart's total momentum when considered from the ground frame. It is not continuously replenished like a table being turned by a motor. To the cart, at this point in time, the difference between the wind speed and ground speed is all represented by ground speed , for the windspeed = 0 and can't contribute (except as a reference point against which to measure the ground energy). The cart wants to accelerate into a head wind. To get the energy to do so, it must, unfortunately, slow the ground. Before it can ever accelerate into the headwind the wind now has become a tailwind. It cannot acquire energy from the ground without simultaneously turning the wind into a tailwind. Slower ground = tailwind. Sure the prop has gained energy! The propeller it now more energetic! Absolutely! But: it has simultaneously increased the energy required to do the work it wants to do, to catch up to and enter that receding headwind.

In principle, I do not believe the cart can do anything better than oscilliate forward and back at this critical transition point between TH and HH. So long as it is using only energy being immediately supplied, nothing stored, I do not see how it can go DDWFTTWPOBTW.

Demonstration videos: there are movies in which it can be plainly seen that wagon wheels are moving in a direction opposite to that required for forward travel. The point being we may simply not understand what we're looking at well enough to draw the right conclusion. That applies to demonstrations conducted right in front of your eyes as well, of course.

 

[schroder]

No, I don't know what you mean with "inverted but equivocal". In fact, you cannot even tell the difference between the two situations and you can have, in your first example, an "external observer" which has exactly the same findings as your external observer in the second case. There is no "independent" observer (which amounts to an "absolute reference frame" which is forbidden by Galilean relativity) which can distinguish between both situations. Place an observer in a balloon, and in the two cases, he will observe exactly the same thing.







Again, the difference comes from mechanically non-essential elements, such as the shore and bottom of the lake (which permit you to say that the water is not "flowing"), or the "waterfall" which permit you to say that it is the water that is flowing.

But when you consider that you are on the ocean, with no shoreline in sight, you cannot distinguish by any means an ocean current and a wind.



Again, I don't understand the concept of "inverted" in the sense of reference frames. Galilean relativity tells us that a mechanical situation will behave identically under galilean transformations, in other words, that the laws of mechanics (and of nature in general) are independent of the frame in which we prefer to look upon them.




I'm sorry, but that is EXACTLY what Galilean relativity forbids you to do. There are no such things as "stationary forces" and "acting forces". It is exactly this kind of thing which is the core of Galilean relativity. You seem not to have grasped what Galilean relativity means.





Well, you have at least given a strong impression that you don't know what's the content of Galilean relativity by making a distinction between concepts which are indistinguishable. And your comment applies to yourself as well, but let us not get personal.

What you are saying is that equivalent = same. That is not what Galilean relativity says at all. What you are saying is that we should completely ignore any real world information we have about a situation and simply allow mathematical abstractions rule our world. I am sorry, but I very strongly disagree with that concept. In the example I gave about the two sail boats, one being pushed by the wind and the other being pushed by the water. Now the captains of those vessels have GPS and they can determine in which actual direction they are moving. One is moving towards a waterfall and the other is moving towards a lake. According to you, the captain of the vessel which will go over the fall, should simply assume that he will also go into the lake because of the equivalency of his reference frame to the other boat which is heading towards the lake. What you are saying is that our real world information is subordinate to abstract mathematical reasoning. You would go over the waterfall, brandishing your copy of “Galilean reference frames” shouting “this is not supposed to happen!” I would turn my ship around that is the difference between practical applications of science and blind dogmatic science.

 

[A.T.]

Well, you have at least given a strong impression that you don't know what's the content of Galilean relativity

You would go over the waterfall, brandishing your copy of “Galilean reference frames” shouting “this is not supposed to happen!” I would turn my ship around that is the difference between practical applications of science and blind dogmatic science.

May I suggest moving the discussion of validity of Galilean relativity to a separate thread, to prevent this one from being locked?

 

[schroder]

May I suggest moving the discussion of validity of Galilean relativity to a separate thread, to prevent this one from being locked?

No one is questioning the validity of Galilean reference frames. I have used those frames in my analysis and have never violated any validity. My point is about the Application of the reference frames to specific events. My point is that a motor driven cart on the TT is equivalent to a wind driven cart on a stationary surface. There is NO disagreement there. But equivalency does not mean they are the Same. We Know there is a motor in one case, we Know there is a wind in the other case. We are not deaf, blind and dumb! In the sail boat example I gave, I did not place them far in the ocean, away from anything which can act as a reference. I can SEE the waterfall and I can SEE the lake. Even though the forces that are acting are indistinguishable, it is undeniable that in one case the boat will move into a SS solution on the lake and in the other case it will move into an oscillatory solution against the waterfall. You cannot simply ignore real world information that you have and make that subservient to a rule of reference frames. No contradiction of Gailiean relativity is being made here, and if the thread is closed on that premise, it is a ruse to escape the fact that what I am saying is correct.

 

[zoobyshoe]

That is why those guys made the models, to test the theory, the captured the results on video for the rest of us to observe.

Are there limits? Yes, it was just posted. The wind can be too slow to start a cart, and it can be too fast for a particular cart to DDWFTTW, although it should be possible to design a cart for any particular speed range, within reason (such as below supersonic speeds).

Ok, this is a yes: you acknowledge that there can be a situation where there is not enough energy for the cart to do what it wants to do despite there being a speed difference in the surrounding media.

Demonstrations: there are many movies that clearly demonstrate that wagon wheels can turn in a direction opposite to the expected direction for forward travel. It is reasonable, then, to conclude that the direction of wheel rotation of a vehicle is irrelevant to its direction of travel, and we should modify our reasoning to explain why this is so. Is that correct?

There is a movie made in the 1960's which demonstrates a large, bipedal, hairy creature walking through the woods in the Pacific Northest. We should therefore conclude such a creature exists and rewrite zoology. Yes?

In 1980 inventor Howard Johnson demonstrated a motor that operated exclusively on the power supplied by permanent magnets arranged in a specific, previously un-thought of configuration. He was able to demonstrate this to a patent examiner and received a patent on it based on the fact the examiner could clearly see it worked, despite the fact he couldn't figure out how it worked. Johnson didn't know how it worked, either, but he was positive it was not perpetual motion and did not violate conservation of energy. Johnson was able to demonstrate the motor to anyone who wanted to see it. It was repeatable, and could be closely scrutinized. We should therefore rewrite physics. Correct?

http://www.newebmasters.com/freeenergy/sm-text.html

We don't always understand what we're looking at, and things aren't always what they look like they are.

 

[vanesch]

What you are saying is that equivalent = same. That is not what Galilean relativity says at all. What you are saying is that we should completely ignore any real world information we have about a situation and simply allow mathematical abstractions rule our world. I am sorry, but I very strongly disagree with that concept. In the example I gave about the two sail boats, one being pushed by the wind and the other being pushed by the water. Now the captains of those vessels have GPS and they can determine in which actual direction they are moving. One is moving towards a waterfall and the other is moving towards a lake. According to you, the captain of the vessel which will go over the fall, should simply assume that he will also go into the lake because of the equivalency of his reference frame to the other boat which is heading towards the lake. What you are saying is that our real world information is subordinate to abstract mathematical reasoning. You would go over the waterfall, brandishing your copy of “Galilean reference frames” shouting “this is not supposed to happen!” I would turn my ship around that is the difference between practical applications of science and blind dogmatic science.

I'm sorry to say so, but you confirm my impression that you don't know how to apply Galilean relativity, and indeed, now I also understand why you cannot accept the equivalence of the turntable test and the "real wind" test. In order to understand (or accept) the equivalence, you have to understand and accept Galilean relativity, because it is the basis of the "proof". Hence my previous allusions to "if the test is performed in the Mid-West, then you wouldn't accept that as a proof that it also works in, say, London.", or "if the test is performed with a red cart, you would also not accept that as a proof that it can work with a blue one".
Indeed, in order for one to accept that, one has to accept a principle of nature. In the first case it is that the relevant physics is independent of absolute *position* and in the second, that the relevant physics is independent of the color of the paint of the cart.

You could then come up with a silly example of trying to find the Tower of London in the Mid West, and demonstrate how silly and futile my attempt would be, or by trying to point out that no matter how hard you try, you will not hear Big Ben in the Mid West. But we should all agree that the *relevant physics* is not dependent on the presence or not of Big Ben, or the Tower of London, even though of course in the real world, it will not be difficult to make the distinction between the two locations.

In the same way, it can be argued that the color of the paint doesn't have a major influence on the relevant physics.

And in the same way, the presence or not of a waterfall 3 km downstream, or a lake shore doesn't have anything to do with the mechanics of the sail boat. So if I have studied a sailboat on a lake with some wind, *I CAN DEDUCE FROM THAT* how it will behave on a river when there's no wind, whether or not there's a waterfall or not. I do not need to do an independent test, and you would probably agree with me that someone claiming that having seen a sailboat on a lake in the wind is no proof that the sailboat will go slower than the river water and that he wants to SEE the sailboat go slower than the water, because he believes it violates one or other principle, is making a rather silly statement. Especially if that person makes an allusion to any waterfall which is present on the river, and not on the lake. Because that would then mean that on a river which doesn't have a waterfall downstream somewhere, maybe the behavior is different.

Well, your statement with "stationary forces" and "active forces" come over just as strange for someone who knows Galilean relativity all the same.

Because *the relevant physical elements* in the two situations are connected through a galilean transformation, and hence should give you identical mechanical responses. Just as much as the *relevant* physical elements are the same for a test in the Mid-West, or in London. Even though you can of course practically distinguish between both, those differences do not intervene significantly in the physics of the situation under study.

 

[vanesch]

But equivalency does not mean they are the Same. We Know there is a motor in one case, we Know there is a wind in the other case. We are not deaf, blind and dumb!

Equivalence means that the solutions to the equations of motion are the same, in other words, that the coordinates of the different objects x(t), y(t) and z(t) are going to be the same functions of time. That's all that is needed here: that the position of the cart as a function of time will be the same. So if that function gives you a derivative which is larger than the velocity of the wind in one case, it will give you the same result in the other case, and that is what we are trying to demonstrate here.
Other philosophical implications over "same" and "equivalent" do not matter, do they ?

In the sail boat example I gave, I did not place them far in the ocean, away from anything which can act as a reference. I can SEE the waterfall and I can SEE the lake. Even though the forces that are acting are indistinguishable, it is undeniable that in one case the boat will move into a SS solution on the lake and in the other case it will move into an oscillatory solution against the waterfall.

But are you claiming that the sailboat will have a *different velocity wrt to the water* in the following cases:

- lake + wind
- river + no wind
- ocean + wind
- ocean current + no wind ?

Do you think that the velocity wrt the water will be numerically different ?

Because Galilean relativity tells you that it will be exactly the same number, in as much as the relevant physics (the interaction air/sails, the interaction water/boat) is not affected by these changes.

So this is the power of that principle: it is sufficient to have studied ONE case, and you *know for sure* how the behaviour will be in the other cases. So the one case is a sufficient demonstration for the others, and you do not need to perform the tests (and it would be a very very bizarre experience if ever it didn't behave that way).
In the same way as another principle (independency of absolute location) allows us to say that if it has been demonstrated in the Mid-West, we *know* that it will work in London too and we can consider it demonstrated if it has been done so in the Mid-West. Someone claiming that it probably won't work in London would be making a strange claim, and has to explain why he thinks that the equivalence is not valid (in other words, what specific part of the physics is dependent on the presence of Big Ben, or, whether in all generality the independence of physics on absolute location isn't true).

You cannot simply ignore real world information that you have and make that subservient to a rule of reference frames. No contradiction of Gailiean relativity is being made here, and if the thread is closed on that premise, it is a ruse to escape the fact that what I am saying is correct.

Contrary to your claim that is *exactly* what is contradictory to Galilean relativity.

 

[schroder]

Because *the relevant physical elements* in the two situations are connected through a galilean transformation, and hence should give you identical mechanical responses. Just as much as the *relevant* physical elements are the same for a test in the Mid-West, or in London. Even though you can of course practically distinguish between both, those differences do not intervene significantly in the physics of the situation under study.

This is the only part of your post which is relevant to the problem at hand. You admit that yes, you can practically distinguish between them. Good! That is my main point. And, I do not claim that the physics in the frames is any different. A person in one frame cannot find anything to distinguish what is happening from a person in the other frame as long as their observations are limited to the frame! But events outside the frame can influence the outcome, do you agree? A waterfall outside the frame that you transition into influences the outcome on your boat as does a lake. The cart can transition into a SS condition where the wheel is constantly being driven by the TT and motor, or it can transition into an oscillatory condition where it keeps bumping into a headwind and is pushed back. All you need to do is broaden your thinking beyond the confines of the reference frame to account for other influences. The truth is right in front of your eyes if you choose to see it!

 

[vanesch]

May I suggest moving the discussion of validity of Galilean relativity to a separate thread, to prevent this one from being locked?

Well, we will see how it evolves, but I would think that Galilean relativity was part of the proof in the OP. Indeed, without Galilean relativity, there's no point in comparing the cart on a turntable to any cart in the open in a wind on an absolutely fixed road.

 

[schroder]

Well, we will see how it evolves, but I would think that Galilean relativity was part of the proof in the OP. Indeed, without Galilean relativity, there's no point in comparing the cart on a turntable to any cart in the open in a wind on an absolutely fixed road.

I think we are starting to find common ground. DDWFTTW may in fact be possible. I have not disproved it. What I have been attempting to show is that the TT evidence does not prove it! With that, I withdraw from the discussion. Thank you once again. With respect, Schroder

 

[rcgldr]

Yes it works great. Just replace air & ground with water & wires and you get this:
http://en.wikipedia.org/wiki/Brennan_Torpedo

(Torpedo speed) < (- wire speed).

Compared to a DDWFTTW-cart the wire is not the wind, it is the ground: air -> water, wheels driven by ground -> reels driven by wires.

You're right, the Brennan torpedo is a DWFTTW (W = water) device.

The wiki article lists the maximum speed of the torpedo at 31mph, it's apparent "head stream" speed so the torpedo's propeller is accelerating the water slightly faster than 31 mph "up stream", say by 4 mph up stream. Say the required water to wire speed is 20 mph. Then the advance ratio is (prop pitch speed = 35 mph) / (torpedo wire speed = 51 mph) < 1 as it should be. If the required water to wire speed was 40 mph, then you'd have a lower advance ratio (35 mph / 71 mph), but it's still > 0 and < 1 and still DWFTTW. (ar < 0 is DWSTTW (S = slower), ar > 1 is upstream).

I corrected my previous post.

“directly down river faster than the river” test (DDRFTTR).

demonstrations

The Brennan torpedo appears to have been a working example of DWFTTW (W = water).

 

[vanesch]

This is the only part of your post which is relevant to the problem at hand. You admit that yes, you can practically distinguish between them. Good! That is my main point.

You can practically distinguish between them, because there are non-essential elements which are different in the two frames, such as the presence of Big Ben, or a waterfall. If you want to base your difference on these, you have to demonstrate how these elements change the free body force diagram on the objects of the system (in other words, how their interaction plays a role).

And, I do not claim that the physics in the frames is any different. A person in one frame cannot find anything to distinguish what is happening from a person in the other frame as long as their observations are limited to the frame!

Well, I guess it is semantics, but the frame is not limited, it extends to all of space, and you can describe all objects in them. It is: as long as their observations are limited to *the system at hand and their relevant boundary conditions*.

Now, what is the system at hand here ? A flat surface, a wheel that doesn't slip on the flat surface, an airmass, a gearing system and a propeller. The wheel, gearing system and propeller are called the "cart". There is a relative motion between the airmass and the flat surface. *that's it*. Do you think that there are any other elements that determine the forces on the cart ? Meaning, do you think that the free body diagram of forces has elements in it that cannot be explicitly calculated from these elements ?

Because it is *this* system we are studying, and the claim is: can the relative velocity of the cart wrt the flat surface in a steady state situation (meaning, without acceleration during a finite time) be larger and of the same sign than the relative velocity of the air mass wrt to the flat surface ?

But events outside the frame can influence the outcome, do you agree? A waterfall outside the frame that you transition into influences the outcome on your boat as does a lake.

I wouldn't think that the existence of a remote waterfall will change the mechanical condition (forces, acceleration, velocity...) of the boat on the water. If you think otherwise, please explain.

The cart can transition into a SS condition where the wheel is constantly being driven by the TT and motor, or it can transition into an oscillatory condition where it keeps bumping into a headwind and is pushed back.

Let us limit ourselves to the steady state situation, which is all we need to demonstrate. After all, you have DWFTTW even if you have to speed up initially to such a situation using a motor or whatever ; the point is that once you are in this situation, you can remain there during a finite (and in principle indefinite) time as long as the experimental setup allows you to (length of available track, length of the river before you reach the waterfall, size of the lake...).

So if we can, say, demonstrate DWFTTW for say 1 second in a steady state situation, that's ok, as long as no energy reservoir (battery, spring, ...) is used, and as long as there is no acceleration, then the principle is demonstrated (again, because I use another principle in physics, which is the independence of physics on absolute time: if I can do it during *this* second, then that means that it can continue the *next* second in the same conditions etc...)

All you need to do is broaden your thinking beyond the confines of the reference frame to account for other influences. The truth is right in front of your eyes if you choose to see it!

It is amazing how much your advice is applicable to your own viewpoint

 

[vanesch]

Let me lead you to yet another way of seeing the equivalence.

Consider a "true outdoor test". We set up a cart on a 100 m long track, in the outdoor wind, and the cart goes DWFTTW for about 80 meters of the track. I suppose that is something that you would accept as proof, no ?

Now, if I take a 100 m long windtunnel, and I do the same test in the wind tunnel, with the wind in the wind tunnel blowing at the same speed as earlier the outdoor wind was (say, 30 km/h), I take it you would still accept that as a proof ? (y/n)

Now, if I place this windtunnel on a train (it is a big train, having a wind tunnel of about 100 m etc... on it) and keep the train still on its track, you would still accept it as a proof that if the cart goes DWFTTW in the windtunnel for about 80 meters, that's sufficient, right ? Consider that the wind in the windtunnel blows from the front to the back of the train. After all, this is almost the same as before, instead of having the windtunnel on the ground, we have it now mounted on a train which is not moving... Do you accept it as a proof ? (y/n)

And now the hardest part: if I redo the test, but the train is driving on a straight track at 30 km/h, do you still accept the test in the windtunnel (which has been mounted on the train) ? (y/n)

Next: given that the train is running at 30 km/h, and the wind in the windtunnel is blowing (from the front to the back) at 30 km/h, in fact, the air inside the windtunnel is stationary wrt the train track, right ? So I do not need any wind tunnel: I can take in the air at the front of the train, guide it into the windtunnel and have it escape at the back and that will give me the same wind conditions in the windtunnel than when I was operation the windtunnel conventionally, right ? So with this wind, do you still accept the test in the wind tunnel (which is now driven by the air inlet instead of the windtunnel ventilator) ? (y/n)

Right. Now, given that we don't need a real windtunnel, we can dismount the roof and walls of it, the air movement will still be about 30 km/h. So we now just have a flat train with a 100 m long track on it, and our cart. If it now still does the same, can you still accept the test ? (y/n)

Ok, so now we have a flat train, running at 30 km/h through the fields, with a 100 m track on it, and a cart doing the test. Let us say that instead of having a straight track, we make a circular track and have the train run on a circle of say, 2 km diameter. This won't change much, so can you still accept the test with the flat train running on this track at 30 km/h ?

Now, we add wagons to the train on the track. We make the train longer and longer. But we don't change the front part where the test track is. I suppose you can accept the test, even if there are more and more wagons attached at the back of the train ? (y/n)

We now add so many wagons, that we fill the entire circular track: the last wagon is now touching the front of the train. Do you still accept the test on the test track ? (y/n)

Instead of driving the train with a locomotive, we now put a central motor with a few 1 km spokes in the middle of the circle, and we drive the train using that motor and those spokes. Do you still accept the test ? (y/n)

We add more and more spokes, and in the end, we have filled up the entire disk within the circular track. Do you still accept the test ? (y/n)

Now, don't we actually have a giant turntable on which we do the test ? If the track is not on a radius of 2 km, but rather 500 m, would that still do the thing ? (y/n)

 

[Subductionzon]

zoobyshoe, I am not an aeronautical engineer or a sailor, but I do know that ice boats can run downwind at a tack with a up to four or five times the speed of the wind. If you do a vector analysis of their velocity vector you will find that the directly downwind portion of it is faster than the wind. For example if the wind is blowing straight north at 10 mph the boat could easily sail north east at 40 mph so its velocity in the northerly direction would be roughly 30 mph. This is widely observed in ice yachting. He is observing a 20mph headwind from the direction the wind is coming from (actually the wind on his face would be higher since there is an easterly component also). The sail is leveraging the power of the wind. The propeller on the boat is doing the same thing, it is working as a sail. I pointed out that the wind would be slower with respect to the ground after it passes through the propeller to show where the power that the cart uses came from. If you think that the propeller is working like a motor driven propeller you will not see how this works, think of it more as a sail on a continuous 45 degree tack.

 

[schroder]

I wouldn't think that the existence of a remote waterfall will change the mechanical condition (forces, acceleration, velocity...) of the boat on the water. If you think otherwise, please explain.

Since you said “please explain” I will. But I can see that once again, the discussion is really leading nowhere. Let me stress, once and for all, that within the reference frame that has been defined (cart, turntable, motor, wind) there can be NO discernable difference detected by any observer who is also within the reference frame. The waterfall and lake are outside, at least as far as the “observable” frame of reference is concerned. But, I have shown that there can be additional information available that is not contained within the frame of reference. A GPS, for example, could indicate that the sailboat driven by the wind is moving East and the sailboat driven by the current is moving West. A map, could show that to the East is a waterfall and to the West is a lake. What I am saying is this: Galilean relativity does NOT forbid you to take into consideration any additional information which may become available to you, in making an analysis of the future state of your sailboat. If I Know, from the GPS information and the map that I am heading towards a waterfall and not the lake, I would be wise not to discount that information simply because it did not come from within my local reference frame! If you Know that a motor is driving the TT in one case, and the wind is driving the cart in the other case, there is nothing in Galilean relativity that says you are forbidden from taking that information into consideration. It would be very close-minded to Not consider that additional information. I have shown, in my analysis, that when the motor is driving the TT, the resulting SS condition is a continuous source of power and a steady state that is Faster than the TT. No one has shown in ANY demonstration that when the wind is doing the driving, that a similar steady state solution exists. What people are doing is claiming that such a SS will result, based upon what is happening when the motor is doing the driving on the basis of Galilean relativity and by disregarding the additional information that is available. The additional information indicates that when the wind is doing the driving, a steady state will not result but an oscillating state of bumping in and out of a headwind will result. Why would you limit your analysis and your conclusion to only the information that is available to you within the reference frames, when you have perfectly good “external” information? That seems a very close minded and indeed dangerous way to do analysis! If there is one thing I have learned over the years, it is to consider all of the available facts. Everyone here is making reference to the fact that this reference frame is equivocal to that so everything has to be the same. Everything is the same so long as you remain in the equivocal frames. Once you exit, stage right in one case, and stage left in the other case, there is no guarantee that the same fate awaits you in both cases!

 

[vanesch]

But, I have shown that there can be additional information available that is not contained within the frame of reference. A GPS, for example, could indicate that the sailboat driven by the wind is moving East and the sailboat driven by the current is moving West. A map, could show that to the East is a waterfall and to the West is a lake. What I am saying is this: Galilean relativity does NOT forbid you to take into consideration any additional information which may become available to you, in making an analysis of the future state of your sailboat.

Sure. The question is: in what way is this information altering the mechanical situation of the sailboat. For instance, do the GPS signals change the force on the sails ? Does the sailboat behave differently when the GPS satellites are switched off (or, would an identical sailboat have behaved differently in the 1960-ies when there was no GPS) ?

If I Know, from the GPS information and the map that I am heading towards a waterfall and not the lake, I would be wise not to discount that information simply because it did not come from within my local reference frame!

Again, are the forces on the sailboat going to be different on a river with a remote waterfall or without ?

If you Know that a motor is driving the TT in one case, and the wind is driving the cart in the other case, there is nothing in Galilean relativity that says you are forbidden from taking that information into consideration. It would be very close-minded to Not consider that additional information.

You can mechanically only take into consideration what will change the interactions and the boundary conditions. What you are claiming is that things like a waterfall 3 miles down the river or a GPS signal is going to change the forces on a sailboat.

Can you answer my mail with the train ?

I have shown, in my analysis, that when the motor is driving the TT, the resulting SS condition is a continuous source of power and a steady state that is Faster than the TT. No one has shown in ANY demonstration that when the wind is doing the driving, that a similar steady state solution exists.

You have shown nothing. You've simply been repeating "but it is not equivalent, there's no motor", while the motor doesn't matter. The motor is simply there to create the boundary conditions, which is a relative motion between a flat surface and an air mass. *how* that boundary condition comes about doesn't influence the problem - in exactly the same way as it doesn't matter whether it is the water that is flowing (towards a waterfall or not), or the wind which is blowing. The mechanical problem, ONCE THESE BOUNDARY CONDITIONS GIVEN, is identical, but is only looked upon "naturally" from different frames. The TT situation is "naturally" looked upon from the reference frame attached to the air, while the "real outdoor test" is naturally looked upon from the reference frame attached to the surface, simply because WE, as EXPERIMENTERS, are at rest in that frame. If we weren't, we wouldn't pick it as a reference frame (hence my train...)

What people are doing is claiming that such a SS will result, based upon what is happening when the motor is doing the driving on the basis of Galilean relativity and by disregarding the additional information that is available.

yes, because that additional information is of no relevance: it doesn't change any interaction between the device and the surface nor the air, and it doesn't change the boundary conditions set to the problem. The name of your grandmother doesn't change the mechanics either, and that extra information, interesting as it may be, doesn't alter anything concerning the mechanical problem.

The additional information indicates that when the wind is doing the driving, a steady state will not result but an oscillating state of bumping in and out of a headwind will result.

This is impossible, or it would mean one of two things:
- galilean relativity is not valid
- some external element is interacting directly with the device, outside of the surface and the air and results in an extra force (like a rope pulling on it, or a magnetic force or whatever) in one case, and not in the other. But that's supposed not to be the case here.

Why would you limit your analysis and your conclusion to only the information that is available to you within the reference frames, when you have perfectly good “external” information?

Because that extra information doesn't change any of the forces on the device, and hence doesn't change the equations of motion. The device is supposed only to interact (exchange momentum with, have action/reaction forces) with two things: the air mass, and the surface. The fact that the surface is moving wrt to the air is the boundary condition. And it doesn't matter what instores these boundary conditions (motors, ventilators, the sun, ...).
These elements specify entirely the interactions with the device, and hence the forces that act upon them. And when you know the forces, you know how it will move, per Newton's equation (if you give it a given initial velocity).

That seems a very close minded and indeed dangerous way to do analysis! If there is one thing I have learned over the years, it is to consider all of the available facts. Everyone here is making reference to the fact that this reference frame is equivocal to that so everything has to be the same. Everything is the same so long as you remain in the equivocal frames. Once you exit, stage right in one case, and stage left in the other case, there is no guarantee that the same fate awaits you in both cases!

Do you take into account also the constellation of the stars, the distance to Big Ben and the color of the paint ? That's also available and important information.

 

[schroder]

Sure. The question is: in what way is this information altering the mechanical situation of the sailboat. For instance, do the GPS signals change the force on the sails ? Does the sailboat behave differently when the GPS satellites are switched off (or, would an identical sailboat have behaved differently in the 1960-ies when there was no GPS) ?
Again, are the forces on the sailboat going to be different on a river with a remote waterfall or without ?

Do you take into account also the constellation of the stars, the distance to Big Ben and the color of the paint ? That's also available and important information.

Let me say this slowly...nothing changes inside the reference frame. The forces velocity and the color of the sails are completely the same in both cases. I repeat INSIDE the reference frame everything is identical, or at least it appears so to the point you cannot detect any difference. Please stop asking me over and over how the conditions inside change. They do NOT!

Now, when you consider the frame in relation to external stimuli, it is important how that frame is interacting or will interact with the external environment. I DID in fact, show that in the case of the sailboats, one is moving East and the other West although everything INSIDE the respective frames is exactly the same. The East and West motion will only become apparent if and when the frame interacts with those external stimuli, whether it be Big Ben a waterfall or your grandmother. If your grandmother is to the East, the boat moving East will encounter her. If my grandmother is to the West, the boat moving WEst will encounter her. If a steady state exists to the East, the boat moving East will enter that SS. If an oscillating state is to the West, the boat moving West will enter that oscillating state. The TT test shoes that when the motor is driving, the cart enters a SS. When the wind is driving, that is an inversion, similar to going West although everything inside the reference frame is the same. You go West you go into the oscillating state of in and out of a headwind. No ss exists there.

 

[vanesch]

Let me say this slowly...nothing changes inside the reference frame. The forces velocity and the color of the sails are completely the same in both cases. I repeat INSIDE the reference frame everything is identical, or at least it appears so to the point you cannot detect any difference. Please stop asking me over and over how the conditions inside change. They do NOT!

So you agree with me that in the frame of the TT, there is an air mass in motion with a certain speed, and that the cart is having a greater speed in the same direction, and that this only comes about because of the forces of the air and the flat surface, right ?

So you agree with me that if there is a flat surface, and a wind blowing over that surface, that the forces "inside this frame" are identical to those on the turntable, right ?
That means, for a given propeller, I calculate, say 12 Newton at a given rotation speed, and a given relative air velocity, and I will find a certain torque on the propeller axle, and hence a certain torque on the wheel, and hence a certain force on the wheels (just gearing ratio) which will result in, say, -10 Newton as reaction force on the flat surface, and the remaining 2 Newton will compensate the drag of the rest of the cart in the air)

And that calculation will be the same in the case of a flat surface and a wind blowing over it (because the same numbers go in the same formula for the propeller, and the gearing ratio and all that).

Now, tell me, if the forces are the same, how come that the motion is different ?

If the forces are the same, the movement is the same (if we start out with the same initial position and velocity). Newton's equations allow for only one single solution (once initial position and velocity given). So how can one solution be a steady motion faster than the wind, and the other an oscillatory motion ?

Now, when you consider the frame in relation to external stimuli, it is important how that frame is interacting or will interact with the external environment. I DID in fact, show that in the case of the sailboats, one is moving East and the other West although everything INSIDE the respective frames is exactly the same.

Yes, but what counts here is the speed of the boat RELATIVE TO the water, and that will be the same number, in both cases. What we are interested in is the speed of the cart wrt the flat surface, and that will also be the same number.

"Eastward" and "Westward" are indications of velocity, and hence are frame-dependent quantities. However, in both cases, the boat is moving eastward wrt the water.

The East and West motion will only become apparent if and when the frame interacts with those external stimuli, whether it be Big Ben a waterfall or your grandmother. If your grandmother is to the East, the boat moving East will encounter her. If my grandmother is to the West, the boat moving WEst will encounter her.

But if your grandmother is floating in a tub in the water, she will see the same motion of the boat. So as seen from the water, the motion is twice the same. And hence as seen from the flat surface, the motion of the cart is twice the same.

If a steady state exists to the East, the boat moving East will enter that SS. If an oscillating state is to the West, the boat moving West will enter that oscillating state.

Huh ? What oscillating state ? Sorry, do you think that the sailboat will start oscillating when the water is flowing and there is no wind ??

 

[zoobyshoe]

zoobyshoe, I am not an aeronautical engineer or a sailor, but I do know that ice boats can run downwind at a tack with a up to four or five times the speed of the wind. If you do a vector analysis of their velocity vector you will find that the directly downwind portion of it is faster than the wind. For example if the wind is blowing straight north at 10 mph the boat could easily sail north east at 40 mph so its velocity in the northerly direction would be roughly 30 mph. This is widely observed in ice yachting. He is observing a 20mph headwind from the direction the wind is coming from (actually the wind on his face would be higher since there is an easterly component also). The sail is leveraging the power of the wind. The propeller on the boat is doing the same thing, it is working as a sail. I pointed out that the wind would be slower with respect to the ground after it passes through the propeller to show where the power that the cart uses came from. If you think that the propeller is working like a motor driven propeller you will not see how this works, think of it more as a sail on a continuous 45 degree tack.

Here is a rough explanation of how this is accomplished in a conventional sailboat:


"Lets say a boat is trying to go directly downwind and has a hull design that will allow it to plane and take maximum advantage of the wind it experiences. Plus the sails / crew are all optimized...

Starting at a beam reach the boat starts to accellerate and generate more apparent wind. Which will be a vector of the true wind speed and the forward direction of the boat. This new apparent wind will be coming from further forwards that the true wind speed and at a faster speed.

From here the boat turns downwind to keep the apparent wind speed on the beam of the boat. As it does so the boat continues to speed up, and the now new apparent wind continues to accellerate while moving further forward.

The boat responds by turning downwind another five degrees and the cycle repeats over and over, until the boat could be sailing completely on its' own apparent wind actually beating into a headwind that exists completely in its' own mind.

The boats that do this the best are ice boats that are always sailing upwind to their apparent wind regardless of their point of sail on the compass, and can reach rediculous SOG due to the effectively zero drag.

The problem with maintaining this type of thing in the real world is that if a boat slowes down and looses the apparent wind, or is effected by an event that robs is of velocity (running into the back of a wave for instance) the boat now has to start the cycle over again. This leads to a practical limit on this, but not a theoretical one."

That was written by "stumble" in this thread on a sailing forum:

http://www.boatdesign.net/forums/sailboats/tacking-downwind-faster-than-wind-24761.html

Quite fascinating!

What people need to do then is scrap the current designs and make a cart with a self tacking iceboat sail sticking out in front of it on a pivoted arm that can swing from side to side. As this little "servo" iceboat zigzags down wind it should drag the cart directly down wind faster than the wind.

 

[zoobyshoe]

The Brennan torpedo appears to have been a working example of DWFTTW (W = water).

I just came up with a better idea to try:

Mount a self tacking ice boat sail on the end of a pivoting arm and stick this out in front of a cart. The sail will be able to tack back and forth with a faster than the wind downwind component dragging the cart with it, directly down wind. The cart fullfills the "directly downwind" criterion.

 

[Subductionzon]

Here is a rough explanation of how this is accomplished in a conventional sailboat:


"Lets say a boat is trying to go directly downwind and has a hull design that will allow it to plane and take maximum advantage of the wind it experiences. Plus the sails / crew are all optimized...

Starting at a beam reach the boat starts to accellerate and generate more apparent wind. Which will be a vector of the true wind speed and the forward direction of the boat. This new apparent wind will be coming from further forwards that the true wind speed and at a faster speed.

From here the boat turns downwind to keep the apparent wind speed on the beam of the boat. As it does so the boat continues to speed up, and the now new apparent wind continues to accellerate while moving further forward.

The boat responds by turning downwind another five degrees and the cycle repeats over and over, until the boat could be sailing completely on its' own apparent wind actually beating into a headwind that exists completely in its' own mind.

The boats that do this the best are ice boats that are always sailing upwind to their apparent wind regardless of their point of sail on the compass, and can reach rediculous SOG due to the effectively zero drag.

The problem with maintaining this type of thing in the real world is that if a boat slowes down and looses the apparent wind, or is effected by an event that robs is of velocity (running into the back of a wave for instance) the boat now has to start the cycle over again. This leads to a practical limit on this, but not a theoretical one."

That was written by "stumble" in this thread on a sailing forum:

http://www.boatdesign.net/forums/sailboats/tacking-downwind-faster-than-wind-24761.html

Quite fascinating!

What people need to do then is scrap the current designs and make a cart with a self tacking iceboat sail sticking out in front of it on a pivoted arm that can swing from side to side. As this little "servo" iceboat zigzags down wind it should drag the cart directly down wind faster than the wind.

Why would they need to scrap their prop cart? It works quite elegantly on an equivalent of a continuous beam reach tack. That is what the propellers are actually doing. The fact that we are used to thinking of propellers only as driven by a motor is prejudicing our perception.

 

[zoobyshoe]

Why would they need to scrap their prop cart? It works quite elegantly on an equivalent of a continuous beam reach tack. That is what the propellers are actually doing. The fact that we are used to thinking of propellers only as driven by a motor is prejudicing our perception.

The trouble, as I see it, is that at windspeed the DDWFTTW cart the propellers are not in any wind. Once the cart reaches windspeed, there is no wind. How does the blade get a beam reach out of 0 wind?

 

[swerdna]

The trouble, as I see it, is that at windspeed the DDWFTTW cart the propellers are not in any wind. Once the cart reaches windspeed, there is no wind. How does the blade get a beam reach out of 0 wind?

A prop that's spinning in no wind creates a wind.

 

[zoobyshoe]

A prop that's spinning in no wind creates a wind.

Yes, which is what is happening in a fan. When you shut the motor off, the wind stops, though.

When the cart reaches windspeed its prop has only momentum left to create the wind. That energy will soon be expended.

 

[Triple_D]

Wow. All I can say is that it's been an interesting read.