Down wind faster than the wind

[John Freestone]

On the treadmill the power to spin the propeller and provide thrust comes from the treadmill motor.

Out in the wind the power to spin the propeller and provide thrust comes from the motion of the cart.

Hi tsig. I know what you're getting at (although I'd have said in the latter case it comes from the wind, but it is so indirectly, via the motion of the cart). It's a weird brainteaser, but Cycrow's question was what forces are different, and the cool thing about equivalent frames is that in these two scenarios (land cart and treadmill cart) the forces when analysed are identical. If the forces are identical, the resulting motions will be identical too.

I know it seems odd, but the more you think about different scenarios, the more you will discover a lot of examples of this principle, with regard to forces and motion. Is gravity holding you to your chair, or holding your chair to you? Nature doesn't make a distinction. The cart is propelled by the difference in velocity between a mass of air and a surface. It doesn't matter which we move past the other. It doesn't matter where the energy comes into the system.

Here's a really good thought experiment to help transcend the frame-stuckness some of us have: imagine that I'm testing my cart in what humber calls the 'real wind', one that blows over the 'stationary ground', and I'm using a rather unlikely wind (to say the least) of 1000 miles per hour. I'm all ready for this event, which happens quite rarely, and only at the equator, and it often blows directly from the east toward the west. Off I go in my land cart, pushed by the wind. This is a proper scenario, I'm sure you and humber will agree (other than being utterly made up, of course). I'm blown west by the wind. Maybe my cart is a bit rubbish, I don't manage to achieve windspeed. Maybe it's just good enough to get to windspeed. Maybe it's well built and beats the wind. Let's imagine it at windspeed.

Now, imagine we look down on the Earth from some distance. The Earth is spinning Eastwards at 1000 mph at the equator (it's actually slightly more, but who's counting?), but the wind is moving 1000 mph westwards [ETA: w.r.t. the land; i.e. not moving relative to our 'fixed' position in space] and the cart is sitting there in place below us. It's moving 1000 mph westward relative to the surface, or staying still w.r.t. the wind.

So, what precisely is the difference between that 'real wind' test and a very large treadmill test? Isn't the ground just a large treadmill moving eastwards in stationary air?

If you don't like 1000 mph winds, because that's impossible, just move the experiment to somewhere nearer the poles, where the earth's surface moves at a reasonable speed. Then instead of just pure fantasy, you could actually create the conditions*. Now tell me what's a treadmill and what's a real wind.

*ETA: I mean, you'd still have to wait for the conditions...but the windspeed would be possible. As you move nearer one of the poles the scenario gradually turns from being like a treadmill to being like ynot's turntable.

 

[CyCrow]

On the treadmill the power to spin the propeller and provide thrust comes from the treadmill motor.

Out in the wind the power to spin the propeller and provide thrust comes from the motion of the cart.

First, power is not a force. Second, how can the cart tell the difference?

There are two relevant reference frames to view the situation. One is stationary wrt the surface, the other to the air. Both are equally valid, and if the cart works in one, it works in the other. But the energy calculations are different. When the earth is moving and the air still, the earth transfers KE and momentum to the air through the cart. Moving air also transfers momentum and KE to the earth. Because the earth is so massive, it does not change rotation speed relative to the air in any detectable way. Likewise, the moving air has much larger mass than the cart, and the average windspeed is not changed significantly.

In both cases, the motion of the cart across the solid surface powers the propeller. The relative motion of the air and surface allow the propeller to generate enough thrust to match or exceed the power required to spin the propeller through the propshaft.

// CyCrow

 

[tsig]

Hi tsig. I know what you're getting at (although I'd have said in the latter case it comes from the wind, but it is so indirectly, via the motion of the cart). It's a weird brainteaser, but Cycrow's question was what forces are different, and the cool thing about equivalent frames is that in these two scenarios (land cart and treadmill cart) the forces when analysed are identical. If the forces are identical, the resulting motions will be identical too.

I know it seems odd, but the more you think about different scenarios, the more you will discover a lot of examples of this principle, with regard to forces and motion. Is gravity holding you to your chair, or holding your chair to you? Nature doesn't make a distinction. The cart is propelled by the difference in velocity between a mass of air and a surface. It doesn't matter which we move past the other. It doesn't matter where the energy comes into the system.

Here's a really good thought experiment to help transcend the frame-stuckness some of us have: imagine that I'm testing my cart in what humber calls the 'real wind', one that blows over the 'stationary ground', and I'm using a rather unlikely wind (to say the least) of 1000 miles per hour. I'm all ready for this event, which happens quite rarely, and only at the equator, and it often blows directly from the east toward the west. Off I go in my land cart, pushed by the wind. This is a proper scenario, I'm sure you and humber will agree (other than being utterly made up, of course). I'm blown west by the wind. Maybe my cart is a bit rubbish, I don't manage to achieve windspeed. Maybe it's just good enough to get to windspeed. Maybe it's well built and beats the wind. Let's imagine it at windspeed.

Now, imagine we look down on the Earth from some distance. The Earth is spinning Eastwards at 1000 mph at the equator (it's actually slightly more, but who's counting?), but the wind is moving 1000 mph westwards [ETA: w.r.t. the land; i.e. not moving relative to our 'fixed' position in space] and the cart is sitting there in place below us. It's moving 1000 mph westward relative to the surface, or staying still w.r.t. the wind.

So, what precisely is the difference between that 'real wind' test and a very large treadmill test? Isn't the ground just a large treadmill moving eastwards in stationary air?

If you don't like 1000 mph winds, because that's impossible, just move the experiment to somewhere nearer the poles, where the earth's surface moves at a reasonable speed. Then instead of just pure fantasy, you could actually create the conditions*. Now tell me what's a treadmill and what's a real wind.

*ETA: I mean, you'd still have to wait for the conditions...but the windspeed would be possible. As you move nearer one of the poles the scenario gradually turns from being like a treadmill to being like ynot's turntable.

We are not looking at different frames of reference but different physical conditions.

"Now tell me what's a treadmill and what's a real wind."

A tread mill is an endless belt device often used for exercise.

Wind is the movement of air from a high to a low pressure.

 

[tsig]

First, power is not a force. Second, how can the cart tell the difference?

There are two relevant reference frames to view the situation. One is stationary wrt the surface, the other to the air. Both are equally valid, and if the cart works in one, it works in the other. But the energy calculations are different. When the earth is moving and the air still, the earth transfers KE and momentum to the air through the cart. Moving air also transfers momentum and KE to the earth. Because the earth is so massive, it does not change rotation speed relative to the air in any detectable way. Likewise, the moving air has much larger mass than the cart, and the average windspeed is not changed significantly.

In both cases, the motion of the cart across the solid surface powers the propeller. The relative motion of the air and surface allow the propeller to generate enough thrust to match or exceed the power required to spin the propeller through the propshaft.

// CyCrow

There are two different situations not different frames of reference

 

[mender]

The cart can't tell the difference between the two situations and doesn't behave any differently as a result. It still wants to move faster than the air around it and gets the energy to do that from the speed of the surface moving underneath the cart turning the wheels that are connected to the prop.

 

[Christian Klippel]

There are two different situations not different frames of reference

Assume a windspeed of 10 mph. Also assume a cart moving on the ground at exactly the same speed and direction as the wind.

1) What is the speed/velocity and the direction of the air relative to the cart?
2) What is the speed/velocity and the direction of the ground relative to the cart?

Edit:
3) What is the speed/velocity and the direction of the ground relative to the air? (or vice versa)

 

[John Freestone]

We are not looking at different frames of reference but different physical conditions.

"Now tell me what's a treadmill and what's a real wind."

A tread mill is an endless belt device often used for exercise.

Wind is the movement of air from a high to a low pressure.

Yes, tsig, of course all of that is true. There would be no need for thought experiments or mathematical proofs that two things were equivalent if they were identical. You are making the mistake of refusing to see the significant sameness and insisting on focusing only on the insignificant differences, I think.

If I had a device that did something interesting or useful for astronauts floating in zero-g, and demonstrated it in the traditional way, in a 'vomit comet' - the flight trajectory that simulates weightlessness temporarily, you could dismiss it by saying that in the simulation we aren't motionless in zero-g, we're experiencing gravity, accelerating due to that force, and just being followed in our freefall by the fuselage of the plane around us, so it doesn't count.

It seems to me that we can fail to understand a correct idea for so long, but at some point we can also choose to refuse to accept it. If you will only accept a land cart as equivalent to a land cart, that's up to you.

I also conjectured earlier that these might not be inertial frames of reference, strictly speaking; I don't know whether they are or not. That term may only apply to different mathematical analyses of the same physical condition, and you are obviously right to say that they are different, or it may be used to include different physical conditions that have certain significant similarities. In the final analysis, this is a semantic distinction: either the treadmill demo is similar in its essential details or it isn't, I'm assured that it is, and I cannot discern any significant error. In the same way, my hypothetical gizmo for astronauts could be demonstrated in freefall to work in all significant details (because, for instance it relies only on the sum forces acting on it, its acceleration, etc., and does not use other things, like air pressure - it is not an altimeter, etc.) - then it is equivalent enough.

Some of the arguments against the treadmill can actually be reversed as ways to support the sloppiness of land tests, BTW. If you insist that wind is caused only by a difference in pressure and does not include still air, it could be argued that real wind also has to be unpredictable in direction and speed, and that the land test, as approximate as it is, is sufficient to demonstrate DDFTTW. Someone objecting that there might have been gusts or variations in direction could be given the reply that unless we have real wind with all its variability, the test isn't real. Constant wind isn't equivalent to real wind. Some might object that we could demonstrate DDFTTW in a perfectly even wind, but that doesn't mean we could do it in a slightly fluctuating one.

Of course, there are those, among whom I am certain humber is one and you may be another, who would use both types of objection, since "debunking" DDFTTW is their only concern.

 

[spork]

There are two different situations not different frames of reference

Once again tsig trots out his patented two word response ("are not") to a carefully thought out analysis. Along those same lines I offer you this tsig:

WRONG

 

[humber]

You should really stop moving the goalposts and thus stop constantly derailing this thread. The question was not if someone could move at the speed of a bullet. The question was what happens to the bullet's KE relative to the person, if a person does move at the same speed & direction. Are you afraid to admit that in such a case the bullet has 0 KE relative to the person?

Cartoon questions have cartoon answers. Even still, the chance to yell 'false' lured some to error. It is irrelevant that KE = 0. True, but trivial.

And no, in that case one would not move to the bullet, but with the bullet. After all, its supposed to be at the same speed and direction. Notice the word "same"? But really, each time you moved yourself into a corner you try to wiggle out by switching the topic. Do you really think that this helps your "credibility", or what is left of it by now?

How do you get there? Your answers do you no credit.

Edit: See, you answered that question from me with:

Let's assume you can do basic math. The formula to calculate the KE has been given more than once, but just for you, here it is a link to it again:
http://hyperphysics.phy-astr.gsu.edu/hbase/ke.html

If you look carefully, you can find out yourself that your statement is absolute nonsense. If it has no velocity relative to me, it also has no kinetic energy relative to me. That is the *v part in the equation. And no matter how much mass you have, multiplied by zero gives zero. Do you really fail to understand that?

Assume what you will. X-X = 0. You need to Wikki that?
And what difference "realtive to you" if 'm' were zero?

 

[humber]

Humber, who employs you as Chief Scientist, by the way, or would posting that put you too much in the, er, line of fire?

Spork claimed that (ROLF), not me.

 

[humber]

Assume a windspeed of 10 mph. Also assume a cart moving on the ground at exactly the same speed and direction as the wind.

1) What is the speed/velocity and the direction of the air relative to the cart?
2) What is the speed/velocity and the direction of the ground relative to the cart?

Edit:
3) What is the speed/velocity and the direction of the ground relative to the air? (or vice versa)

When at the suipermarket, do you see the items on the chechout belt as in a different "frame of reference"?
I hope that you note the Spork has been unable to answer any of the questions that I put him regarding the treadmill. His response is the usual football chant of "wrong".

 

[Christian Klippel]

It is irrelevant that KE = 0. True, but trivial.

So, you agree now that in that case the bullet's KE would be zero relative to the person moving at the same speed and direction?

Assume what you will. X-X = 0. You need to Wikki that?

Wikki? What Wikki? But thanks for showing so clear that you don't even bother to read the links that people give you.

Again, do you agree that there is 0 KE in the bullet in that example?

 

[Modified]

When at the suipermarket, do you see the items on the chechout belt as in a different "frame of reference"?

This seems to be another of your misunderstandings. Things are not "in" or "out of" a frame of reference. Any inertial frame of reference may be used to analyze a physical situation. If two scenarios are identical in any two arbitrarily chosen inertial reference frames, then they are identical.

In the case of the checkout belt, the frame in which the belt moves at a chosen constant speed would be a valid frame. A frame in which the belt moves upward at a chosen constant speed would also be valid. The frame in which the store moves NNE at 41.3 mph would also be valid.

 

[Christian Klippel]

When at the suipermarket, do you see the items on the chechout belt as in a different "frame of reference"?
I hope that you note the Spork has been unable to answer any of the questions that I put him regarding the treadmill. His response is the usual football chant of "wrong".

That's not an answer.

Why should spork answer your gibberish if you constantly fail to answer even simple things, let alone answer them consistently? Do you really think that people will take you serious, or respond to your demands if you don't do either?

 

[Michael C]

Would it be possible to get back to the original topic of this thread? Remember? It had something to do with traveling down wind faster than the wind.

The merits of generally accepted principles of physics, e.g. equivalence of different inertial reference frames, can be debated in threads of their own by people so interested.

Indeed. This thread seems now to be devoted almost entirely to debating basic principles of classical mechanics with (1) somebody who floods the thread with endless arguments to support his total misunderstanding of these basic principles, but is incapable of listening to reason because he simply cannot be wrong, and (2) somebody who doesn't offer any arguments at all, just unfounded statements which I imagine are supposed to be pithy and to the point, but which are in most cases totally miss the point or are just simply wrong.

There are a many interesting people here with whom I'd very much like to continue discussing aspects of DDWFTTW travel (questions of propeller efficiency, possible designs for a larger cart, possible designs for a cart that not only travels DDWFTTW but in any chosen angle to the wind...). Is there a way we could split this thread into two, one for debating frames of reference and the like and one for discussing DDWFTTW travel?

 

[Michael C]

I just saw that Dan posted his message a few minutes before mine. I'm for the split as Dan suggests.

 

[Subduction Zone]

When at the suipermarket, do you see the items on the chechout belt as in a different "frame of reference"?
I hope that you note the Spork has been unable to answer any of the questions that I put him regarding the treadmill. His response is the usual football chant of "wrong".

I don't but the vegetable do. Vegetable (like humber) are not too bright and once the belt starts moving (vegetables also like humber do not have the best memories) they are shocked to see the whole world moving slowly past them.

 

[Christian Klippel]

If the participants in this thread agree to the split, all further discussion of the validity of alternate inertial reference frames, the special nature of an earth based reference, the treatment of Kinetic energy being an absolute property of an object and not relative to the frame of reference and any other claims that invalidate classical physics shall be split to the new thread.
--- cut here ---
Title: The validity of classical physics [split from Down Wind Faster Than The Wind]

Hello Dan,

yes, that would be a good idea. And then, preferably make the split about 40 or so pages ago ....

Greetings,

Chris

 

[Subduction Zone]

One good thing that humber may have done with his constant ranting is to give doubt to tsig. I think he is starting to realize that anyone who spouts so many humberisms cannot possibly be right and as a result he is starting to distance himself from his beliefs. At least tsig seems to be a bit more amenable to education.

 

[Michael C]

If you have questions about props and other details, why not post them, instead of saying that you'd like to but other people are swamping the thread with trivia about physics? You don't have to compete for thread space to discuss those things.

I've already tried that. Such questions quickly get left behind by the flood of lessons in incorrect physics, bickering, justifications and other nonsense. If anybody can suggest a place where I can actually discuss DDWFTTW, I'm going there.