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

[schroder]

Now, what are the elements which exert a force onto the cart in the treadmill experiment ?

The air ?
The surface of the treadmill ?
The ground ?
Big Ben ?

edit: next: what are the equivalent elements which exert the same forces on the cart in the outdoor experiment ?

The air ?
The treadmill surface ?
The ground ?
Big Ben ?

The Only thing that is importasnt here for YOU to finally grasp is that the surface of the tread, although it is moving wrt the cart does not offer any more rolling resistance to the cart than any other flat surface. I have clearly demonstrated in my thought experiment that you can rotate the tread in a complete circle and at no time is it working against the cart.
It offers the exact same resistance to the wheels rolling as the floor does. You are allowing yourself to think that because the tread is moving backwards, it is also dragging the cart backwards with it. It isn't because the prop is working into the air. Once the static resistance is overcome by the prop, the tread offers the same resistance to the cart as the floor. Rolling resistance is velocity independent or at least can be considered as such. I choose the floor as the reference because it is less confusing to use it than the tread. If you use the tread, you must disregard the tread velocity altogether, NOT add it to the velocity of the cart to get a FTTW number. Just forget the tread is moving and calculate with respect to the floor and all this incredible nonsense goes away once and for all!

 

[vanesch]

The Only thing that is importasnt here for YOU to finally grasp is that the surface of the tread, although it is moving wrt the cart does not offer any more rolling resistance to the cart than any other flat surface.

That's evidently wrong. You didn't demonstrate that at all. As the propeller is doing work on the air, it needs to take that power from the wheel, and hence there is a torque on the wheel that is balanced by the force the surface exerts on the wheel (all these considerations in the cart frame of course). If you now add a second force (with the vertical tread), then this is providing the torque, and no extra force is needed anymore by the horizontal surface. The total torque on the wheel must be the same, and given that the vertical treadmill is providing it, the horizontal surface isn't, anymore. This changes the forces on the system. Instead of your vertical treadmill, you could just as well have added a motor on the wheel which provides just that power which the propeller needs. In THAT case, indeed, the wheel would behave as if it were free and not connected to the propeller, and there would only be the very small rolling resistance (which we can neglect here).

I have clearly demonstrated in my thought experiment that you can rotate the tread in a complete circle and at no time is it working against the cart.

You have demonstrated no such thing. You just said it.

It offers the exact same resistance to the wheels rolling as the floor does. You are allowing yourself to think that because the tread is moving backwards, it is also dragging the cart backwards with it.

Of course. Otherwise, what would drive the propeller ? And where would that power for the prop come from ?
If you have a bicycle, and you flip on the dynamo (old-fashioned light on the bicycle), don't you have to pedal a bit harder ? Now, if you are running down a slope and you're not pedaling, doesn't flipping on the dynamo amount to some braking ?

 

[schroder]

That's evidently wrong. You didn't demonstrate that at all. As the propeller is doing work on the air, it needs to take that power from the wheel, and hence there is a torque on the wheel that is balanced by the force the surface exerts on the wheel. If you now add a second force (with the vertical tread), then this is providing the torque, and no extra force is needed anymore by the horizontal surface. The total torque on the wheel must be the same, and given that the vertical treadmill is providing it, the horizontal surface isn't, anymore. This changes the forces on the system.



You have demonstrated no such thing. You just said it.



Of course. Otherwise, what would drive the propeller ? And where would that power for the prop come from ?
If you have a bicycle, and you flip on the dynamo (old-fashioned light on the bicycle), don't you have to pedal a bit harder ? Now, if you are running down a slope and you're not pedaling, doesn't flipping on the dynamo amount to some braking ?

I honestly think you now realize your mistake but you persist. Give it some more thought. Ask opinions of others and listen rather than argue. Have a drink if it helps. I am taking a break.

 

[vanesch]

I honestly think you now realize your mistake but you persist.

You are totally clueless on this, and I don't think you realize (or will ever realize) your multiple mistakes here, so I know you will persist. I know you don't understand a word of what I write, but I do this essentially for the sake of others here who might have their doubts on how this goes.

Nevertheless, I want to get this back on track, and not delve too deeply in your erroneous thought experiments.

If you can answer the question about the cars and the train, that would be nice (post #630). The crux of the demonstration is in fact that if I have two objects, A and B, and in an arbitrary frame, they have velocity vA and vB, then the relative velocity of A wrt B is the velocity A would have in a frame attached to B, is given by the vector subtraction: vA - vB, and this result is the same independently of the frame in which it was calculated.

That contradicts then your claim that there is a "meaningful reference for two cars passing in opposite directions in order to solve for the velocity of either car with respect to the medium the car is driving on".

If I know, in just any frame, 1) the velocity of the car and 2) the velocity of the ground, then their subtraction (algebraically of course) gives you the velocity of the car on the ground. So there is no "meaningful frame" in which to do that, more than any other.

Do you agree with that ?

 

[vanesch]

BTW, just to show the free-body force diagram on the cart in the "normal" case and the case of the thought experiment by schroder, we see that in the normal case, there is a horizontal force balance between the force exerted by the air on the cart (via the propeller) and the force exerted by the horizontal surface (via the wheel). The force by the wheel is determined by the torque needed on the axle of the wheel to drive the propeller.

With the vertical force on the wheel, this torque is provided for, and the only horizontal force that remains (and is not represented) is the small rolling resistance by the wheel on the surface.

The sum of forces in the horizontal direction balances to 0 in the first case (so steady state), while there is a net forward force component in the second case, hence acceleration of the cart.

Note that in the first picture, I didn't add explicitly the force of gravity which is compensated by the vertical binding force exerted by the surface on the wheel, while I did show that force in the second (so the vertical binding force is diminished by F_vertical, because F_vertical + binding force = force of gravity (weight).

 

[schroder]

BTW, just to show the free-body force diagram on the cart in the "normal" case and the case of the thought experiment by schroder, we see that in the normal case, there is a horizontal force balance between the force exerted by the air on the cart (via the propeller) and the force exerted by the horizontal surface (via the wheel). The force by the wheel is determined by the torque needed on the axle of the wheel to drive the propeller.

With the vertical force on the wheel, this torque is provided for, and the only horizontal force that remains (and is not represented) is the small rolling resistance by the wheel on the surface.

The sum of forces in the horizontal direction balances to 0 in the first case (so steady state), while there is a net forward force component in the second case, hence acceleration of the cart.

Note that in the first picture, I didn't add explicitly the force of gravity which is compensated by the vertical binding force exerted by the surface on the wheel, while I did show that force in the second (so the vertical binding force is diminished by F_vertical, because F_vertical + binding force = force of gravity (weight).

That is a helpful analysis; there may be a happy ending here for both of us, but NOT for DDWFTTW. I was saying you could disregard the tread velocity altogether, and use the floor reference only. But you are right in that the cart must be doing some work at the drive wheels to generate the torque. But this cannot be the amount of work to cancel out the motion of the tread altogether. If it were, that by definition would be at least a unity device and the prop pushes it over unity and we KNOW that cannot be happening.
Let’s look at the grab bag numbers I picked before: tread 10 m/sec and cart 2 m/sec with respect to the floor. OK I was saying the cart does NO work on the tread, so we only need to consider the velocity wrt the floor so we have a 2 m/sec cart. That assumption disregards any work being done by the wheels to provide the Force to the propeller and so it is not correct.
You were basically saying the cart is working against the full 10 m/sec and so we can use the tread as the reference and we have a 12 m/sec cart. That is impossibly high, as it is 120% efficiency and it can’t be happening so your assumption is not correct.
I obviously need to take into consideration the work being done by the wheels, convert that to a velocity and add that to the velocity measured wrt the floor and will get a higher number than what I measured.
You would need to subtract that same number from the velocity you calculate wrt the tread.
In any case, I am convinced there is no DDFTTW going on here. The damn thing is just very confusing. We need to isolate the wheel/tread interface and the prop/air interface and take them both into consideration to get the complete solution.
I am taking a rest. I suggest you do the same!

 

[vanesch]

I was saying you could disregard the tread velocity altogether, and use the floor reference only. But you are right in that the cart must be doing some work at the drive wheels to generate the torque. But this cannot be the amount of work to cancel out the motion of the tread altogether.

Stop. An amount of work doesn't cancel a motion. You constantly mix kinematical and dynamical considerations, and as I said many times before, they are distinct.

Kinematics = description of MOTION

Dynamics = description of FORCES

The link between both is through Newton's equation, which links forces to acceleration. From that, one can also derive other techniques, such as energetic considerations. But any state of motion (position/velocity) is compatible a priori with any dynamical state (set of forces).

If it were, that by definition would be at least a unity device and the prop pushes it over unity and we KNOW that cannot be happening.

You go way too fast here. There's no link between the premise and the conclusion. You are jumping to conclusions. Don't. Go 1 step at a time.

Let’s look at the grab bag numbers I picked before: tread 10 m/sec and cart 2 m/sec with respect to the floor. OK I was saying the cart does NO work on the tread, so we only need to consider the velocity wrt the floor so we have a 2 m/sec cart. That assumption disregards any work being done by the wheels to provide the Force to the propeller and so it is not correct.

Right.

You were basically saying the cart is working against the full 10 m/sec and so we can use the tread as the reference and we have a 12 m/sec cart. That is impossibly high, as it is 120% efficiency and it can’t be happening so your assumption is not correct.

Again, you go too fast. You always want to jump to your conclusion, without making a step-by-step argument.

The wheels are exerting a force on the treadmill, and the treadmill is exerting a force on the wheels. As per action = reaction, both forces are equal and opposite. This holds in any (inertial) frame, as forces are invariant under change of inertial frame. So whatever is the force that the wheels of the cart exert on the treadmill, that is that force, and that is independent of what VELOCITY the treadmill has (in no matter what frame). There is a force acting upon the treadmill. And that same (opposite) force is acting upon the wheels. In any frame.

Can you agree with that ? It is elementary mechanics.

I obviously need to take into consideration the work being done by the wheels, convert that to a velocity and add that to the velocity measured wrt the floor and will get a higher number than what I measured.

Mechanical power done on something = the velocity of that something x the force done on it, right ?

So this is dependent on the frame in which you express the velocity. So work is a quantity which is frame-dependent. We know that there is conservation of energy, but in order to do so, we need to express all work done in one and the same frame. No matter what frame. But one and the same for all contributions.

Now, if the cart is exercising, say, 12 Newton on the treadmill, then the treadmill is exercising 12 Newton (in the opposite direction) on the (wheels of) the cart.

Now, in the ground frame, the direction of these 12 Newton is opposite to its velocity (which is 10 m/s), so the treadmill is not gaining, but delivering 12 x 10 = 120 watt (our values are not realistic, but no matter). But again, this power is frame dependent: we did it in the ground frame. We'll stick to it.

Let us look at the cart. We already know that there is a force of 12 Newtons on it dragging it in the sense of the treadmill. If, by assumption, the cart is in steady state, that means that no (horizontal) force is acting net on it, so this means that the only other thing acting upon it, the air, must be pulling 12 Newton in the other direction (in the direction of the motion of the cart).

The work done by the air, however, is 0 (in the ground frame), because the air is not moving, and work = force x displacement.

So no power delivered by the air. 120 Watt delivered by the treadmill. No gain in kinetic energy anywhere (actually, there is, momentaneously, in the air stream behind the propeller, but this will eventually be dissipated).

So net power balance of the system: 120 Watt IN. This is pure dissipation.

Let us do the same thing now from the frame of the cart. In the frame of the cart, the air is coming in at 2 m/s. The treadmill is coming in at a velocity of 12 m/s.

The forces are the same (don't change under a change of reference frame, as long as they are inertial).

So in this frame, the energetic balance is:
on the air, there is a force of 12 N in the sense of the treadmill (because this is the reaction on the fact that the air is pulling on the cart with that force). The air is moving in that same direction at 2 m/s.

So in this frame, there is work done on the air, the air is receiving 2 x 12 = 24 Watt.

On the other hand, in this frame, the treadmill is running at 12 m/s. We already knew that it was undergoing a force in the opposite direction of 12 N. So the treadmill is delivering 144 W in this frame.

Net balance: the treadmill is delivering 144 W, the air is taking 24 Watt. Net goes into the system: 120 Watt. Again, dissipation.

Finally, let us look upon it from the frame of the treadmill. In this frame, the treadmill is of course at rest. The air is coming in at 10 m/s, but in the sense of the cart. The cart is moving at 12 m/s.

Here, the force on the air is 12 N, but in the opposite direction of the motion of the air. So the air is DELIVERING 12 x 10 = 120 W of power. As the treadmill is standing still, the 12 N acting upon it don't do any work. Net balance, again: 120 W in.

It is interesting to return to the frame of the cart, and look upon the energy balance inside the cart.

The wheel is turning and at its outer side which turns at 12 m/s, a force of 12 N is acting, which gives a torque on the axle. It can easily be established that this wheel is receiving 144 W. This power is transmitted through the gearing system to the air, where only 24 W is delivered to do mechanical work on the air which establishes the force of 12 N. All the rest (120 W) is dissipated. It is mainly dissipated by accelerating the air until it settles down (through turbulence), and also in the gears and wheels. The large dissipation comes from my arbitrary choice of 12 N of course.

So we are still a long way away from a potential over-unity device. We dissipate a lot of power here.

 

[ThinAirDesign]

I have ONE BIG FACTOR on my side: Plain old fashioned common sense!

I will see your "old fashioned common sense" and raise you one device which goes DDWFTTW repeatedly and upon demand.

In science, the latter wins over the former every single time.

JB

 

[ThinAirDesign]

I always find it interesting that those who question a treadmill as a valid way to create wind never want to address the following (repost):

If you have a problem with the motor on the threadmill, you're only arguing about what creates the 'wind'.

There are two ways to create 'wind' -- move the air over the surface, or move the surface under the air.

Stand on the deck of an aircraft carrier in the dense fog -- stiff breeze flowing from bow to stern. Fly a kite. Set up a turbine powered generator. Listen to the repeated crack of the flag. Run a small land-yacht around on the deck. Test a DDWFTTW cart. Blow drifting bubbles with the kids. Hell, flood the deck and hold a tiny regatta with tiny sailboats.

Is there wind? Every wind powered device in the world says yes and behaves normally. How can you know?

Is the carrier:

A: sitting still in the ocean with a breeze blowing across it?
B: cruising on it's engines on a calm day?
C: floating engines off and drifting with a strong water current on a calm day?
D: sitting on a rollers with large electric motors winching it across the ground?
E: floating down a man made recirculating river where large electric pumps move the water?
F: inside a foggy wind tunnel where large electric fans create the wind?
G: I'm lying to you -- there is no carrier, just a giant treadmill with a carrier movie set on it.

It's a serious question and I'd love for those who think there is a difference between the street and the treadmill to answer it.

Let's invert that -- same deck, same fog, no wind. Flags don't flap. Bubbles don't drift. Wind turbine doesn't rotate. Is it calm, or is the carrier steaming with the wind? Or is the water drifting with the wind? Or is the wind tunnel turned off? Or is the carrier on rollers and being winched with the wind. You get the idea.

That's the entire point of Galilean relativity, something established without fail for four centuries now -- of two objects moving relative to each other, one can't tell which is 'moving' and which is 'still' without external reference -- reference wind carts and sailboats don't have.

Isn't there a single treadmill doubter willing to take a stab at what's creating the wind for the sailboat regatta described above? Better yet, find the option that *can't* create the wind for the regatta.

Aircraft carriers don't just sitting waiting for the wind to blow to commence deck operations -- no, they steam ahead creating their own relative wind. Do the aircraft taking off know the difference? -- well, neither does any other object on the deck.

Rather than ignore it, someone tell me which option fails to 'power' the sailboat regatta on the flooded deck. Which option won't fly a kite? Which option will not generate electricity through a plane jane wind turbine?

JB

 

[vanesch]

I always find it interesting that those who question a treadmill as a valid way to create wind never want to address the following (repost):

Problem is, I asked a similar question (sailboat on a fast river in a calm day, or sailboat on a lake with wind over it), and I got as an answer that with a GPS, you can discriminate between both, and if you wait long enough, the boat on the river will go down the waterfall on it, while the boat on the lake will end up ashore

 

[zoobyshoe]

...

And now we come to the crux of the business...

...That is what constitutes the demonstration.

Thanks for the excellent explanation. I believe I understood it all, and I don't see anything I have a problem with.

The reason I avoid discussing the motorized tests is that the boost, does, indeed make it all more complex to sort out: there are more things to keep track of and, as you have seen, each new indoor demonstration set up (and torpedo) that is introduced generates more sub-discussions which only act to strain everyone's ability to focus. The videos of outdoor demonstrations are the simplest to grasp and discuss.

There are other problems with demonstrations: the under the ruler video for example is set up such that the cart instantaneously achieves faster than wind speed because it is locked to the wind in a way that no real cart can be. Likewise, as I pointed out at the start, swerdnas table is probably throwing air off at an angle that changes the wind direction or speed or both. None of you cared to check and see if that might be a significant amount of air. (The table is not spinning very fast, but it is large in area.)


Regardless, it's good that you explained the relativity so disputes about this or that element of a demo can be looked at in context.

 

[vanesch]

The reason I avoid discussing the motorized tests is that the boost, does, indeed make it all more complex to sort out: there are more things to keep track of and, as you have seen, each new indoor demonstration set up (and torpedo) that is introduced generates more sub-discussions which only act to strain everyone's ability to focus. The videos of outdoor demonstrations are the simplest to grasp and discuss.

The problem with outdoor demonstrations is that you have no controlled air velocity, which may be variable, in space as well as in time. The advantage of course is that it is the most direct test. The other setups (to me, the most convincing is not the turntable, but rather the threadmill with a slight slope) have at least a cheap way of a controlled and steady (relative) air motion. The next step is a wind tunnel, which is cranking up the experimental gear by a few orders of $$ magnitude. The reason why I find the most convincing test the treadmill with a slight slope is that the the other two (advancing on a treadmill and turntable) have imperfections. Advancing on the treadmill makes for a short demonstration, so steady state is not evident. The turntable has the problem of the not 100% inertial reference frame.
The treadmill with a slope proves that in a strictly inertial situation, at a steady state, when the cart is going exactly at wind speed (and maybe even a bit faster, if air is dragged along with the treadmill), there is a net forward force, which is compensated by the component of the force of gravity parallel to the treadmill. This shows that the cart CAN accelerate beyond wind velocity. Maybe only a tiny bit (this the test doesn't show), but in any case a little bit. Otherwise, it would slowly fall backward/downward if it couldn't compensate for the force of gravity.

There are other problems with demonstrations: the under the ruler video for example is set up such that the cart instantaneously achieves faster than wind speed because it is locked to the wind in a way that no real cart can be. Likewise, as I pointed out at the start, swerdnas table is probably throwing air off at an angle that changes the wind direction or speed or both. None of you cared to check and see if that might be a significant amount of air. (The table is not spinning very fast, but it is large in area.)

The ruler thing is not totally equivalent to the air thing. You could make it more equivalent by dipping the ruler in slippery oil. However, what the ruler thing proves is that there is no over-unity problem in the steady state solution.

Concerning the air on the large table, in fact you are right, but this goes even more in the sense of DWFTTW, in that the air dragged along with the table, makes the apparent head wind for the cart even larger. You DO have a point however with the sidewise air motion, in the sense that the cart is not going to go exactly downwind, but with an angle to it. All that depends of course on the exact amount of the air motion, which can be measured by having a bubble generator fixed on the turntable.

Regardless, it's good that you explained the relativity so disputes about this or that element of a demo can be looked at in context.

That would indeed be nice.

 

[ThinAirDesign]

This seems to be one of the longer threads on this topic in this forum that I have seen. Maybe it has not been locked because the powers that be have recognized the validity of spork and JB's work. At least no one is getting overly excited yet, though shcroder seems a bit trollish to me and he can be a bit provocative.

Subduction, I've not told Spork about this thread -- I'm pretty sure that the combo of him and Schroder would be more than any moderator could take.

I find it entertaining when a Schroder invokes the collective "Academy" and screams "impossible" while it's done every day, all over the world -- Spork finds it insulting.

If the "Academy" only knew how Schroder was taking their name in vain.

JB

 

[vanesch]

Subduction, I've not told Spork about this thread -- I'm pretty sure that the combo of him and Schroder would be more than any moderator could take.

I find it entertaining when a Schroder invokes the collective "Academy" and screams "impossible" while it's done every day, all over the world -- Spork finds it insulting.

If the "Academy" only knew how Schroder was taking their name in vain.

JB

We would like to avoid this...

 

[ThinAirDesign]

The problem with outdoor demonstrations is that you have no controlled air velocity, which may be variable, in space as well as in time. The advantage of course is that it is the most direct test.

Agreed. We do feel that we have a plan that will cover many of the concerns related to an outdoor test. It will take time, a fair amount of money and effort and a bit of luck related to timing the weather.

We are in the design/build process of a ride-along version of the cart with a 15ft prop. Such a large prop will allow us to float an RC hot air balloon (or equivalent) at the same level as the prop (~10ft) and race it. The problem with the small design is you simply can't reliably keep a balloon 6inches off the ground to hold the race. If you allow the balloon to rise above prop level, the gradient nullifies the test. If the balloon touches the ground, it also nullifies for obvious reasons.

We think with our large cart and something short of a mile long course and a 10-15mph mph wind should do the trick:

Parallel to the course set out tethered helium balloon streamers every 100ft or so -- with say 20ft long tethers so that with the angle of the wind the balloons hang out at ~prop level. During filming, this will establish at regular intervals along the course that the wind is indeed blowing downcourse during the entire run.

Place two tethered balloons on long thin 'whips' up and out to the sides of the cart -- this will show relative airflow in the regions near the cart but outside the influence of the prop.

Place a small smoke generator on the cart ahead of the prop. This of course will highlight airflow through the prop.

Upon start, release the RC balloon from perhaps 1/3rd the way down the course and allow the cart to chase directly towards it.

Film from two angles -- a chase vehicle stays even with the cart and from the side films a broad view of the onboard streamers and smoke while the fixed streamers along the course flash by every few seconds. A second fixed camera looks from the start downcourse and shows that the cart is always chasing the floating RC balloon directly -- this will validate the DDW portion of the test.

Of course plot the course of the RC balloon and the cart with onboard GPS and perhaps both cameras show a GPS receiver set to show a timestamp in the corner of the shot -- this will establish that both videos 'reels' were shot simultaneously.

Now by combining the shots into a left/right split screen, you can establish TW (fixed balloons and drifting balloon), relative wind (onboard balloons and smoke) and that the cart is going DDW (chasing drifting balloon).

Over something like a mile course (4 minute vid) if each of the tethered balloons is still tilting downwind as the cart goes by ... if the balloons onboard the cart tilt to the rear continuously (after start period) ... if the smoke through the prop is moving backwards relative to the cart but forwards relative to the course ... If the cart and the drifting balloon stay in alignment relative to the upwind camera ... If the cart catches and then passes the drifting balloon ... THEN, I'd be interested to know what the critics say.

Shroder can always say we faked something, but let's hold for sake of discussion that we didn't fake anything and it's not a HOAX (his word). I'd be interested to hear where folks see the weakness in the described test procedure.

Clearly we must have a day with steady breeze and a low gust factor, but over 4 minutes and a mile of test course, any small gust should be equalized between the cart and the drifting balloon.

If the balloon and cart were released at the same point and the cart barely won, it could arguably be attributed to minor wind differences, but with the balloon given say a 1/4mile or so head start and with the cart passing the balloon on course and arriving at the finish line well ahead (which much to the consternation of "the Academy" is EXACTLY what will happen) it will be hard to argue "gust differential".

Looking for feedback from the skeptics and others alike. Good test?

JB

PS: Here in CA, we have dry lake beds galore which is where we intend to perform said test.

 

[ThinAirDesign]

We would like to avoid this...

You really think we shouldn't tell "the Academy" that Schroder is taking their name in vain?

JB

 

[Subductionzon]

Subduction, I've not told Spork about this thread -- I'm pretty sure that the combo of him and Schroder would be more than any moderator could take.

I find it entertaining when a Schroder invokes the collective "Academy" and screams "impossible" while it's done every day, all over the world -- Spork finds it insulting.

If the "Academy" only knew how Schroder was taking their name in vain.

JB

Agreed, spork can get just a tad, well let's say acerbic when he encounters someone like shroder. Besides he is getting more than enough of this out of humber on the JREF. Vanesch if you want to see someone that will give you hope for schroder check out this thread:http://forums.randi.org/showthread.php?t=131646"

 

[swerdna]

Concerning the air on the large table, in fact you are right, but this goes even more in the sense of DWFTTW, in that the air dragged along with the table, makes the apparent head wind for the cart even larger. You DO have a point however with the sidewise air motion, in the sense that the cart is not going to go exactly downwind, but with an angle to it. All that depends of course on the exact amount of the air motion, which can be measured by having a bubble generator fixed on the turntable.

Here is a short video with bubbles to show air movement while the cart and turntable are operating . . .

http://nz.youtube.com/watch?v=vvrf78dNihk

 

[Subductionzon]

Another nice video swerdna, though I don't think the people who don't understand the frame of reference equivalence will understand what it illustrates. But it does show that on the open road there may be some problems getting enough bubbles to show the motion of the cart relative to the wind. They dropped a bit faster than I hoped they would.

 

[ThinAirDesign]

I present that the primary value of that video is in demonstrating how difficult it is to present a video that demonstrates something of value.

Swerdna will defensively take that as in insult and none intended -- people just don't realize how hard it is to present testing videos that don't raise more questions than they answer.

A good example of the above in our library is our video showing the cart self starting in the wind. For this test we didn't care about DDWFTTW -- we only wanted to show the cart taking off from a standstill in a tailwind. All over the internet we now see comments regarding that video saying "it wasn't faster than the wind -- there were leaves passing it" etc. Of course there were leaves passing it ... it was taking off from a standstill and we couldn't run along side and film it on a busy street with cars coming. It proved one point, but just provided opportunity for more doubt in another.

We should have never released that one as it just adds to the DDWFTTW confusion while only proving a point that isn't central to our claim.

JB

 

[A.T.]

I have ONE BIG FACTOR on my side: Plain old fashioned common sense!

"Common sense is the collection of prejudices acquired by age eighteen."

Albert Einstein

 

[swerdna]

Another nice video swerdna, though I don't think the people who don't understand the frame of reference equivalence will understand what it illustrates. But it does show that on the open road there may be some problems getting enough bubbles to show the motion of the cart relative to the wind. They dropped a bit faster than I hoped they would.

I made the video very quickly a few days ago and intended to make a better version. Unfortunately the cart has been dismantled to use some of the bits for a small outside wind test cart I’m building so I can't make another. I thougt posting it would better than nothing.

ETA - This doesn’t mean that I don’t think the video has a value however. You can see bubbles landing and popping on the spinning turntable surface showing how relatively little wind is created on the surface (also off the edge of the surface). When I upload to Youtube the quality always reduces considerably and perhaps I can see things more clearly on the origional.

 

[zoobyshoe]

The problem with outdoor demonstrations is that you have no controlled air velocity, which may be variable, in space as well as in time. The advantage of course is that it is the most direct test. The other setups (to me, the most convincing is not the turntable, but rather the threadmill with a slight slope) have at least a cheap way of a controlled and steady (relative) air motion. The next step is a wind tunnel, which is cranking up the experimental gear by a few orders of $$ magnitude. The reason why I find the most convincing test the treadmill with a slight slope is that the the other two (advancing on a treadmill and turntable) have imperfections. Advancing on the treadmill makes for a short demonstration, so steady state is not evident. The turntable has the problem of the not 100% inertial reference frame.
The treadmill with a slope proves that in a strictly inertial situation, at a steady state, when the cart is going exactly at wind speed (and maybe even a bit faster, if air is dragged along with the treadmill), there is a net forward force, which is compensated by the component of the force of gravity parallel to the treadmill. This shows that the cart CAN accelerate beyond wind velocity. Maybe only a tiny bit (this the test doesn't show), but in any case a little bit. Otherwise, it would slowly fall backward/downward if it couldn't compensate for the force of gravity.

Yeah, it's clear there are pros and cons to any test.

However, what the ruler thing proves is that there is no over-unity problem in the steady state solution.

Yes, it removes that doubt.

Concerning the air on the large table, in fact you are right, but this goes even more in the sense of DWFTTW, in that the air dragged along with the table, makes the apparent head wind for the cart even larger. You DO have a point however with the sidewise air motion, in the sense that the cart is not going to go exactly downwind, but with an angle to it. All that depends of course on the exact amount of the air motion, which can be measured by having a bubble generator fixed on the turntable.

The reason it occurred to me is that in the 1960's one company was experimenting with tesla turbines as vacuum cleaner rotors. I suppose they didn't turn out to be any better or cheaper, but the point is a spinning disc can potentially throw out a lot of air. They use spinning discs, also, to impell concrete slurry to pump it from one place to another.

 

[Subductionzon]

The problem with a wind tunnel test is that most testing in them is of stationary objects. They do not have a very great length so the cart might not get up to wind speed before it runs out of wind tunnel. The cart on the treadmill test is very similar to a test of a wing in a wind tunnel. A wing is tested in a wind tunnel for its reaction to the wind when it is flying. The cart is tested on the treadmill to test its action on the road with a tailwind at treadmill speed.

 

[schroder]

Again, you go too fast. You always want to jump to your conclusion, without making a step-by-step argument.

The wheels are exerting a force on the treadmill, and the treadmill is exerting a force on the wheels. As per action = reaction, both forces are equal and opposite. This holds in any (inertial) frame, as forces are invariant under change of inertial frame. So whatever is the force that the wheels of the cart exert on the treadmill, that is that force, and that is independent of what VELOCITY the treadmill has (in no matter what frame). There is a force acting upon the treadmill. And that same (opposite) force is acting upon the wheels. In any frame.

Yeah, yeah, my methods are unconventional but the end result can be easily verified using more conventional analysis. That’s what you are here for, I can’t do this all by myself!
I don’t know if you realize it or not, but this problem is solved!
I was not giving any credit to the tread/wheel interface and focusing on the propeller/air interface. This led me to numbers that were too low.
You, on the other hand, were forgetting about the propeller/air interface and assigning too much importance to the tread/wheel interface. This gave you the impossibly high numbers and the faster than the wind solution.
If we use the numbers we were using in the example, 10 m/sec tread, 2 m/sec cart both wrt the floor. I was getting 2 m/sec too low! You were getting 12 m/sec too high!
At the wheel/tread interface, as you and Newton say and I agree, they are working on each other, it is not an all or nothing proposition as we were using (I was giving it nothing while you were giving it all)
So I need to add 5 m/sec to my 2 and the correct velocity is 7 m/sec
You need to subtract 5 m/sec from your 12 and the correct velocity is also 7 m/sec.

When we were looking at Hor/Ver comparisons, the answer was staring us in the face as it is an RMS solution. 10 m/sec x .707 = 7 m/sec
It all fits perfectly. The cart is running at 70% efficiency and will do 7 m/sec down wind in a 10 m/sec wind.
You can do your more conventional analysis with wattage and velocities and I am sure you will find this is correct.
I am happy with this, maybe you will not be because it is definitely not DWWFTTW but it is the correct solution.
All the Thin Air Designs can offer up here is insults and personal attacks, as usual. At least you worked with me to get through this, even if you also threw your share of barbs.
I think this forum has redeemed itself by letting us work this out.
NOW, I am finished with this subject, hopefully forever!

 

[Subductionzon]

It's not that easy schroder, your methods are still wrong. Even though the equivalence of the cart on the treadmill to a cart on the road already going the speed of the wind has been explained countless times you still don't get it. Let's look at the forces on the two carts in the two situations. The cart on the treadmill has a zero wind speed relative to it, the cart on the road going at wind speed has a zero wind speed relative to it. Both the same so far. The wheels of the cart on the treadmill are rotating to that the cart is going 10m/sec relative to the treadmill, the cart on the road has its wheels spinning so that it is going 10m/sec relative to the road. Again, identical. The propeller on the cart which is driven by the treadmill forces the cart down the treadmill so that its wheels are rotating faster than treadmill speed, the propeller on the cart on the road forces the cart forward faster than the wind, again same result. The cart on the treadmill is equivalent to a cart on the road going at wind speed. When the cart advances down the treadmill its wheels are rotating faster than treadmill speed, that much is obvious, it is also the same as the cart moving faster than the wind.

 

[ThinAirDesign]

All the Thin Air Designs can offer up here is insults and personal attacks, as usual.

LOL -- there's two problems with that statement:

1. I offer up an absolute rock solid working version of a DDWFTTW device -- that's got to count for something.

2. You're the one insisting that I'm perpetrating a "HOAX" and "SCAM", even though I offer up a working model that I will demonstrate to anyone who wishes to see it.

And considering the above, you think *I'm* the one throwing insults?

I think this forum has redeemed itself by letting us work this out.

Well, unless you are conceding, the above does leave one little niggling tidbit -- the existence of numerous working models of DDWFTTW

NOW, I am finished with this subject, hopefully forever!

You will never ever be free of this burden Schroder.

Any news on that "Academy" assertion? Anything? Anything at all?

Hey, here's a thought ... perhaps as a member of "the Academy", you would be willing to be the science "expert" standing on the dry lake bed exclaiming "impossible" as the MythBusters films the cart running past the balloon. Yes? If you're interested, I'd be happy to suggest you for the part -- MythBusters is pretty well known for soliciting the experts opinion before those breakthough moments and you'd be absolutely PERFECT.

Seriously. You should think about it -- it's just the opportunity you would love to put the FRAUDS and HOAXTERS in their (our) place.

JB

 

[ThinAirDesign]

You can see bubbles landing and popping on the spinning turntable surface showing how relatively little wind is created on the surface (also off the edge of the surface).

I've always thought that smoke would allow folks to see this effect better than bubbles -- just as someone focuses on a bubble to see where it goes ... it pops. Smoke has a much longer dwell time.

Now I understand that some have concerns regarding the turntable 'slinging' air and perhaps influencing the results, but as far as a straight up treadmill is concerned, any air movement with the belt (wind gradient) simply makes the carts job harder.

JB

 

[rcgldr]

Just to show the free-body force diagram on the cart in the "normal" case and the case of the thought experiment by schroder, we see that in the normal case, there is a horizontal force balance between the force exerted by the air on the cart (via the propeller) and the force exerted by the horizontal surface (via the wheel).

That diagram has the tread related force vector pointing in the wrong direction. Force from the tread to the wheels is backwards opposing the larger forward force from the propeller.

 

[Subductionzon]

A good example of the above in our library is our video showing the cart self starting in the wind.

We should have never released that one as it just adds to the DDWFTTW confusion while only proving a point that isn't central to our claim.

JB

I have to disagree with you here JB. You already showed clearly for those with eyes to see that DDWFTTW works with your cart. The treadmill tests show that at speeds just below or at the wind the cart can accelerate to speeds faster than the wind. The starting from a stop test held outside showed that in a "real" wind the cart would start off from a speed of zero and at least approach the speed of the wind. It also showed it you watched it closely two different ways that the cart could start. With a gentler wind it ran just like it did on a treadmill. When the wind gusted it acted as a turbine at first and then once it got to a certain speed it acted like a proper prop cart (sorry couldn't help myself). People who would have a problem with the cart on a treadmill (like scrorder) would have a problem with any outdoor test, even if you had an anemometer that showed wind speed and a radar that showed cart speed.

 

[rcgldr]

... various long descriptions of the physics ...

... keeps stating that cart speed is not greater than tread speed ...

Cart speed versus tread speed seems to be schroder's issue, until that gets resolved there's no point in discussing the physics.

Once again the claim for a DDWFTTW cart is cart speed greater than wind speed not cart speed greater than -1 x ground speed. There are no claims about the magnitude of cart speed versus the magnitude of ground speed. The claim is:

|v_cart - v_ground| > |v_wind - v_ground|

But you are right in that the cart must be doing some work at the drive wheels to generate the torque. But this cannot be the amount of work to cancel out the motion of the tread altogether.

The relative motion of the tread which is backwards, is increased as the carts forward speed increases. The only time cart speed == tread speed is at the initial start up. The tailwind then propels the cart forwards against the tread, and once there is motion, the cart speed is never equal to the tread speed. Put some signs on the numbers and this will make sense.

Using your floor based reference on a very long treadmill:

Startup states
treadmill speed -10 mph
cart speed -10 mph
wind speed 0 mph
cart speed - tread speed = 0 mph
cart speed - wind speed = -10 mph

cart at -5 mph
treadmill speed -10 mph
cart speed -5 mph
wind speed 0 mph
cart speed - tread speed = +5 mph
cart speed - wind speed = -5 mph

cart at 0 mph
treadmill speed -10 mph
cart speed 0 mph
wind speed 0 mph
cart speed - tread speed = +10 mph
cart speed - wind speed = 0 mph

cart at +2 mph
treadmill speed -10 mph
cart speed +2 mph
wind speed 0 mph
cart speed - tread speed = +12 mph
cart speed - wind speed = +2 mph (faster than the wind)

The tread as a power input. The backwards moving tread causes the driving wheels to rotate forwards. The forwards rotation of the wheels is used to turn the propeller which resists this rotation via an opposing torque. This opposing torque is transferred back to the wheels, and results in a forwards force against the tread. In compliance with Netwond 3rd law, the tread exerts an equal and opposing backwards force against the wheels. The power input from the tread equals the backward force at the tread times the relative speed of the tread (tread speed - cart speed). At start up, cart speed == tread speed and there is no power input from the tread, just power output at the prop acting as a bluff body.

The prop as power output. The torque at the prop is converted to a thrust and speed depending on the characteristics of the prop. For the DDWFTTW carts, the prop is configured to generate a high amount of thrust at a low amount of speed.

Comparason of forces: The thrust from the prop is greater than the opposing force from the tread unless the the relative headwind speed of the cart is too close to the thrust speed of the prop for the prop to be able to generate sufficient thrust. As long as the relative headwind is sufficiently below the prop thrust speed, the thrust from the prop is greater than the opposing force from the tread (or ground). The claim for the DDWFTTW carts is that this can occur even with some apparent headwind (cart speed greater than wind speed). In the case of a relative tailwind (cart speed < wind speed), the force at the prop is a combination of bluff body drag + prop thrust effects.

Comparson of power: The power output from the prop is less than the power input from the treads because of the much lower speed (thrust speed - cart speed) of prop's thrust versus the speed (tread speed - cart speed) at which the treadmill applies a force to the driving wheels. Even though the prop produces more force, it produce less power because of the lower speed.

 

[ThinAirDesign]

I have to disagree with you here JB. You already showed clearly for those with eyes to see that DDWFTTW works with your cart. The treadmill tests show that at speeds just below or at the wind the cart can accelerate to speeds faster than the wind. The starting from a stop test held outside showed that in a "real" wind the cart would start off from a speed of zero and at least approach the speed of the wind. It also showed it you watched it closely two different ways that the cart could start. With a gentler wind it ran just like it did on a treadmill. When the wind gusted it acted as a turbine at first and then once it got to a certain speed it acted like a proper prop cart (sorry couldn't help myself). People who would have a problem with the cart on a treadmill (like scrorder) would have a problem with any outdoor test, even if you had an anemometer that showed wind speed and a radar that showed cart speed.

Thanks Sub. I certainly agree that for those who already 'get it', those self start videos are an interesting demonstration. As you point out, they DO get into some of the subtleties of the device (like which way the prop spins and when) that tend to confuse a lot of people.

I got very, very tired of explaining the physics of those those starts on a second by second basis over and freakin' over. LOL This silly little thing can't just be "the wind blows and away it goes" ... no, it's "first it tries to do this and then it gets overpowered and tries to do that and then after a few seconds it goes into that mode and and and.

JB

 

[vanesch]

At the wheel/tread interface, as you and Newton say and I agree, they are working on each other, it is not an all or nothing proposition as we were using (I was giving it nothing while you were giving it all)
So I need to add 5 m/sec to my 2 and the correct velocity is 7 m/sec
You need to subtract 5 m/sec from your 12 and the correct velocity is also 7 m/sec.

Hahaha! We cut it in half ! The "correct" reference frame in which to compare the cart velocity to windspeed is in the frame midway between cart and treadmill !

I would actually prefer 1/3 versus 2/3, what do you think ?

In fact, no, you were right all along. We have to compare the velocity of the cart in the ground frame of course. The treadmill has nothing to do with it.
Wait, what was the velocity of the cart in the ground frame ? 2 m/s.
What was the velocity of the "wind" in the ground frame ? 0 m/s

Ha, my cart is going faster than the wind here: cart: 2 m/s, wind 0 m/s.

When we were looking at Hor/Ver comparisons, the answer was staring us in the face as it is an RMS solution. 10 m/sec x .707 = 7 m/sec
It all fits perfectly. The cart is running at 70% efficiency and will do 7 m/sec down wind in a 10 m/sec wind.

I will use your logic: you are forgetting that the wind is not doing 10 m/s, but only 5 m/s because we have to measure it half way (the propeller is only acting for half of the force in this setup, the other half comes from the treadmill), so here we have "true" wind velocity of 5 m/s, and an RMS corrected "true" cart velocity of 7 m/s, so it goes 2 m/s faster than the wind

Listen, if the trick is to just spout random mathematical operations until some numbers come up with a random inequality which suits you, I can play that game too !

You can do your more conventional analysis with wattage and velocities and I am sure you will find this is correct.
I am happy with this, maybe you will not be because it is definitely not DWWFTTW but it is the correct solution.

Ok, if you are happy, so am I. We have a many worlds solution to the problem here: we have generated enough meaningless quantities here so that you can pick two numbers from the lot and compare them, and find whatever you want to find, and so can I. This is post-modern mechanics! After all, numbers are relative. They are there to bring us happiness.

Seriously, you seem to suffer from a mental block on this issue. You seem to be able to do some elementary mechanics with cars and trains and so, but you have no trouble spouting any kind of incoherent nonsense reasoning when it touches upon DWFTTW. I give up. (vanesch hands in his black belt in Art of Zen - he failed the ultimate test of "bringing to reason the unreasonable")

EDIT: that said, I would like to ask you a favor. Next time I'm getting a speeding ticket, would you like to argue my case ? I'm sure you will be able to find a way to tell the judge I was not speeding. They simply measured my velocity in the wrong frame... as my wheels push on the road as much as the road is pushing on me, the correct speed of my car is half of what it is in the ground frame.
I never speed twice the speedlimit.

 

[vanesch]

Cart speed versus tread speed seems to be schroder's issue, until that gets resolved there's no point in discussing the physics.

I'm starting to realize that the issue is that for schroder, the starting axiom is that "DWFTTW is impossible" and from this, all else follows. "(a + b n)/n = x, donc Dieu existe".

( http://everything2.com/index.pl?node_id=105498 )

 

[vanesch]

Agreed. We do feel that we have a plan that will cover many of the concerns related to an outdoor test. It will take time, a fair amount of money and effort and a bit of luck related to timing the weather.

...

.

Seriously, why do you take all that trouble ? Is it a kind of hobby or so ?

I'm actually amazed at how much debate this thing can generate. 2 weeks ago I never even gave this DWFTTW any thought, and as I said, if people would have asked me on the street if it were possible to make a device that does it, I would probably even have guessed "no" (as I've done some sailing and had courses on it and all that).

When I saw the issue raised here, it took a few minutes to understand that there's no principle prohibiting it (it took somewhat longer, with the help of Jeff, to get all the forces and so on right). But once the thing is understood, it is a simple application of classical mechanics - even to the point of getting bored about it.

In fact, the treadmill test is, on the mechanics level, more interesting than an outdoor test. It is a very good exercise in classical mechanics, and I would even suggest that it is taken up in a regular curriculum of first year mechanics, because it illustrates many aspects without being intuitively obvious.

But once it is understood, what's the use of spending a lot of effort doing a controlled outdoor test ?