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

[Clive]

Humber, as far as I can see, there are two problems in this diagram.

1. In the "real world" version at the top, you've shown a part of the boundary layer that is moving at 7 m/s (relative to ground). In the treadmill version you've shown a 7 m/s vector in the same direction as the belt. But to compare apples with apples that 7 m/s vector needs to be 3 m/s (which actually makes it 7 m/s relative to the belt), because it "nearer the top of the boundary layer than the bottom" if that makes sense!

2. You've then tried to calculate the velocity of that part of the boundary layer relative to the cart. That's done correctly in the "real world" diagram, but in the treadmill version you have another mistake. To get the velocity of the boundary layer relative to the cart, you need to subtract the cart velocity from the boundary layer velocity. You've done that part wrong. If A and B are vectors (say drawn on paper), then to subtract B from A, you need to reverse the direction of B and then add that to A (adding by connecting the arrow on A to the tail of the reverse B). So to see the direction of the boundary layer on the treadmill relative to the cart you need to subtract the cart vector (0 m/s) from the boundary layer vector (which should really be 3 m/s to the left) and that will give you 3 m/s to the left which is the same as the real world version.

Hope this makes sense (and that I've interpreted your diagram and intentions correctly).

ETA: Belatedly I see now that mender already covered a lot of this (and mentioned it below also). I'm clearly far too slow (and error prone)!

 

[mender]

I take the cart observer's view in each case.
But it's not too difficult to picture this one. If you attach a red smoke-flare to a real cart in real wind, which way is the smoke going to go?

Good. This will be easy then. All I have to do is follow another vehicle that is driving in front of me at 10 mph in a 10 mph tailwind when it is cold out and get a video of how the exhaust from their car moves. I get to see that a lot up here!

Maybe I can make this even easier: a camera crew filming the cart in action at 10 mph in a 10 mph tailwind would be able to hold the camera on a boom just behind the cart as they were driving behind the cart. A red smoke flare would show a cloud of red smoke around the cart, with a layer of red smoke right near the ground (boundary layer) moving back at the camera.

Using the exact same camera position to get the cart's "eye view" (right behind the cart) on the treadmill, using the fog machine (steady blue light as per your preference) will show a cloud of fog around the cart, with a layer of blue fog right near the treadmill belt moving back at the camera.

Same for both cases. I'll still get the fog machine because I want to see what the air flow is like around the propeller at speed.

ETA: I did some editing on my other post with comments about your diagram. I just read Clive's comments which point out essentially the same errors as I did, though in a slightly different way. Probably so you can tell us apart.

 

[spork]

Don't bark, explain. Teacher.

Well, I've explained myself to this brick wall plenty of times. And I'm even willing to explain again. But I can't have an exchange with you when you speak in partial sentences that are completely unresponsive. If you care to describe what you're trying to show us in your diagram, I'll be happy to tell you exactly where you're right and where you're wrong. But frankly, I don't see that happening, because it's predicated on you becoming able to communicate all of a sudden.

I fear for your students.

Perhaps I would too if I actually had any students. In your world have you learned something about me that I don't know?

 

[humber]

I don't recall whether we've talked about it on this forum, but that uses several components of arguments we've used. In particular, we talk about a boat tacking upwind into a relative wind when it's on a river on a day with no wind. This tells us the boat can go up-current, faster than the current.

meaning;
I cited Dr Drela in support, but ignored his calculations or reasoning.

But I realize your scenario is a bit different. Certainly, people have described exactly the craft you envision, but I've not seen them put it on a river on a windless day (although I'm sure we can agree that's identical to putting it on calm water in a tailwind). My feelings are:

1) Yes, it works just fine in theory. If you can get prop, hull, turbine of sufficiently high performance, it would work fine in practice.

Meaning;
Of course, there are two independent power sources, but your "intuitions" should tell you that if you remove one of them, you should immediately appreciate that it makes no difference.

2) It's hard to imagine that would bring any sceptics over; but I'm becoming hardened to the "thought processes" of sceptics.

Meaning:
They are always asking for evidence and reason.

3) Completely contrary to my point #2, this is the first time I've ever seen humber agree with something that's actually true - despite the fact that his reasoning is completely flawed.

Meaning:
◊◊◊◊, humber has found something I can't deny. I am willing to contradict myself in order to suggest that he is an idiot.

4) It seems there are many approaches to explaining this thing and convincing people. Yours is sound. If it changes minds, it's good.

Meaning;
Consider the lily. If you don't buy my ice-cart analogies, then there is little else I can offer.

 

[humber]

Well, I've explained myself to this brick wall plenty of times. And I'm even willing to explain again. But I can't have an exchange with you when you speak in partial sentences that are completely unresponsive. If you care to describe what you're trying to show us in your diagram, I'll be happy to tell you exactly where you're right and where you're wrong. But frankly, I don't see that happening, because it's predicated on you becoming able to communicate all of a sudden.

To a real brick wall perhaps.
OK, but you do now have the diagrams, right?
There is also a written description I posted to Clive.
Plenty to work with.

Perhaps I would too if I actually had any students. In your world have you learned something about me that I don't know?

Many times you have referred to your teaching skills. So, which way is it today? Oddly enough, I am not a teacher.

 

[spork]

Meaning:
◊◊◊◊, humber has found something I can't deny. I am willing to contradict myself in order to suggest that he is an idiot.

You don't even understand your own meaning humber, and you sure as hell don't understand mine.

Again, I challenge you to find anyone, anyWHERE, that will back you up. It would seem surprising that you couldn't find a single sole anywhere that would be able to follow your immaculate logic and in-depth knowledge of physics. It must be very lonely to be the only one in the entire world that understands a subject such as this. And it must be terribly frustrating when so many out there THINK they understand it, but actually have it all wrong.

 

[humber]

Good. This will be easy then. All I have to do is follow another vehicle that is driving in front of me at 10 mph in a 10 mph tailwind when it is cold out and get a video of how the exhaust from their car moves. I get to see that a lot up here!

That would not be the case I posed. I can work that up later, perhaps.

Maybe I can make this even easier: a camera crew filming the cart in action at 10 mph in a 10 mph tailwind would be able to hold the camera on a boom just behind the cart as they were driving behind the cart. A red smoke flare would show a cloud of red smoke around the cart, with a layer of red smoke right near the ground (boundary layer) moving back at the camera.

Cameras are a surrogate observer, and pander to our senses on the matter. That is why they work. They do not measure velocity.

Using the exact same camera position to get the cart's "eye view" (right behind the cart) on the treadmill, using the fog machine (steady blue light as per your preference) will show a cloud of fog around the cart, with a layer of blue fog right near the treadmill belt moving back at the camera.

Too figurative. I need to know how you see it before I can make head nor tail of it Diagram please. Mender.

Same for both cases. I'll still get the fog machine because I want to see what the air flow is like around the propeller at speed.

ETA: I did some editing on my other post with comments about your diagram. I just read Clive's comments which point out essentially the same errors as I did, though in a slightly different way. Probably so you can tell us apart.

OK

 

[Clive]

There is also a written description I posted to Clive.

Just for the record: you seem to have referred to mender as "Clive" a bit earlier, see #1378 (more or less also implying some of his words were mine, not that I'm really complaining), and then again mentioned my name when I think you meant mender again!

 

[spork]

They do not measure velocity.

I wish you could realize there is no instrument or method on earth for measuring absolute velocity (and of course this is because there's no such thing as absolute velocity). The only velocity we can measure is velocity relative to something else - because that's what velocity IS.

 

[mender]

Cameras are a surrogate observer, and pander to our senses on the matter. That is why they work. They do not measure velocity.

Replace "cameras" with eyes; in fact that is exactly what a camera attempts to do!

I can't lend you my eyes; the best I can do is record what either of us would see if we were right behind the cart watching what was happening in both cases. It's up to you to see what is happening.

Are you saying a video wouldn't constitute sufficient evidence?

 

[Brian-M]

Or a stationary cart, in still air, with a belt passing under its wheels. Which is it?

Like much of what you say, this question appears to be a non-sequiter. Could you rephrase the question with greater detail? I'm not sure what you're asking here.

That is not quite correct. The real person (you) standing on the belt, has those experiences, because you are moving on a beltat a velocity w.r.t those objects. One of those objects is the cart.
It's a stretch to equate that with a cart moving w.r.t aroad.

If you're standing on the belt (being moved backwards by the belt) then from your perspective, the cart is racing away downwind from you, just like you'd expect to see the cart race away downwind from you on a real road. It doesn't seem like any kind of stretch to me. Exact equivalence is the term I'd use to describe the situation.

That is information Brian-M. The light reflected off the cart and so interpreted by your mind, leads to that conclusion.

No. Assuming the cart is stationary to you, direct perception of the cart leads to the impression that the cart isn't actually moving at all. It's only when you take into acount the speed of the belt that it can be logically deduced that the cart must be moving rapidly along the belt (relative to the belt) in order to maintain that "stationary" position.

It may be correct, but is so in fact?

Since I feel I've already covered this question above, I'd like to bring up another point about your posts.

At first glance, your question seems to be asking "It may be true, but is it true?" It took me a minute or so to understand what you were actually trying to ask.

Why not simply ask: "But is that what is really happening"? This would have made it much easier to understand.

Twisting the language into tortured knots only leads to confusion and misunderstanding. I'm sure you don't do it intentionally, but you do do this frequently. A great deal of misunderstanding could be avoided if you phrased your posts more clearly.

Where are the expected accompanying physical phenomenon, that support that claim? I find they are missing, or inconsistent.

Here is another example of the confusing ambiguity of your posts. I have no idea as to what physical phenonomenom you are referring to. If you had taken the effort to add, "(Such as ..." at the end of your question along with a few examples, I might actually be able to attempt to answer your question.


On to an irrelevent side note...

Inductance needs no voltage to sustain a current.

Actually, the inductor does need voltage to sustain a current, as current cannot exist without voltage (except in a superconductor). It generates it's own voltage as it's magnetic field collapses. If the circuit is open circuit, the inductor won't produce any current, just a very high voltage.

I know, I know. What you meant to say was that an inducuctor can sustain current (briefly) without an external voltage source. I'm just feeling in a pedantic mood at the moment.

 

[mender]

Evidence always does that.

Will it in this case though?

 

[spork]

I can't lend you my eyes...

Of course what he needs is your brain. But I shudder to think of the condition it would be returned in. He clearly has never used one.

 

[Brian-M]

(3) In the real wind case, the boundary layer still moves with the wind, but slower. Dropping and object from the cart in the same way as above, will see that object, recede into the past, and therefore lend the impressionof going backwards, (the apparent reversal), though it is not.

You're forgetting that while the belt may by pulling the in the boundry layer in the same direction as the belt, the boundary layer is still moving slower than the belt. As someone standing on the belt will be moving backwards faster than the boundary layer, from their perspective the boundary layer will be moving forward, in the same direction as the wind, just slower.

For someone standing on the belt, the boundary layer appears to be moving in the same direction as the wind.

For someone standing on the road, the boundary layer appears to be moving in the same direction as the wind.

There is no difference.

If you attach a red smoke-flare to a real cart in real wind, which way is the smoke going to go?

Try drawing a dotted diagonal line on a piece of paper, and cover the line with another piece of paper. Now slowly slide the second piece of paper off. If you watch the line as a whole, this creates an illusion[/i] of movement. If you watch the individual dots, you can see there is no actual movement.

To avoid confusion caused by this illusion, let's have the smoke flare produce the smoke in distinct seperate puffs, and have the observers only watch individual puffs of smoke.

To someone travelling beside the cart at cart speed (possibly on a bicycle), the smoke would appear to move backward.

To someone travelling at wind speed (possibly in a hot-air balloon) the smoke would appear to move straight up.

To someone stationary on the road (possibly in a deck-chair) the smoke would appear to move forward, in the same direction as the cart.

Now here is the trick question: Which perspective is the correct one?

Answer: They all are!

The smoke is moving in different directions relative to the different observers.

Now let's try the treadmill...

When the cart is moving along the treadmill, and you're standing next to the treadmill you are at wind speed. Like the people in the hot air balloon you will see each puff of smoke rising straight up because, from your perspective, there is no wind.

But if I was standing on the treadmill belt itself, behind the cart, the smoke would appear to be chasing after the cart, because from my perspective a wind is blowing in the same direction as the cart.

 

[humber]

Humber, as far as I can see, there are two problems in this diagram.

1. In the "real world" version at the top, you've shown a part of the boundary layer that is moving at 7 m/s (relative to ground).

Yes, it moves left to right at 7m/s.
3m/s slower than the driving wind.

2. You've then tried to calculate the velocity of that part of the boundary layer relative to the cart. That's done correctly in the "real word" diagram, but in the treadmill version you have another mistake.

Good, you agree the real wind is correct.

In the treadmill version you've shown a 7 m/s vector in the same direction as the belt. But to compare apples with apples that 7 m/s vector needs to be 3 m/s (which actually makes it 7 m/s relative to the belt), because it "nearer the top of the boundary layer than the bottom" if that makes sense!

I prefer treadmill carts and oranges, but...
I think see what you are doing here, Clive. You are assuming that the treadmill is already like the real thing and adapting the treadmill to that, in order to make the 3m/s vector in the way just did above. That is not what actually happens on the treadmill. That is what I am trying to point out.

It's double-dipping again! The cart is given "windspeed" status, allowing it to be said that there is no relative velocity between wind and cart. Correct?
Therefore, is not possible to get more or less relative velocity between that cart and belt at windspeed, because it is fixed at belt-speed. It is a "bubble" going at exactly windspeed, with the wind. If that is so, then the boundary layer would simply follow that, but 3m/s slower. This is not the case. There are two possible interpretations at windspeed on the treadmill

(1) The boundary layers are similarity "frozen", so there will be a static boundary layer w.r.t the cart too. So no effective flow.

(2) The driving wind and cart are as one, but moving relative to the boundary wind, in which case, the latter will simply lag behind the cart a bit, so from the perspective of the carteer, it is like it moving backwards at 3m/s.

However, in the example case I offered, and after one second, the object will be actually 7m behind the cart not the apparent 3m or the real wind case. It is nearly an object stuck too the road, where as the real flow moves forward over the road. Big difference.

You expect the road surface to move 10m in that same time, so it must be true of the boundary, but at different rates according to the profile of that flow. It "works" for the limiting cases of beltspeed and windspeed, but not for others.

Also, there cannot be " multiple winds".
(1) "Wind" for someone moving back with the belt
(2) "Boundary Wind" for the same
(3) "Still wind" just behind the cart
(4) "Boundary Wind" for the cart.

 

[humber]

Will it in this case though?

This case is certainly part of that set, Mender.

 

[humber]

Just for the record: you seem to have referred to mender as "Clive" a bit earlier, see #1378 (more or less also implying some of his words were mine, not that I'm really complaining), and then again mentioned my name when I think you meant mender again!

Yes. Sorry, about that Mender, Clive

 

[humber]

I wish you could realize there is no instrument or method on earth for measuring absolute velocity (and of course this is because there's no such thing as absolute velocity). The only velocity we can measure is velocity relative to something else - because that's what velocity IS.

Heard the news? Speed of light is absolute.
An inherent value of differential measurement, is that you need no absolute.

 

[Brian-M]

Humber, let's get beck to basics. After all these posts, you still haven't presented a clear argument (at least, not clear to us) as to why the treadmill-with-no-wind and road-with-wind situations aren't the same.

Currently, you're enthusiastically advancing your boundary layer idea, which everyone but you percieves as clearly mistaken. The strange part is, even if you are right, the effects of this on the cart would be small enough not to invalidate the treadmill model.

I'd like to know exactly where our views differ, and how. In order to achieve this, I'm going to set up a hypothetical situation, and gradually alter it step by step, explaining the expected outcome as I go.

What I want you to do is point out exactly where you disagree with my expected outcome, and tell me what you think would happen in that instance, and why.


Stage 1: Run the cart in a wind-tunnel

You've already stated on several occasions that a wind-tunnel would be the same, so I expect you'll agree that the cart will behave the same in the wind-tunnel as it would on the road in the wind.


Stage 2: Relocate wind-tunnel

I don't see that it matters how high or how low the wind tunnel is, or in which directon it is facing, as long as it remains level. In this case, I'd like put the wind-tunnel about 10 or 15 feet in the air, by strapping it to the top of a semi-trailer truck, with the end that holds the fan that blows the air directly above the driver's cabin.

The cart should perform exactly the same if the wind-tunnel is strapped to the top of a stationary truck.


Stage 3: Drive the truck around

We drive the truck while running the cart in the wind tunnel. The truck has got really good suspension, so there's no vibration. It's on a perfectly level and straight highway with a computer-controlled cruise-control keeping the truck at a constant, unchanging velocity (ie. no acceleration or decelleration.)

The cart should perform exactly the same while the truck is moving as when it's stationary, as long as there is no acceleration from the truck while the cart is being tested.


Stage 4: Syncronise velocities

If the air in the wind tunnel is blowing at 10mph. Just for fun, let's drive the truck at 10mph too. As the velocity of the truck isn't affecting the experiment, this shouldn't make any difference either.


Stage 5: Open the wind-tunnel

Wait a sec... there's no wind today. So, if the truck is moving forward at 10mph and the fan in the truck is pushing the air backward at 10mph, then the air in the wind-tunnel is moving at the same speed as the air outside!

So let's get rid of that fan, and open up the front and back of the wind-tunnel to the outside air.

As far as the cart is concerned, nothing has changed from stage 4. The truck is moving at the same speed as it was before, and the air is still blowing through the wind-tunnel at 10mph.


Stage 6: Comparison

The cart is now on a moving surface in still air, just like the treadmill.
The situation it is now in is identical to having it on the treadmill.
The cart on the treadmill should perform the same as the cart in the open-ended wind-tunnel, which in turn performed the same as as the cart in the close-ended wind tunnel, which in turn performed the same as the cart on the ground in a "real" wind.

There is no difference.


Stage 7: Humber replies

This is the point where you, Humber, tell me at exactly which stage you think I went wrong, and give me a clear, detailed explaination as to why.

 

[Brian-M]

It's double-dipping again! The cart is given "windspeed" status, allowing it to be said that there is no relative velocity between wind and cart. Correct?
Therefore, is not possible to get more or less relative velocity between that cart and belt at windspeed, because it is fixed at belt-speed. It is a "bubble" going at exactly windspeed, with the wind. If that is so, then the boundary layer would simply follow that, but 3m/s slower. This is not the case. There are two possible interpretations at windspeed on the treadmill

Wait, Humber...

Do you think we're just arbitrarily assigning wind-speed equivalent to the cart, and therefore it must remain fixed at that speed?

The cart is only at wind speed because it's been held stationary to the air (and stationary relative to the air is windspeed), but that's only the starting point. A little like giving the cart on the road a push, and letting it go at wind-speed.

The point is to find out what the cart does next.

As you can clearly see in Sporks videos... the ones where the belt is level... the cart's velocity changes soon after he lets it go, and so it is no longer at windspeed any more.

Your statement: "Therefore, is not possible to get more or less relative velocity between that cart and belt at windspeed, because it is fixed at belt-speed." is clearly wrong.