Merged DDWFTTW in a “Real” Wind

[CORed]

How so? Quoting a formula does nothing to support your position.

Humber thinks that if he throws in some random forumulas and phsyscisy words, it will make him look smart and nobody will notice he's spouting nonsense. It hasn't worked so far, but humber is an eternal optimist.

 

[jadebox]

Quoting again, you need to make an application. As quoted your claim only applies to a balloon starting below wind speed, not to one towed to a speed higher than wind speed and then released. Also you are making exactly the same "mistake" that you claim students are when it comes to charging a capacitor, in fact the equations are almost identical.

Once again, the big question here is "Will a neutrally buoyant balloon towed above the speed of the wind travel with a velocity greater, equal, or less than the speed of the wind after it is released?"

According to the formula, the difference between the speed of the balloon and the wind gets smaller over time. And the speed at which the difference gets smaller also increases over time. Therefore, the difference quickly approaches zero.

This means that, in the real world, a balloon will quickly reach windspeed after being released.

It will soon be travelling at the same speed as the wind. It won't have any memory of how fast it was moving before being released, so that's irrelevant.

If the balloon were somehow able to continue to travel at less than the wind's speed, it would be like someone swimming upstream. There would need to be an extra source of energy (and an external frame of reference to determine the direction to travel) for that to happen. To the balloon there is no wind, so there is no upwind or downwind.

You could argue about how it'll really be moving a random speed above or below wind speed because of the Brownian motion of the particles of air, or whatever. But, I'd never resort to being pedantic .....

-- Roger

 

[recursive prophet]

Well, forget that Platt isn't an academic, Forget that Camp now openly states that DDWFTTW violates no natural laws. Forget that Allain, Whiteman and Kamman will now no longer say it's impossible but rather avoid the topic like the plague.

Gee, I wonder why?

Apparently, humber is now conceding that the wheels drive the prop, and abandoning his ridiculous assertion that the "headwind" is driving the prop as a turbine. Then again, maybe not. Humber has repeatedly shown himself able to believe two mutually exclusive things at the same time.

The sign of a true artist, that bit I underlined. As to the part I colored can you provide a link to where he made this claim? Thanks. Enjoy reading your comments.

A free-floating balloon, however, has nothing interacting with a medium with relative motion to the air. It will, in short order, be moving at the same velocity as the air it is floating in.

If one wanted to split hairs, the balloon's approach to ws is asymptotic, yes? I recall an unrelated debate at RDF between spork and Asymptotic Freedom about the forces in play with balloons. What I best remember is that they disagreed on some fundamental aspect and that made me wonder about how many disputed elementary facts there are out there in the wide world of physics.

@mender: Actually I think only sleeping dogs lie in Cartville. I guess the question is at what point does cherry-picking morph into lying. Fine line distinctions like this are best ignored imho. Semantics disputes seldom get very far.

 

[humber]

If one wanted to split hairs, the balloon's approach to ws is asymptotic, yes?

Actually, it isn't. That is only a convenient approximation.

I recall an unrelated debate at RDF between spork and Asymptotic Freedom about the forces in play with balloons. What I best remember is that they disagreed on some fundamental aspect and that made me wonder about how many disputed elementary facts there are out there in the wide world of physics.

Yes, AF balked at Spork's super-sonic force transmission internal to a flexible membrane. Even he wouldn't go with that one.
AF tried to apply Stokes' law, which is used when the transit time of the fluid over the object, is small compared to the flow of the fluid itself. That is, very small particles, and not balloons. The problem with that ruse, is that such particles never take a straight path, but a random walk.

 

[humber]

According to the formula, the difference between the speed of the balloon and the wind gets smaller over time. And the speed at which the difference gets smaller also increases over time. Therefore, the difference quickly approaches zero.

The "formula" is a general case. To apply that to balloons, other factors must be added. Friction can never be zero, for without that, the air would simply slide over the balloon, and not apply any force. That friction does not suddenly become zero.

It will soon be travelling at the same speed as the wind. It won't have any memory of how fast it was moving before being released, so that's irrelevant.
If the balloon were somehow able to continue to travel at less than the wind's speed, it would be like someone swimming upstream. There would need to be an extra source of energy (and an external frame of reference to determine the direction to travel) for that to happen. To the balloon there is no wind, so there is no upwind or downwind.

So falling objects never reach terminal velocity? A state of dynamic equilibrium? The simple fact is that weather balloons never reach 100% of WS. They are designed to accelerate rapidly, so have a large area and small mass, allowing them to track the changes in the wind, without a lot of lag.
Or do you have another reason why they are over 1m in diameter and about 20gr?
They are specifically made that way.

 

[Jack by the hedge]

"Will a neutrally buoyant balloon towed above the speed of the wind travel with a velocity greater, equal, or less than the speed of the wind after it is released?"

No, it will fall back to below WS.

I still want to know why humber believes this nonsense. The balloon is not in contact with anything except the air it's suspended in. If it is released while travelling faster than the air, why would it slow down to less than the speed of the air? Consider the moment the balloon was travelling at windspeed: what force would make its velocity continue to change?

 

[Modified]

The "formula" is a general case. To apply that to balloons, other factors must be added. Friction can never be zero, for without that, the air would simply slide over the balloon, and not apply any force. That friction does not suddenly become zero.

It's fun getting back to this, since even someone with no physics knowledge at all can see how insane it is.

Humber, how does the balloon interact with the ground? There must be some interaction, otherwise how does it orient itself to the movement of the Earth? Or does the earth's gravity drag the balloon backward?

In other words, if I release a balloon at wind speed above a thick layer of clouds, how does it "know" which way the earth is moving, so that it can accelerate to below wind speed?

If I release it above a large river, is the wind direction with respect to which it will slow down relative to the earth or the water in the river?

 

[CORed]

That's nice. Rather than face the fact that I do know what all those formulas mean, you choose bar-stool rhetoric. If you think you know more, then you can challenge them directly, or ape Spork and the way he covers for his ignorance.

If a ball is tossed in the air, what is its acceleration at the top of its trajectory? You have previously asserted that it is zero, demonstrating that you have no clue what the formulas mean.

 

[humber]

I still want to know why humber believes this nonsense. The balloon is not in contact with anything except the air it's suspended in. If it is released while travelling faster than the air, why would it slow down to less than the speed of the air? Consider the moment the balloon was travelling at windspeed: what force would make its velocity continue to change?

How did it get there? Meteorologists in the field, use rule-of-thumb calculations. The force on the balloon is calculated according to Rayleigh's drag formula F = 1/2Cd.p.A(V^2). The force on balloon moving downwind is related to the relative speeds of balloon and wind so the force is
F = 1/2.Cd.p.A(Vwind - VBalloon)^2, but that is only half of the equation.

Newton says that F should be opposed by an equal force. That part of the equation is missing. That opposing force is not just due to the acceleration of the balloon, its inertia, F=ma, but drag components as the air flows over the balloon. Contact of the wind, with the balloon slows it, and it builds up in front of the balloon, as well as all sorts of parasitic drag effects. The balloon can never get to the state where (Vwind - VBalloon)^2. Simple as that.

It's vaguely true that at wind speed, in a hot air balloon, there may be no wind on your face, but above your head, is an enormous balloon. The basket hangs below the center of the balloon. The wind that may pass directly by you by in the basket, has to go all the way around the balloon up above and in in the same time. How does that happen if the wind in your face is the same speed as the wind going around the balloon?

 

[temporalillusion]

I'm speechless.

 

[Jack by the hedge]

How did it get there?

Irrelevant.

I presume this diversion is as close as it's possible to get to your announcing that you recognise you were wrong.

 

[humber]

It's fun getting back to this, since even someone with no physics knowledge at all can see how insane it is.

Something with Cd=0, can be dragged by friction?

Humber, how does the balloon interact with the ground? There must be some interaction, otherwise how does it orient itself to the movement of the Earth? Or does the earth's gravity drag the balloon backward?

It's your problem if you don't understand relative motion, and think the ground must be involved, if something is to go backwards relative to the air itself. Funny how you say upwind and downwind, isn't it, but don't see your error.

In other words, if I release a balloon at wind speed above a thick layer of clouds, how does it "know" which way the earth is moving, so that it can accelerate to below wind speed?

Who told you that things "know" or not? That is snake oil..
An object can, and has always been able to move in a medium, without reference to another. Known before Galileo....well before Galileo.

Let's say that medium is the wind. What if I first make a bogus claim to the motion of the Earth, and then later, in a support of a claim, tell you that "objects can't tell the Earth is there", I may be able to sell you some BS about things "not knowing", so that I may claim energy from the ground.

That was the original claim. The cart moved relative to the wind, and then to the ground, so the ground would be able to drive the wheels, and that you know was the claim that one was supposed "to get".

If I release it above a large river, is the wind direction with respect to which it will slow down relative to the earth or the water in the river?

Irrelevant. There is no force between the river and the balloon, so the river can't possibly influence it.

 

[humber]

Irrelevant.

I presume this diversion is as close as it's possible to get to your announcing that you recognise you were wrong.

You find one reference that says balloons travel at WS. No "hot air balloon anecdotes" but a text book. You won't find one, and for the same reason there are no references to tacking props.

 

[humber]

I'm speechless.

If you like, I can give you a formula. Would that help?
With your remark in mind, I doubt that would be fruitful.

Thought about the fact that the shortest path, that would be through the balloon, is no longer available to the wind?
Why airfoils have lift? Isn't the general pressure argument; that the wind must travel further over the top, than underneath? That the air over the wing is faster? What does happen at the basket? The basket is at WS, is the balloon slower relative to the wind? Or is that faster?

 

[Jack by the hedge]

You find one reference that says balloons travel at WS. No "hot air balloon anecdotes" but a text book. You won't find one, and for the same reason there are no references to tacking props.

Still irrelevant. The question was about a balloon released at above windspeed and you said it would slow to below windspeed. Why, please?

 

[humber]

Keep them coming:
No textbook support for:
Objects needing to know or not.
Influence without force
Tacking props
WS Balloons
Magic gears.

 

[Jack by the hedge]

Give us your textbook support for a balloon released at above windspeed slowing to below windspeed. You do have one, don't you?

 

[Jack by the hedge]

Influence without force

A balloon, suspended in a body of air, accelerates in the direction which the ground is moving relative to the air, despite there being no interaction between the air around the balloon and the ground. Please explain.

 

[humber]

Still irrelevant. The question was about a balloon released at above windspeed and you said it would slow to below windspeed. Why, please?

Any book on general fluid dynamics, will contain the theory that says that objects do not travel at the speed of the medium.

You won't find an example of where a text says that they balloons do, because of the above fact. You also won't find one where they don't say that balloons don't, because of the above the first statement.

In short, a text book from may give an example of where a brick falls to the Earth, but that does not mean that because it was not specifically mentioned, that an i-pad, won't.

 

[temporalillusion]

Isn't the general pressure argument; that the wind must travel further over the top, than underneath? That the air over the wing is faster?