Split Thread The validity of classical physics (split from: DWFTTW)

[John Freestone]

Clive, I am not dyslexic. Trust me, I am right. I build models, all the time.

Start again. It is agreed that the cart is propelled by an effective tailwind, a wind moving left to right. So let's agree that the observer, has that wind to his back, so he would be looking to the right.
To make the model complete, moving with the belt must be the equivalent of being stationary in the real world, with the wind to his back.

Realwind:
Wind from the back, with the laminar wind flowing from heel to toe. Like the wind. Correct?
Now, become an imaginary observer, which way does the flow move?
From heel to toe. It passes from heel to toe, like the wind.

Treadmill:
Wind to the back, with laminar wind flowing from heel to toe. Correct?
Now, become an imaginary observer.
Which way does the laminar flow go?
From toe to heel. It passes from heel to toe like the belt (road), not the wind.

Which part do you not agree with?

Two separate "winds" that are only "correct" in two limiting cases. When fixed to the belt, or at "windspeed". At other velocites, the ratio will change.

This distinction is very important.

Ah now I have bothered to read the actual, erm, thought experiment (LOL), I see that humber's mistake is much more simple than I thought. The fronts and backs all appear to be in order. In both examples we are facing the same way, towards the front of the cart, wind at our backs. All he has done YET A-FREAKING-GAIN is to "jump" into being an "imaginary observer", but in the first situation he has remained in the same frame as he did so, and in the treadmill frame he has jumped and boosted without noticing it. If he jumped, but also kept his "imaginary observer" travelling with the belt (as was proposed by him), the wind would still be moving heel to toe. Because the bit at the belt is dragged, it moves SLOWER wrt to his heels and toes than does the upper portions of the air. However, because he has unwittingly shifted his frame of reference with the sneaky slight of hand of "jumping" and "imagining observing" to now being an observer hovering in mid air, in the room, stationary wrt the room, the air accelerated by the belt is moving from toe to heel.

As has been explained to him, he will continue to get wrong answers to these things if he measures different quantities from the same task wrt different frames.

It is entirely possible, however, that humber thinks that if he jumped on a moving train he would suddenly stay still while the train moved backwards past him.

 

[Christian Klippel]

When the observer going back with the belt jumps he keeps moving backwards*. Jumping can't suddenly change his speed from whatever it is to zero** and back to whatever it was as soon as he hits the belt again. His lateral velocity does not change, so he keeps backing up as he goes through his up-and-down motion, and the boundary layer air continues to move heel-to-toe, though at a speed somewhat slower than that on the back of his head.

*I was going to add "you moron", but that would violate the terms of agreement.
**As viewed from someone standing on the floor.

Hello jjcote,

i was about to post the same, but then gave up because he wouldn't understand it either.

Oh, and not to mention that when such a person would indeed instantly "stop" when jumping, uhm, he would suddenly be at the same speed as the air. That would be the equivalent of someone outside with wind, jumping up and then suddenly get to the same speed as the wind. And guess how that windspeed at the ground would be, and what direction ....

Oh well, but he surely won't see that either....

Greetings,

Chris

 

[humber]

When the observer going back with the belt jumps he keeps moving backwards*. Jumping can't suddenly change his speed from whatever it is to zero** and back to whatever it was as soon as he hits the belt again. His lateral velocity does not change, so he keeps backing up as he goes through his up-and-down motion, and the boundary layer air continues to move heel-to-toe, though at a speed somewhat slower than that on the back of his head.

*I was going to add "you moron", but that would violate the terms of agreement.
**As viewed from someone standing on the floor.

Yes, you can jjcote. If you can have an imaginary observer on the belt or by the cart on the treadmill, you can have them in the real world.
I knew this would be a problem, which is why I did not post the full explanation. This is a conceptual barrier.

Please do as I just posted to Clive. #615. Any questions will be addressed, jjcote. This is not trivial, please don't trivialise it.

ETA:
Try again Christian. What have you to lose? Do you think I can fool you?

 

[jjcote]

It is entirely possible, however, that humber thinks that if he jumped on a moving train he would suddenly stay still while the train moved backwards past him.

Yeah, this really is the crux of the matter.
humber, a guy is standing in the middle of a boxcar rolling down the track at a constant speed. He makes a chalk mark between his feet. At the exact moment that he passes a phone pole, he jumps straight up in the air. When he lands, is he still even with the phone pole, or does he land on the chalk mark?

 

[humber]

Yeah, this really is the crux of the matter.
humber, a guy is standing in the middle of a boxcar rolling down the track at a constant speed. He makes a chalk mark between his feet. At the exact moment that he passes a phone pole, he jumps straight up in the air. When he lands, is he still even with the phone pole, or does he land on the chalk mark?

No, now you are accelerating the observer. That will fail too, but do not do so.

Just observe. Then see if you can find a problem in my argument.

All you need do is not worry about being blown by the wind. Just be consistent in the use of the observer.
Do it for both cases; stationary and windspeed, real and treadmill. If you are consistent, you will see a difference.

 

[Clive]

Treadmill:
Wind to the back, with laminar wind flowing from heel to toe. Correct?
Now, become an imaginary observer.
Which way does the laminar flow go?
From toe to heel. It passes from heel to toe like the belt (road), not the wind.

Which part do you not agree with?

You still have this part wrong. It is from heel to toe. I suggest you draw the velocity vectors and subtract that of the person moving backwards ("with the belt") from that of the laminar flow (same direction but of lesser magnitude) to get the velocity of the laminar flow relative to the person on the belt. The result is a vector that points "from heel to toe", opposite to the direction of the belt movement. That might sound wrong, but it's because it is relative to the belt. Relative to the ground that the treadmill is resting on, the airflow is in the other direction, but we're not talking about relative to the ground. We're talking about relative to the foot of the person standing on the belt... but because the person is at rest relative to the belt you might as well just talk about velocity relative to the belt instead.

Two separate "winds" that are only "correct" in two limiting cases. When fixed to the belt, or at "windspeed". At other velocites, the ratio will change.

This distinction is very important.

I don't understand this last part of your post. Perhaps you can recast it?

By the way, I made a mistake earlier when I suggested standing on my head would make difference. It wouldn't! (At least not unless I was horribly twisted.)

 

[CORed]

Yeah, this really is the crux of the matter.
humber, a guy is standing in the middle of a boxcar rolling down the track at a constant speed. He makes a chalk mark between his feet. At the exact moment that he passes a phone pole, he jumps straight up in the air. When he lands, is he still even with the phone pole, or does he land on the chalk mark?

So, does Humber get the wrong answer, ignore the question, or get the right answer?

Regardless, he still won't understand either the treadmill, the balloon drifting with the wind, or the canoe drifting with the current.

 

[jjcote]

humber, a guy is standing in the middle of a boxcar rolling down the track at a constant speed. He makes a chalk mark between his feet. At the exact moment that he passes a phone pole, he jumps straight up in the air. When he lands, is he still even with the phone pole, or does he land on the chalk mark?

No, now you are accelerating the observer. That will fail too, but do not do so.

"No" is not one of the available choices. Phone pole, or chalk mark?

 

[humber]

You still have this part wrong. It is from heel to toe. I suggest you draw the velocity vectors and subtract that of the person moving backwards ("with the belt") from that of the laminar flow (same direction but of lesser magnitude) to get the velocity of the laminar flow relative to the person on the belt.

That is very much the point, Clive. When fixed to the belt, the laminar flow is at least in part, "apparent", it is the effect of being dragged through the still air of the room.
Now, when you are fixed to the belt, and look somewhat to the right of you out of your wake, is the still air of the room. The belt-generated laminar flow, is moving with the belt, with a relative velocity of beltspeed w.r.t that air.
Under the still air.
But not at your feet. There it is effectively zero, the difference between the belt-generated laminar flow, and the "apparent" wind.

Simply decoupling the observer, so as to become imaginary, reveals the component of the "wind", that is actually flowing with the belt.

That is a primary distinction between real wind, and an artificial wind, because one is the product of being dragged through the air and one is a real moving mass. They have the same relative velocities, but are not the same.

The treadmill "wind" is conditional upon friction. No friction to the belt, no "wind", save for the belt-generated laminar flow.
This is not the case for real wind. It does not care about friction to the road. It may influence the outcome, but it does not cause the wind.

Try this:
If when fixed to the belt, you actually "jump", you are magically suspended but a real body. What happens?
You continue in the direction of the belt until your kinetic energy is expended against the air. In the direction of the belt (road).
Do that in the real wind. You are accelerated to some speed less than the wind, (not windspeed!) in the direction of the wind, not the road per se.

This has consequences, that allow the cart to sit on the belt doing nothing.
KE can show that too.

 

[humber]

"No" is not one of the available choices. Phone pole, or chalk mark?

humber, a guy is standing in the middle of a boxcar rolling down the track at a constant speed. He makes a chalk mark between his feet. At the exact moment that he passes a phone pole, he jumps straight up in the air. When he lands, is he still even with the phone pole, or does he land on the chalk mark?
No, now you are accelerating the observer. That will fail too, but do not do so. "No" is not one of the available choices. Phone pole, or chalk mark?
[/QUOTE]

OK, jjcote, but I have a catalogue of your next objections, but if the answer helps.
The answer:
Chalk mark. If quick.

These things are not the same. I keep telling you. These thought experiments may apply, to some other situation, but not to the treadmill.!

 

[spork]

That difference is minor, but in a roundabout way, keeps the cart where it is.

If by "minor" you mean "nonexistent" and by "keeps the car where it is" you mean "has no effect" then I guess we agree.

This is not trivial, please don't trivialise it.

Your thinking is trivial humber. That's why you won't address my assertion that 2 + 2 = 4. Just give it a try. Do you agree with my assertion? Is there any way that you can find common ground with anyone?

So it seems everyone* has come to the same (and correct) understanding about the boundary layer. Perhaps the easiest way to think of it is this. Far from the belt the wind speed is simply the belt-speed (by definition). At the belt the air will have zero velocity relative to the belt. This is true of belt or ground or any surface at all with wind passing over it (or with it passing beneath still air). If there's any confusion just make the belt really long. Plant some tress on the belt. Build houses on it. Mount a camera on the roof of one of those houses and watch the cart from that camera. That becomes your real world - and it's no different from "our real world". This includes the profile of the boundary layer.


*except humber of course


DISCLAIMER: If you want to start talking about the fact that our earth rotates, orbits the sun etc., that's fine. In that case the ground is not truly an inertial frame. We can just as easily make a really bizarre treadmill that duplicates those non-inertial accelerations but it would be unbelievably silly to model such small effects particularly when they don't do us any good or harm.

 

[spork]

The treadmill "wind" is conditional upon friction. No friction to the belt, no "wind", save for the belt-generated laminar flow.
This is not the case for real wind. It does not care about friction to the road. It may influence the outcome, but it does not cause the wind.

Well, there it is folks. That's as close as we can get to 2 + 2 does not equal 4

NASA will be intrigued to hear that gradient and boundary layer don't "care about friction".

This has consequences, that allow the cart to sit on the belt doing nothing.

humber, I absolutely do not believe a word you say about anything you claim to have done. I don't think you've ever been in a canoe, had a job, built a model, or perhaps been outdoors. It seems the one thing you can relate to is "sitting there doing nothing".

 

[Dan O.]

But is this really a conceptual problem for humber or is he faking that too?!

 

[humber]

UO

If by "minor" you mean "nonexistent" and by "keeps the car where it is" you mean "has no effect" then I guess we agree.

I mean something quite different from those.

Your thinking is trivial humber. That's why you won't address my assertion that 2 + 2 = 4. Just give it a try. Do you agree with my assertion? Is there any way that you can find common ground with anyone?

You could try addressing the matter directly. The floor is open.

So it seems everyone* has come to the same (and correct) understanding about the boundary layer.

Let's say I am testing that. Are you a good teacher?

Perhaps the easiest way to think of it is this. Far from the belt the wind speed is simply the belt-speed (by definition). At the belt the air will have zero velocity relative to the belt. This is true of belt or ground or any surface at all with wind passing over it (or with it passing beneath still air).

Yes, by definition.
The bottom most layer, physically stuck to the belt, zero velociy w.r.t the belt/road, yes. But the real road does not carry that layer. The wind blows over it.

Itapproximates the real wind over a road in other ways. One obvious difference is that belt surface is not like tarmac. To be an accurate equivalent, that should be the same, or the friction will not be correctly modeled. The other matters necessary to that, the wheels and gravity, are common to the treadmill and road, so naturally they are accurately modeled.

If there's any confusion just make the belt really long. Plant some tress on the belt. Build houses on it. Mount a camera on the roof of one of those houses and watch the cart from that camera. That becomes your real world - and it's no different from "our real world". This includes the profile of the boundary layer.

Nice image, but off topic

*except humber of course

Of necessity.

DISCLAIMER: If you want to start talking about the fact that our earth rotates, orbits the sun etc., that's fine. In that case the ground is not truly an inertial frame. We can just as easily make a really bizarre treadmill that duplicates those non-inertial accelerations but it would be unbelievably silly to model such small effects particularly when they don't do us any good or harm.

Those, I denied as relevant. They can be excluded. They would be of little consequence in relation to local effects.

 

[CORed]

humber, a guy is standing in the middle of a The answer:
Chalk mark.

Truly amazing. He got it right

If quick.

Oops! If only he knew when to stop. So near, and yet, so far.

 

[jjcote]

The answer:
Chalk mark.

Let's have a round of applause for humber! He got something right. Welcome to our universe!

If quick.

Oh dear, I spoke too soon.
He's in a Hi-roof boxcar. It's not a tiny hop, it's a Michael Jordan quality basket-dunking vertical leap, and he nearly manages to touch the ceiling of the boxcar. Inside height of the boxcar is 13 feet. He's a short guy, and so he stays off the floor for (hang on...) 1.8 seconds (somebody feel free to check my math on that one). We can make a taller custom boxcar if you like, and give him a trampoline. He's still going to land on the chalk mark.

Make a chalk mark on the belt. Jump. Check your wind directions again.

 

[spork]

Are you a good teacher?

Yes I am. But you need a doctor... or a magician.

 

[Subduction Zone]

I am not too sure if spork is a good teacher. He does not suffer fools gladly. Sorry spork, gotta call em as I see em.

 

[humber]

Well, there it is folks. That's as close as we can get to 2 + 2 does not equal 4

NASA will be intrigued to hear that gradient and boundary layer don't "care about friction".

I did not say that. The "wind" that is said to drive the cart, is conditional upon friction to the belt. Real wind can move objects independently of friction to the road. Big difference, right there.
This error is masked by declaring 'whatever does not move with the belt' as being at windspeed. Cart, observer, the room....
The cart balances instantly at windspeed, as I have said, so you never see the intermediary steps that show that error.

humber, I absolutely do not believe a word you say about anything you claim to have done. I don't think you've ever been in a canoe, had a job, built a model, or perhaps been outdoors.

Nonetheless, true.

It seems the one thing you can relate to is "sitting there doing nothing".

OK

 

[humber]

Let's have a round of applause for humber! He got something right. Welcome to our universe!

If quick = No loss of KE.

Now, you were saying?