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

[schroder]

Ok, so let put a cart outdoors facing west at about latitude 89.44 of the Earth. Relative to the north pole, the ground is moving eastward at 10 mph. Say the wind is moving at 0 mph relative to the north pole. Initially the cart experiences an apparent tailwind of 10 mph and accelerates west. As the cart speeds up, the apparent wind transitions to 0, and then into a headwind. Say the cart reaches terminal velocity at 4 mph west, relative to the north pole. At this point the westward facing cart experiences an apparent wind of -4 mph, and an apparent ground speed of -14 mph. How is this significantly different than the turntable?

It is significantly different because it is only a product of your imagination! Nothing of that sort has ever been demonstrated. If you would like to perform such a demonstration, I would be happy to witness and sign off on it!

 

[vanesch]

The closest I can come to making sense of schroder's objection is: have the forces been correctly transformed in switching frames?

When you switch inertial frames, forces don't change (that is, the vectors remain the same - their component values can change of course if there is a rotation of axes).

When you switch from an inertial frame to a rotating frame, you have to add centrifugal and coriolis forces. (hence my caveat).

 

[vanesch]

It is significantly different because it is only a product of your imagination! Nothing of that sort has ever been demonstrated. If you would like to perform such a demonstration, I would be happy to witness and sign off on it!

But, if that demonstration doesn't work, we have established a case in which galilean transformations do not work! We have established that Newtonian mechanics is basically flawed if the demonstration would work on a treadmill but not in a "real wind" situation. It would mean that physics is different in different reference frames.
We could use then such a device to find out what is the "absolute 0 speed".

(up to the caveat of the rotating frame).

 

[atyy]

When you switch inertial frames, forces don't change (that is, the vectors remain the same - their component values can change of course if there is a rotation of axes).

Is this also true for velocity dependent forces?

 

[zoobyshoe]

There's a spinning prop producing thrust when the apparent wind is 0. The prop rate of rotation is related to ground speed, not apparent wind speed.

The ground speed when the wind is 0 is the same speed as the former tailwind speed over the ground. The ground has a v = TWS but the ground's mass is deceptively small in this case: it is only equal to the mass of the cart. It will only apply torque to the wheels equal to the torque the cart exerts on the ground. It will slow the ground down pretty quickly once you press that torque into service accelerating air.

The prop is slowing the air down to a speed less than that of the cart's forward speed.

Extracting energy from the wind is not accomplished by slowing the wind down relative to your own speed. It is accomplished by slowing the wind relative to the wind's previous speed. It is slower because it has imparted some of its energy to the blades of the windmill. To give energy to the windmill it must lose something: speed.

 

[schroder]

But, if that demonstration doesn't work, we have established a case in which galilean transformations do not work! We have established that Newtonian mechanics is basically flawed if the demonstration would work on a treadmill but not in a "real wind" situation. It would mean that physics is different in different reference frames.
We could use then such a device to find out what is the "absolute 0 speed".

(up to the caveat of the rotating frame).

Well, I see two ways of looking at this:
1) If we are correctly interpreting what is happening on the treadmill/turntable, and the outdoor test fails to duplicate the result, then we have proved an inconsistency with Newton mechanics and Galilean reference frames.
2) If the outdoor test fails to duplicate the result obtained on the TT, it means we have not been accurately interpreting what is happening on the TT.

I do not for one second think that we are about to discover something inconsistent with Newtonian mechanics or reference frames. I do firmly believe, as I have said from day one on this subject, that we are not correctly interpreting what is happening on the TT or TM. The “ground” being driven by a motor in one frame can only be reciprocated by a motorized cart in the corresponding frame in my opinion. I don’t see how anyone can say a cart with no motor can advance against a stationary ground and an effective headwind. It has no power source. In my opinion, what is happening here is we are making a misjudgment of what is happening on the TT/TM and now trying to explain it within the laws of physics. Unfortunately, the laws of physics cannot explain it, so we are inventing parameters such as energy from the ground air interface and we cannot see the forest any more because of all the trees we have planted! I suggest we fall back and re-evaluate what is happening on the TT/TM and I believe you will all find that there has been a serious misinterpretation there. I believe that is what Atyy is alluding to.

 

[A.T.]

I do not accept these as evidence of a transition because...

Because you still haven't mastered http://en.wikipedia.org/wiki/Galilean_relativity"

It is simulated because the ground is moving and being powered by a Motor! In the true H-H situation, the ground is stationary,

What do you mean by "stationary"? The Earth moves around the sun at 30 km/s. There is no such thing in physics as absolute rest. The ground outside can be assumed to be stationary just like the conveyor belt of the treadmill can be too. Both views are valid. Neither one is 'simulated' or less real.

Let’s assume the cart is somehow pushed into a true transition to H-H. What exactly is supposed to keep it there?

The same thing that keeps http://de.youtube.com/watch?v=9Yt4zxYuPzI&feature=related" going against the ground and the ruler: translation.

You absolutely...

Stop thinking in absolute terms. Movement is relative.

... cannot say that because it is happening on the treadmill, it will happen in the real world situation because there is NO MOTOR!

Yes there is a motor called sun. It drives the air masses just like the electric motor is driving the conveyor belt of the treadmill.

 

[zoobyshoe]

zoobyshoe, the air still has a different speed than the ground in your frame of reference. The air is still and the ground is moving backwards at 10 mph (for a ten mph tailwind and the cart matching windspeed). So if the cart changes the speed of the wind so it is closer to the ground speed then is has taken some of the potential energy out of that wind and it can be used to raise the speed of the cart, enabling it to go downwind faster than the wind. The carts energy comes from the difference between air speed and ground speed, something that will exist regardless of the cart's speed.

But to change some of the wind speed so it is closer to ground speed it must take energy from the ground by slowing it down. The ground now has less speed. As soon as that happens we're back in a TH.

 

[vanesch]

Is this also true for velocity dependent forces?

Yes, because in order for forces to have a physical meaning and be velocity-dependent, they have to be velocities BETWEEN objects, which are themselves invariant under change of reference frame. There are no forces which are dependent on velocities wrt the reference frame.

That is, for instance, the force on the propeller will be dependent on the velocity of the air wrt the propeller. That velocity is independent of the frame in which it is observed, because it is calculated by the difference of the two velocities (wrt the frame) of the two objects. If you switch frames, you add the same "transformation" velocity to the two velocities, and this drops out in the difference.

In just any frame, the velocity of the air wrt the propeller is the same (but it is calculated differently of course).

 

[schroder]

Because you still haven't mastered http://en.wikipedia.org/wiki/Galilean_relativity"

Yes, that must be the explanation! I was wondering when the thread would degenerate into this sort of mud slinging.

A.T. : “What do you mean by "stationary"? The Earth moves around the sun at 30 km/s. There is no such thing in physics as absolute rest.”

So this cart harvests power from the rotation of the Earth around the sun? It is an even bigger breakthrough than I thought!

A.T. : “Yes there is a motor called sun. It drives the air masses just like the electric motor is driving the conveyor belt of the treadmill”

So now it is extracting solar energy as well? Where are the solar panels? Why not just put a battery and DC motor in it as well.

I don’t see that anything you have posted here has any relevance to this thread.

 

[vanesch]

Well, I see two ways of looking at this:
1) If we are correctly interpreting what is happening on the treadmill/turntable, and the outdoor test fails to duplicate the result, then we have proved an inconsistency with Newton mechanics and Galilean reference frames.
2) If the outdoor test fails to duplicate the result obtained on the TT, it means we have not been accurately interpreting what is happening on the TT.

I do not for one second think that we are about to discover something inconsistent with Newtonian mechanics or reference frames.

Ah, that's a relief

I do firmly believe, as I have said from day one on this subject, that we are not correctly interpreting what is happening on the TT or TM. The “ground” being driven by a motor in one frame can only be reciprocated by a motorized cart in the corresponding frame in my opinion.

But in the "true windtunnel test" the wind is motor-driven. OF COURSE the energy comes from the motor.

I don’t see how anyone can say a cart with no motor can advance against a stationary ground and an effective headwind.

Well, nevertheless, that's what a sailing car can do.

It has no power source. In my opinion, what is happening here is we are making a misjudgment of what is happening on the TT/TM and now trying to explain it within the laws of physics. Unfortunately, the laws of physics cannot explain it, so we are inventing parameters such as energy from the ground air interface and we cannot see the forest any more because of all the trees we have planted! I suggest we fall back and re-evaluate what is happening on the TT/TM and I believe you will all find that there has been a serious misinterpretation there. I believe that is what Atyy is alluding to.

But you only formulate "opinion" here, and you give no indication as to where, looking from a different reference frame, there is the slightest difference between the situation you want to see "stationary ground and real wind" and the actual situation "moving ground and steady air mass". Do you actually know what it means to change reference frames ? No offense, but you insisting so much on this "difference" makes me wonder.
There *are* a few differences, such as the fact that it is a rotating frame, and maybe that there are some aerodynamical differences (there might be some air dragged along with the turntable, so that the air mass is not entirely stationary etc...).

 

[schroder]

But in the "true windtunnel test" the wind is motor-driven. OF COURSE the energy comes from the motor.






Do you actually know what it means to change reference frames ? No offense, but you insisting so much on this "difference" makes me wonder.
There *are* a few differences, such as the fact that it is a rotating frame, and maybe that there are some aerodynamical differences (there might be some air dragged along with the turntable, so that the air mass is not entirely stationary etc...).

But once the cart exceeds the windspeed (assuming it ever does) the force of the wind and the motor driving the wind, is gone. However, in the simulation on the TT, the motor force is always present. That, to me, is a very BIG difference.

Yes, I actually do know quite a lot about reference frames.

 

[vanesch]

But to change some of the wind speed so it is closer to ground speed it must take energy from the ground by slowing it down. The ground now has less speed. As soon as that happens we're back in a TH.

As I said much earlier in this thread, you are running into a Zeno-type paradox here. That is because you are analyzing things which happen *simultaneously* and you apply them *sequentially*.

What happens is that you TAKE power from the link with the ground (from the wheels), and you GIVE it to the air (with the propeller), and that for a given power, this causes a SMALLER braking force on the wheels than it gives you a "pulling force" with the propeller. These forces are applied simultaneously, and because the propeller "wins", the net force is forward, in the wind direction.

And that's because the power you extract from the wheels equals the velocity of the ground (wrt the cart) times the "braking force" which is the reaction of taking away power, while (ideally) the force you can give to the air is equal to the air velocity (wrt the cart) times the "pulling force" by the propeller (minus the losses due to drag and so on).

When the air is "at rest", that force can actually be as big as you want (with a big enough propeller which moves slowly enough - think ultimately of a giant paddle wheel with huge sails on them).

 

[vanesch]

But once the cart exceeds the windspeed (assuming it ever does) the force of the wind and the motor driving the wind, is gone.

Once the cart exceeds windspeed, it takes its power from the wheels, and puts it into the air, like a propeller-driven airplane. In the overall power balance, this has slowed down some air mass (which has been projected UPWIND by the propeller), and this is the ultimate energy source in the "fixed ground and flowing air" system. So the wind is moving slower in the wind tunnel than it would have been without the propeller blowing upwind. That's the source of the energy. Even if the propeller itself is moving faster than the wind, it STILL blows the air somewhat upwind.

Remember that kinetic energy is frame-dependent. So which medium "wins" energy and which one "looses" is dependent on from which frame you look upon it.

In the turntable test, as seen from the lab system, it is the "braking power" of the wheels which brakes the table, and this is compensated by the motor of the turntable. The air WINS some power here, because it was stationary, and now it is set into motion by the propeller (in the same direction as the turntable).

But if you do the energy balance of this turntable thing in the frame that moves with the turntable, then the air had initially a higher velocity than afterwards, and so now it is the air that delivered the energy, given by the motor which turned the Earth (and the air) underneath the stationary turntable.

The motor (rotating the Earth and the room) is acting as the ventillator in the windtunnel.

 

[rcgldr]

It will only apply torque to the wheels equal to the torque the cart exerts on the ground. It will slow the cart down pretty quickly once you press that torque into service accelerating air.

Except in this case the the torque pressed into service to accelerate the air ends up being converted by the propeller into thrust that is greater than the torque related opposing force from the ground. This works because the propeller interacts with a headwind that is much slower than the ground speed, and although force is higher, speed is slower still, and power output is less than power input.

Extracting energy from the wind is accomplished by slowing the wind relative to the wind's previous speed.

Which is what is happening with the DDWFTTW cart. Repeating my previous example, the wind speed is 10 mph, the cart speed is 14 mph, and the prop accelerates the air by 6 mph upwind. The air speed at the prop is 14 mph - 6 mph = 8 mph. The prop is slowing down the wind from 10 mph to 8 mph, even though it's moving forwards at 14 mph. This 2 mph reduction in wind speed is what is powering this example cart. The videos we've seen appear to confirm that these DDWFTTW cart work.

 

[schroder]

Once the cart exceeds windspeed, it takes its power from the wheels, and puts it into the air, like a propeller-driven airplane. In the overall power balance, this has slowed down some air mass (which has been projected UPWIND by the propeller), and this is the ultimate energy source in the "fixed ground and flowing air" system. So the wind is moving slower in the wind tunnel than it would have been without the propeller blowing upwind. That's the source of the energy. Even if the propeller itself is moving faster than the wind, it STILL blows the air somewhat upwind.

OK. It takes its power from the wheels. Would you kindly tell me where the wheels are taking their power from? Ultimately it has to come from somewhere. Let's trace it back to the source, please.

 

[vanesch]

OK. It takes its power from the wheels. Would you kindly tell me where the wheels are taking their power from? Ultimately it has to come from somewhere. Let's trace it back to the source, please.

I edited my previous post a bit in the mean time.

But the source is always the source of power that establishes the difference in velocity between the air mass and the ground. In the turntable test, this is the motor that runs the table, in the windtunnel test, it is the motor that drives the ventilator, and outdoor, it is the sun that drives the wind.

 

[schroder]

I edited my previous post a bit in the mean time.

But the source is always the source of power that establishes the difference in velocity between the air mass and the ground. In the turntable test, this is the motor that runs the table, in the windtunnel test, it is the motor that drives the ventilator, and outdoor, it is the sun that drives the wind.

It seems to me that the power budget is making a complete circle, and you know what that means...pmm. I'm sorry, I don't have any more time today to discuss this. I don't think the issue is settled. Thank you for your time and insights. I do respect your opinion and analysis.

 

[rcgldr]

I was wondering when the thread would degenerate into this sort of mud slinging.

Come on guys we can discuss this without the implied insults.

So this cart harvests power from the rotation of the Earth ... and the sun?

The cart harvests power from the wind. There are all multiple reasons why there are winds on the earth, including the Earth's rotation, and the heat from the sun. It doesn't really matter, as we know that winds exist, and we know that a large wind turbine can generate mega-watts of power. This would seem to prove that power can be extracted by slowing down the wind.

I'm assuming that you agree that slowing down the wind is suffcient to extract power from the wind? If so, then the only remaining issues are if a propeller on a cart can slow down the wind even though the cart and propeller are moving DDWFTTW, and do so without consuming more power than the wind is providing.

 

[vanesch]

It seems to me that the power budget is making a complete circle, and you know what that means...pmm.

So we are back to your initial claim that a DWFTTW is an over-unity device, and hence must be able to be proved against classical mechanics. As I said, I would like to see your proof of that - as I've given gedanken experiments that comply with classical mechanics' postulates and are nevertheless moving DWFTTW (by planting windmills and all those other strange examples).

But your claim is not true. There is an external source of power, which is whatever establishes the difference in velocity between the two media (air and ground). I don't see why you find this obvious in the turntable setup (YES, the power comes from the motor), but refuse to acknowledge that in a "fixed ground but moving air" frame (such as the lab frame in a wind tunnel experiment) the power comes from whatever makes the air move - such as the ventillator that drives the wind tunnel.

After all, as Jeff said, with a windmill it is also possible to extract power from this - so is this then also an over-unity device ?

 

[rcgldr]

It seems to me that the power budget is making a complete circle.

It might appear that way, but the power output can be significantly less than the power input because of the difference in the speed of the wind versus the speed of the ground. The power input is related to ground speed that the wheels interact with, while the power output is related to air speed that the prop interacts with. When there is a wind and the cart is doing downwind, the air speed is slower than the ground speed. Power equals force times speed, so the difference between air and ground speed allows the force on the air to be greater than the opposing force from the ground, but with an air speed much lower than the ground speed so that the power output, thrust times air speed, is less than the power input, ground force times ground speed.

 

[rcgldr]

I offer this extreme example of taking advantage of differing velocties in the same media, the air. By simply traveling back and forth across a reasonably small shear boundary between two differing air streams while circling in an angled loop, a radio control glider can achieve extreme speeds. This is called dynamic soaring.

In this youtube video a glider reaches 333 mph, and it's averaging over 270mph, with a wind flow differential around 50 mph or so. There's a high speed wind, around 50 mph flowing over a narrow ridge, so that on the downwind side of the ridge, there's a somewhat turbulent but relatively stagnant volume of air below the ridgeline, and a fast moving air stream above the stagnant air, separated by a reasonably small shear boundary.

http://www.youtube.com/watch?v=Vi0hrjqU15I&fmt=18

Last I read, the record for rc glider dynamic soaring was 365mph.

 

[A.T.]

A.T. : “What do you mean by "stationary"? The Earth moves around the sun at 30 km/s. There is no such thing in physics as absolute rest.”
So this cart harvests power from the rotation of the Earth around the sun?

I never said that, and you know it. That's why you sniped the relevant part:

"The ground outside can be assumed to be stationary just like the conveyor belt of the treadmill can be too. Both views are valid. Neither one is 'simulated' or less real."

I tried to explain to you that neither the ground not the conveyor belt are at absolute rest. But observers on either one will observe the same laws of physics. If the observer standing on the conveyor belt sees the cart moving faster that the air in the same direction as the air, the observer on the ground in a wind will see the same.

A.T. : “Yes there is a motor called sun. It drives the air masses just like the electric motor is driving the conveyor belt of the treadmill”
So now it is extracting solar energy as well?

Yes. No sun -> no wind -> no DDWFTTW

I don’t see that anything you have posted here has any relevance to this thread.

Because you have deliberately ignored the relevant parts of my post, in order not to have to address them:

When you look a http://de.youtube.com/watch?v=9Yt4zxYuPzI&feature=related" you see that not all parts of the cart are moving faster than the ruler. The top of the big gear has the same speed as the ruler, so it experiences no headwind. The same is true for the front surfaces of the propeller blades on the wind powered carts: they don't experience a headwind, even if the rest of the cart does.

 

[rcgldr]

I tried to explain to you that neither the ground not the conveyor belt are at absolute rest.

I don't think the frame of reference is the issue, it's the fact that the belts or turntable are powered. However the wind on the Earth is also powered (something had to accelerate the wind), so I'm not sure why the fact that the turntable is powered is an issue. The key thing here is that the carts are outruning the powered turntables and treadmills, while operating in an unpowered apparent headwind. Change this to unpowered ground and powered wind, and it should be the same.

video wheel ruler ... prop forward edges

I'm not sure what the point here is. The top wheel rolls forward on the "ruler" and doesn't move at the same speed. When the air based carts are going DDWFTTW, the prop experiences a headwind, but it's not an issue.

 

[A.T.]

I'm not sure why the fact that the turntable is powered is an issue.

It isn't an issue. It just the last straw of someone who said that he would accept a wind tunnel test. As if wind tunnels were not powered by a motor.

I'm not sure what the point here is. The top wheel rolls forward on the "ruler" and doesn't move at the same speed.

Read again what I wrote: "The top of the big gear has the same speed as the ruler".

If the ruler is not sliding over the big wheel, the very top of the big wheel (where it touches the ruler) has to move at the same speed relative to the ground as the ruler.

When the air based carts are going DDWFTTW, the prop experiences a headwind, but it's not an issue.

What I meant is the perspective of someone sitting on the surface of a propeller blade that faces (skew) forward. If he drills a small hole into the blade the air will flow from back to front trough that hole. So the propeller blade experiences a tailwind.

 

[zoobyshoe]

Except in this case the the torque pressed into service to accelerate the air ends up being converted by the propeller into thrust that is greater than the torque related opposing force from the ground. This works because the propeller interacts with a headwind that is much slower than the ground speed, and although force is higher, speed is slower still, and power output is less than power input.

I find it difficult to believe the prop will be anything but a useless fan in this situation.

The problem as I see it is that, although we still have a speed difference between the air and ground, the only energy available from that difference at this point is that represented by the cart's momentum. This is like a gust: it will only afford a brief period of continued forward motion till it is bankrupted.

The cart can extract energy from the relative speeds of the air and ground regardless of it's own speed, but that does not guarantee that the relative speeds of the ground and air always contain enough energy to do anything the cart wants it to do. Whittled down now to the cart's momentum, I think we may not have enough energy to proceed.

Therefore your claim that thrust greater than the slowing by friction will be generated does not strike me as what will actually happen.

Consider swerdna's set up. We replace the table with an ultra-lightweight table exactly equal in mass to the cart's mass. This can rotate but is not motorized. We lift the cart and set the table rotating (by some separate power source: a rubber wheel chucked into a drill, say.). Once it's up to speed, we remove the power source. Then we set the cart back down. Regardless of the speed we give the table it, and the cart, will come to a stop eventually, because it's running on stored energy being used up by the propeller. The propeller won't start to push the table adding energy to it.

Which is what is happening with the DDWFTTW cart. Repeating my previous example, the wind speed is 10 mph, the cart speed is 14 mph, Now it's encountering slow air and leaving it faster in its wake. The whole cart system is losing energy. And the prop accelerates the air by 6 mph upwind. The air speed at the prop is 14 mph - 6 mph = 8 mph. The prop is slowing down the wind from 10 mph to 8 mph, even though it's moving forwards at 14 mph. This 2 mph reduction in wind speed is what is powering this example cart. The videos we've seen appear to confirm that these DDWFTTW cart work.

It seems like you're stretching to show that somewhere some air was slowed. In a windmill energy is transferred by air slowing at the rotor. At your rotor, air was accelerated. The cart's propeller received no energy from the air; it gave energy to the air. The cart system lost energy (which you acknowledge).

 

[rcgldr]

The top of the big gear has the same speed as the ruler.

OK, the surfaces at the contact patch aren't moving during the time they are in contact (except there's motion perpendicular and compression related movment), but the "top" or contact patch of the big gear is moving.

Prop ... headwind ... What I meant is the perspective of someone sitting on the surface of a propeller blade that faces (skew) forward. If he drills a small hole into the blade the air will flow from back to front trough that hole. So the propeller blade experiences a tailwind.

The hole isn't needed. I'm guessing you're referring to the "upwash" that occurs at the separation point in front of the leading edge of the prop blades, or the tip vortices that include an upwash component.

I still don't get what this has to do with explaining how DDWFTTW carts work (but then again, it's late and I'm tired). When the carts are going DDWFTTW, the prop operates in an apparent headwind, but it's not an issue because the ground speed is faster still. The prop's purpose is to accelerate an apparent headwind faster still, an upwind acceleration of the air.

 

[vanesch]

It seems like you're stretching to show that somewhere some air was slowed. In a windmill energy is transferred by air slowing at the rotor. At your rotor, air was accelerated. The cart's propeller received no energy from the air; it gave energy to the air.

Yes, the propeller GAVE energy to the air *in the frame of the cart*. Now, in that same frame (I pointed out elsewhere that energetic relationships are frame-dependent, so one must not change frames while doing an energy balance: the balance only holds if we do it in one frame ; the balance will hold in any frame, but the different contributions will be different from frame to frame).

But in that same frame's reference, the ground is moving, and the wheels on the ground act like an old-time's water mill, extracting energy from the floor's motion.

So in the cart's frame, the air, which was initially at rest, won energy, and the floor lost some. However, the energy of the floor was replenished by the motor (which, from the cart's perspective, delivers power to the floor). And as the cart gave less power to the air than it took from the floor, it gained kinetic energy which allowed it to accelerate.

Now, from the *floor's perspective*, we have a different balance. Here, the motor is not delivering energy to the floor (which is not moving), but rather to the Earth and the room (which is rotating, powered by the motor). In this frame, the cart is *taking* some energy from the air, which is now moving less rapidly (but this is then compensated again by the motor speeding up the air again by rotating the room and the earth, which make the air blow over the fixed floor). With a part of that energy, the cart is accelerating.
It may seem strange that the motor "powers the wind through rotating the room and the earth", but in fact, what happens is that the air which was rotating with the room, has been slowed down by the cart's propeller, and is now sped up again by the room (which is powered by the motor). This would be similar in a doughnut-shaped wind tunnel with a ventilator. The cart's propeller would slow down the airflow, and the ventilator would have to spend extra power to speed the air up again. Here, we have a funny "wind tunnel" which is a rotating room with air in it, rotated by a motor which is fixed to the "fixed turntable".

In any case, the power comes from the motor.

The cart system lost energy (which you acknowledge).

 

[rcgldr]

The cart can extract energy from the relative speeds of the air and ground regardless of it's own speed, but that does not guarantee that the relative speeds of the ground and air always contain enough energy to do anything the cart wants it to do.

If wind turbines can extract mega-watts of energy from the wind, then it would seem that the potential energy from the wind would be more than enough for these relatively small DDWFTTW carts.

Consider swerdna's set up. We replace the table with an ultra-lightweight table exactly equal in mass to the cart's mass. This can rotate but is not motorized. We lift the cart and set the table rotating (by some separate power source: a rubber wheel chucked into a drill, say.). Once it's up to speed, we remove the power source. Then we set the cart back down. Regardless of the speed we give the table it, and the cart, will come to a stop eventually, because it's running on stored energy being used up by the propeller.

You don't need an ultra-light table. Without the motor driving the table, everyting will eventually come to a stop. However the stated conditions for the DDWFTTW cart is an existing tailwind that doesn't diminsh during the time the cart is in operation. Wind farms can suck hundred's of megawatts of power from the wind, and yet the effect is small compared to the total potential energy of the wind. The winds powersource (rotating earth, sun, ...) is huge and the DDWFTTW carts only require a tiny fraction of this potential energy.

Repeating my previous example, the wind speed is 10 mph, the cart speed is 14 mph, and the prop accelerates the air by 6 mph upwind. The air speed at the prop is 14 mph - 6 mph = 8 mph. The prop is slowing down the wind from 10 mph to 8 mph, even though it's moving forwards at 14 mph. This 2 mph reduction in wind speed is what is powering this example cart. The videos we've seen appear to confirm that these DDWFTTW cart work.

It seems like you're stretching to show that somewhere some air was slowed.

It's not a stretch, the videos are evidence that the carts work, which wouldn't be possible unless the carts slowed down the wind, the same as any wind powered device.

In a windmill energy is transferred by air slowing at the rotor. At your rotor, air was accelerated.

Relative to the prop yes, accelerated upwind. Relative to the ground, the air is decelerated by the prop.

The cart's propeller received no energy from the air; it gave energy to the air.

The wheels are receiving the energy, and they in turn drive the prop, which adds energy to the apparent wind. Kinetic energy includes a velocity factor, so it's sensitive to the frame of reference. From the prop + cart frame of reference the kinetic energy of the air is increased. From the wind's frame of reference the kinetic energy in also increased (speed changes from 0 relative to the wind to a non-zero upwind value). From the ground frame of reference the kinetic energy of the air is decreased.

If you're going to compare the cart to a windmill, you need to use the ground as a frame of reference, and my example situation from above describes the situation (10 mph wind, 14 mph cart, 6 mph upwind acceleration of air, 8 mph net wind at the prop, so the wind was slowed down).

 

[A.T.]

I still don't get what this has to do with explaining how DDWFTTW carts work

I was responding to schroder: "Let’s assume the cart is somehow pushed into a true transition to H-H. What exactly is supposed to keep it there? The ground is stationary, the wind is a headwind against the cart"

The key here is to understand that not all surfaces of the cart experience a headwind. Just like the top of the big gear doesn't experience the ruler as staying behind.

 

[rcgldr]

I still don't get what this has to do with explaining how DDWFTTW carts work.

The key here is to understand that not all surfaces of the cart experience a headwind.

I was referring to apparent headwind (relative to prop and cart here). Almost all surfaces of the cart experience an apparent headwind, and anything aft of the prop experiences even more of an apparent headwind. The point of the cart is to travel DDWFTTW, so an apparent headwind is expected. The point of the prop is to accelerate air upwind, so the apparent headwind is increased (relative to cart and prop).

From the ground frame of reference the only thing not traveling faster than the wind is the air accelerated upwind from the prop, unless you want to consider the tread surface of the wheels which cycle between zero to double the cart speed and back, but average the same speed as the cart. The net effect is all the parts of the cart go DDWFTTW and only the air at the prop is going DDWSTTW (S=slower).

 

[zoobyshoe]

If wind turbines can extract mega-watts of energy from the wind, then it would seem that the potential energy from the wind would be more than enough for these relatively small DDWFTTW carts.

Let's just take this point. The power available from the wind is dependent on its speed^3. The slower the wind, the power available drops off exponentially. At 1 mph a wind probably does not have the energy to start the cart. At 2 mph the wind is 8 times more powerful, and so on. The point is that the idea that the cart can extract energy from the relative motion of surrounding media regardless of its own speed breaks down when the cart wants to do something that requires more energy than either medium can supply. Below a certain wind speed the mega-watt turbines won't budge, despite the relative motion of ground and wind, and there is some speed at which the same turbine can only generate one watt.

 

[zoobyshoe]

Yes, the propeller GAVE energy to the air *in the frame of the cart*. Now, in that same frame (I pointed out elsewhere that energetic relationships are frame-dependent, so one must not change frames while doing an energy balance: the balance only holds if we do it in one frame ; the balance will hold in any frame, but the different contributions will be different from frame to frame).

But in that same frame's reference, the ground is moving, and the wheels on the ground act like an old-time's water mill, extracting energy from the floor's motion.

So in the cart's frame, the air, which was initially at rest, won energy, and the floor lost some. However, the energy of the floor was replenished by the motor (which, from the cart's perspective, delivers power to the floor). And as the cart gave less power to the air than it took from the floor, it gained kinetic energy which allowed it to accelerate.

Now, from the *floor's perspective*, we have a different balance. Here, the motor is not delivering energy to the floor (which is not moving), but rather to the Earth and the room (which is rotating, powered by the motor). In this frame, the cart is *taking* some energy from the air, which is now moving less rapidly (but this is then compensated again by the motor speeding up the air again by rotating the room and the earth, which make the air blow over the fixed floor). With a part of that energy, the cart is accelerating.
It may seem strange that the motor "powers the wind through rotating the room and the earth", but in fact, what happens is that the air which was rotating with the room, has been slowed down by the cart's propeller, and is now sped up again by the room (which is powered by the motor). This would be similar in a doughnut-shaped wind tunnel with a ventilator. The cart's propeller would slow down the airflow, and the ventilator would have to spend extra power to speed the air up again. Here, we have a funny "wind tunnel" which is a rotating room with air in it, rotated by a motor which is fixed to the "fixed turntable".

In any case, the power comes from the motor.

I am pretty sure I agree with this analysis of what is going on with swerdna's set up. (However, once you got to the floor's perspective I was afraid I might be drunk: the room was spinning.)

 

[A.T.]

Jeff, I agree with you. I just wanted to show the analogy to the simple mechanical cart propelled by the ruler:

Ruler: The very top of the big gear is pushed forward by the ruler, despite the fact that the ruler is moving back from the carts perspective.

Air: The rear surfaces of the propeller blades are pushed forward by the air, despite the fact that the air is moving back from the carts perspective.

Yes I know that it's the propeller that pushes back the air, but since action=reaction you can also see it this way to examine the forces: In both cases this forward force is transferred to the ground, where it creates a backward force on the cart. But due to translation via gears the backward force is smaller. So the net force on the cart points forward and accelerates it.

 

[vanesch]

Let's just take this point. The power available from the wind is dependent on its speed^3. The slower the wind, the power available drops off exponentially. At 1 mph a wind probably does not have the energy to start the cart. At 2 mph the wind is 8 times more powerful, and so on. The point is that the idea that the cart can extract energy from the relative motion of surrounding media regardless of its own speed breaks down when the cart wants to do something that requires more energy than either medium can supply. Below a certain wind speed the mega-watt turbines won't budge, despite the relative motion of ground and wind, and there is some speed at which the same turbine can only generate one watt.

The problem is again that your v^3 is dependent on the reference frame in which you are looking. If you are looking in the ground reference frame, then the only "moving medium" is the wind, but there's no problem: the wind is blowing at a steady pace.

Now, when you are looking at the situation from the cart's perspective (when it is exactly at the wind speed), then it is right that the *wind* medium doesn't allow any extraction, but in this reference frame, the floor is moving, and from *that* movement you can extract energy (like you can when you would have a running treadmill and you would put a wheel with a dynamo on it: you can extract power from this moving floor.

The problem is that you are applying your energy balance by using movements as seen in two different reference frames: you say that you cannot extract power from the floor (which is correct in the floor reference frame) AND you say that you cannot extract power from the air (which is correct in the cart's frame). So you say: there's no power source anymore (the floor is not moving, and the air is not moving). But you should do your balance in one and the same frame. It doesn't matter which one (although there will always be one where things are simpler), but once you've chosen it, you have to stick to it.

As pointed out already several times, when you reach the point where the cart is at (or beyond) windspeed, the propeller is DELIVERING power to the wind in the cart's frame, and TAKING that power from the motion of the floor (again, in the cart's frame). That's because it is *accelerating* the wind in its frame.

You get a similar analysis when you look at it from the wind's frame (but the numbers are different).

When looking at is from the floor's frame, the propeller is TAKING power from the wind (because in *this* frame, it is slowing down the wind). As in this frame the floor isn't moving, there' s no power to or from the floor.