Down wind faster than the wind

[ynot]

All other arguments aside, the only question being asked is can the propeller develop enough thrust to overcome the rolling resistance between the wheels and surface when the energy for that thrust is only coming from the wheels rolling over the surface.

 

[humber]

Humber...

Yes, Brian-M !
Works both ways, all points. The problem is not with the principle, or my understanding, but with interpretation and its consequences.

I'm going to make one last attempt at explaining the frames of reference thing to you. If you still don't get it after this, I just don't care any more. You're on your own.

Yes, I have appreciated your effort. No problems engaging with you.

There is no such thing as absolute velocity.
There is no such thing as zero velocity either.

Velocity only exists relative to something else.
When we measure velocity, we have to choose what velocity we want to call zero when measuring the velocities of other objects. Normally we choose to call the velocity of our local environment zero, for convenience. On the ground, we use the velocity of the surface of the earth. In a plane, we use the velocity of the plane. In a train, we use the velocity of the train.
<snip>

Agreed. This argument is for the "principle of equivalent relative velocities". I have no problem with that.

Situation A:
A skateboarder is standing stationary to the road on his skateboard. A West-bound car is headed toward him at 60 mph. Because the skateboarder is moving at 0 mph relative to the road, we'll set our frame of reference to 0 mph relative to the road (reference A).

Situation B:
A skateboarder is travelling 60 mph East on his skateboard, heading toward a stationary car. Because the Skateboarder is moving at 60 mph East relative to the road, we'll set our frame of reference to 60 mph East, relative to the road (reference B).

In situation A from reference A, the skateboarder appears to be stationary and a car appears to be travelling West at 60 mph toward him. In situation B from reference B, the skateboarder appears to be stationary and a car appears to be travelling West at 60 mph toward him.

In situation A from reference A, the skateboarder is struck by the car, which imparts exactly enough kinetic energy into him so that he is now travelling West at 60 mph. In situation B from reference B, the skateboarder is struck by the car, which imparts exactly enough kinetic energy into him so that he is now travelling West at 60 mph.

You'll notice that in both situations, using a frame of reference equal to the skateboarder's initial velocity, the exact same thing happens, and the exact same amount of energy is imparted to the skateboarder from the car.

You'll also notice that in situation B, when the skateboarder struck the car, the frame of reference continued to move at the skateboarder's initial velocity. A frame of reference does not change velocity, even if the object it was originally based on does.
Newton's laws of motion are symmetrical. No matter what frame of reference you look at an event, the results are always the same.

Your idea that a car colliding with a person is somehow different from a person colliding with a car, or that a ball colliding with a house is somehow different from a house colliding with a ball, violates the symmetry of these laws.Newton's laws of physics work the same no matter what frame of reference you are using, and identical situations produce identical results.

If a cart is moving downwind at wind speed in a 10 mph Westerly wind, using 10 mph West relative to the ground as your frame of reference: the air is motionless, the cart is motionless and the ground is moving 10 mph East. If a cart is stationary on an indoor treadmill with a belt moving 10 mph East, using 0 mph relative to the ground as your frame of reference: the air is motionless, the cart is motionless and the ground is moving 10 mph East.

The situations are identical and the results will be identical.

This is where we differ.
Of course I agree that Newton's laws apply in both cases, and from the view of relative velocities, A hits B is the same as B hits A, but not any A hitting any B.

A large mass hitting a smaller mass is not the same as a smaller mass hitting a larger one. The difference in momentum alone means that the outcome will be different.

1.With respect to their relative velocities, when a person hits a car, each has an equivalant view.

2.With respect to their relative velocities, when a car hits a person, each has an equivalant view.

But not that (1.) and (2.) are equivalent. They are different events.

This appears to be the problem. The conclusion seems to be 'If two objects have equivalant velocities, they are equivalent in all properties'.

Other poster's opinions;

Mr Seymour Butz,
"The treadmill eliminates 1 variable found in a real world, wind driven situation:"

@gmaster ( you tube),
"To the cart a moving road in still air is *identical* to being on a still road in moving air."
NO! Oversimplification. Torque != Speed. Fluid dynamics of momentum emparted on a vehicle by wind moving against a vehicles surfaces does not equate to momentum emparted by friction between tiers and ground!

@Gunstick ( you tube),
My conclusion is that the "controlled environment" of the threadmill is not equivalent to an outdoor street with wind blowing.
This is a version of "plane on threadmill" where the wheels are fixed to the propeller speed. And in this case the plane won't take off.
The myth is different: rear wind blowing the car faster than the wind itself. In your experiment there is no wind. Take it outside or in a long
windtunnel. Don't use a threadmill.

Roadside observer and driver may claim to have "equivalent relative velocities", but that does not mean that the driver can insist that his
velocity is zero, and the ground is moving beneath at 60mph, because the car would have to be capable of rotating the entire planet.

"Equivalancy" means that the car is bound to the Earth, by the same means and forces, as the Earth is bound to the car. Our participants will
also agree that the Earth is a planet, a car is car, and that a road is not a treadmill.

"Two things may be relative to each other but common to a third, and at the same time"

Practically though, the problem is not about "frames of reference" but of the accuracy and completeness of the model.

Other treadmills:
A dyno does not consider or model the car's kinetic energy. It is recognised that it is excluded from the model (frame?). The only concern is the engine's power, so it is a valid simplification to dissipate the engine's power as heat, while the car is at rest. But it does model a realistic load.

In aerodynamic tests, the vehicle is tied down, so the engine's power and the vehicle's kinetic energy are not modelled. This too is a valid simplification, because the only concern is airflow over the body. The tunnel may not completely emulate wind, and have boundary conditions, but these innacuracies are minimimized or accountable. But it does model air flow over a car. (F1 car designers are after the tiniest improvements, so they may well run their aerodynamic tests on a dyno at the same time.)

This treadmill is quite different, so where was the math, the structure that validates the simplification? However, it really did turn out to be the simple arithmetic subraction of V_wind.!

What exactly does the treadmill demonstrate, then?

The thought experiments;
The treadmill is in a van travelling at windspeed. See, it predicts the correct behaviour.

What would indicate a failure, and how would it be explained?

I have said it N times. The cart on the treadmill is no a model of the real cart, even in principle, because there is no wind, and it cannot be inferred to exist.

The treadmill;
If you are still not convinced, then perhaps the difference is best described as a phylosophical.

But what effect does that have on the treadmill? It causes you to come to the wrong conclusion regarding the working of the cart.

(One of the first questions asked by new posters "Which way do the prop/wheels turn?")

It is not possible to conduct a valid test without kinetic interaction with the wind. A real wind.

I can show you why.

Both you and Myriad, think that I am deceiving you in some way with my force balance idea. Well, no.

How does is move up the belt? Here's one way.

Thrust 2n <-- X --> Drag 12n
\
\
---------------
| Gears |
---------------
0 0
-------------------------------------------------
Belt <---------- 10n (friction)
​

(The wheel/prop forces are locked int opposition, due to the gears.
I divided the prop force into two forces this time; "more drag" alone qualifies, perhaps clearer this way)

The cart is in balance. Now, reduce the friction with the belt. Now the "thrust" component of the propeller's output can push the cart to the right. But if the wheels are not providing power, where does the thrust come from? The prop will continue to turn from momentum ( as seen on the video) and that is more than enough.

The cart moves up, friction is regained and lost momentum restored, until the next time. There is no "next time", because the process is essentially continuous. You can see that the cart is barely in contact with the belt. It's floating on friction, and the laminar flow of the air close to the belt surface. (The bigger one is similar, but its mass makes a difference in how it behaves in detail.)

Because of the relation between friction and available (useful) torque, I would expect the cart would move faster on a steeper slope because the load is greater. If that results in a bigger residual imbalance, then it would go faster. If you want a complete force-by-force analysis, then that would be complex analytical task, and more data would be needed...by someone else.

Perhaps you agree, or think that is OK, because the prop does provide thrust. Yes, but it is limited. The treadmill tries to maintain the driving force at a minimum, obviously (hopefully) not like the real cart.

Do another 'experiment'. Place your finger on the prop shaft to generate a little friction, so as to do some work. Now, the power to meet that work will come from the belt. The forces within the transmission and at the wheels will rise, but all of the power taken from the belt does work only at the finger, so the car should not move. Generally, the only forces within the cart's transmission are due to residual frictions and drag.

Why is this happening? I implied that the only source for the work done on the finger comes from the treadmill, but it shouldn't be; I should be able to get it from the wind, but it just isn't there.

 

[humber]

Understood - completely. I've been through the same thing too many times. I should quit doing this altogether because as everyone can see I now have a hair trigger. I don't mind curiousity, or even ignorance, at all. But I just get ultra fed up with the folks that don't know how it works telling me either that it doesn't work or that I don't know how it works.

Maybe it is the quality of the evidence that it the problem. That's how science works. No gurus. Many questions are raised about what is actually being claimed. There seems to be different opinions as to which way the prop turns. Surely a matter easily settled. If posters "don't understand", then the explanation is inadequate. If you don't like the heat of the kitchen...

I agree that explaining it to people like Humber can have two positive effects. As JB points out, his answers to people like Humber aren't generally intended for Humber. They're intended for the other readers.

Intended only for converts, then.

If I may be so bold... it's pretty bizarre that you're willing to engage humber in a discussion about physics. At the very best he simply doesn't have the capacity to follow a simple line of logic (which he demonstrates with each post) - and at worst, he's simply a troll with one hand on the keyboard and the other on his johnson as he intentionally tries to twist us all around with his truly bizarre "theories".

I applaud your patience and effort, but I suspect your energy would be better spent teaching your dog calculus.

This is "equivalence" at work. I have my hand on my johnson, when I read the posts written by someone thinking with theirs.

 

[spork]

Maybe it is the quality of the evidence that it the problem.

Good point. Physical evidence with videos taken from all angles, posted descriptions of the parts involved and how to build one, along with the offer to send a working model to skeptics is pretty darn flimsy.

There seems to be different opinions as to which way the prop turns.

True. There's your opinion and everyone else's

I have my hand on my johnson, when I read the posts written by someone thinking with theirs.

That's actually pretty clever. But it'd be more effective if I were the idiot.

 

[ThinAirDesigns]

I see you saw your mistake about inverted flight for a conventional plane and have decided to gloss over the fact that you didn't read the question I answered before you jumped in and started spewing about acrobatic planes...... Niiiiice.

I see you still realize that when I quote *you* I am responding to *you* and what *you* say. You continue to want to blame the content of your blanket statements on others.

Also, I think what the OP was about was a plane that could maintain inverted flight. So I think a sailplane, that just trades energy to hold inverted for a short while hardly qualifies.

By your stated logic above, sailplanes can't fly *at all* since in any orientation they are only "trading energy" to hold *any* flight. Upside down or right side up a sailplane only flies until it's kinetic and potential energy relative to the ground is spent

I never said that a plane with lotsa power means it isn't conventional.

Yes you did:

You say that conventional planes can't "knife edge", and we all know that "knife edge" is about "lotsa power" -- therefore it totally follows that conventional planes can't have "lotsa power".

Your words and simple logic.

What you're doing is trying to set up a strawman in order to make it look like you're correct. Tsk tsk....

As everyone (but you likely) can see from the above, no strawman involved -- simple as 1+1=2

A: sailplanes can't fly
B: conventional planes can't have lotsa power.

Very special logic you've got there.

So what other planes can knife edge then?

Any plane with sufficient thrust and the ability to control it can 'knife edge'

Did I say that there were no other planes, other than acros, that can knife edge?

No, what you stated was that conventional planes can't have 'lotsa power' and can't knife edge. Then you tried to say that you didn't say it. False on both counts.

Fighter jets come to mind. And there's your high power/weight. Which prove me right.

Here's a great bit of twisted logic from your own words:

A: Conventional planes can't knife edge.

B: Fighter jets can knife edge.

C: "B proves A is right". ROFLAO!!!

And prove that you again need a strawman to try and make yourself look better.

What's being proven is you need to be more careful what you write as it may come back to haunt you.

JB

 

[humber]

Yay, you made a real version of my animated cart, and suspiciously quickly too. Did you already have one lying around?

Sleight of hand. The average velocity of the hand, must equal the average velocity of the cart. If the operator does not apply force with 100% duty cycle,but in short bursts, that will lead to the given effect.

 

[ThinAirDesigns]

spork:

I agree that explaining it to people like Humber can have two positive effects. As JB points out, his answers to people like Humber aren't generally intended for Humber. They're intended for the other readers.

Humber:

Intended only for converts, then.

They need not be converts, but they do need to be folks who can string together a simple bit of logic without straying.

JB

 

[ThinAirDesigns]

humber:

There seems to be different opinions as to which way the prop turns.

Here is an important point to remember -- our cart agrees with us as to which way it's prop should be turning.

JB

 

[Thabiguy]

A large mass hitting a smaller mass is not the same as a smaller mass hitting a larger one.

...

 

[humber]

Good point. Physical evidence with videos taken from all angles, posted descriptions of the parts involved and how to build one, along with the offer to send a working model to skeptics is pretty darn flimsy.

Why would I need to build one if your evidence is solid? Where is the evidence that the cart actually travels at windspeed? The video in the street does not indicate the speed of the wind or cart.

True. There's your opinion and everyone else's

Check this very thread.

That's actually pretty clever. But it'd be more effective if I were the idiot.

Both true.

 

[humber]

...

You have a problem with that?

 

[humber]

humber:


Here is an important point to remember -- our cart agrees with us as to which way it should be turning.

JB

Well, enlighten us. The treadmill shows clockwise prop, clockwise wheels.
The street video seems to support this. It changes here and there, but when full flight, it seems to be counterclockwise, so the wheels are running backwards at this point?

 

[humber]

spork:


Humber:


They need not be converts, but they do need to be folks who can string together a simple bit of logic without straying.

JB

Repetition of unsupported assertions is not logic, nor evidence. The burden of proof is with you.

 

[spork]

Yes please I would like to see that!

Alrighty, here's your self start video. It's pretty darn unimpressive, but it does exactly as advertised.

http://www.youtube.com/watch?v=QTAd891IpRs&fmt=18

 

[spork]

The burden of proof is with you.

The burden of proof is with us only if we give a crap whether you believe. Guess what...

 

[humber]

The burden of proof is with us only if we give a crap whether you believe. Guess what...

Your response suggests that you give a crap. So proof please.

 

[spork]

Your response suggests that you give a crap. So proof please.

You just wait right there for my custom Humber proof. Then let me know what that suggests.

 

[ThinAirDesigns]

Well, enlighten us. The treadmill shows clockwise prop, clockwise wheels.
The street video seems to support this. It changes here and there, but when full flight, it seems to be counterclockwise, so the wheels are running backwards at this point?

When viewed from the rear with a tailwind, the prop will always turn CW when the wheels are engaged with the rolling surface and moving downwind. This is without exception.

When the wheels are *not* engaged and moving downwind, (in other words, when you lift the cart up and hold it in a tailwind), the prop rotates CCW. This is without exception.

We and the cart both agree on the above. Also, the cart doesn't seem to care if someone other than us feels differently -- it still agrees with us.

JB

 

[humber]

You just wait right there for my custom Humber proof. Then let me know what that suggests.

I may be open suggestion, but not enough to accept yours. I post a critique of your design and reasoning, and that is your response. Thabiguy managed to post an ellipses.

 

[Dan O.]

humber, you seem to still have a problem with simple basic physics.

Lets take the person/vehicle collision and simplify it one step further by taking the ground away.

In case A, An astronaut floating in free space hits a space ship at 25 m/s.
In case B, The space station hits the astronaut at 25 m/s.

What is the difference between these two cases other than the frame of reference?

What difference does it make if there happens to be a surface we call the ground?