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

[recursive prophet]

Yo humbert: how’s tricks? I try and avoid interrupting the ‘adult conversations’ in this thread, but I doubt if anyone follows it more closely than I. It has become my all time favorite. And now it seems you’re attracting some disciples! Keep it going, and start training the Magister Ludi that will follow. I prophesize that in 2048 humbert.4.0 will be the primary source for physics references. Dan’s list with 1,024 disputed claims will remain unresolved. He will need a ‘walker’ for his mouse to keep up.


Very difficult for me to keep in any kind of focus all that’s been covered in this debate, but I’ll keep going back and working on it. So many questions I’d like to ask you-and others-but don’t want to be the guy holding up the class cause he didn’t do his homework. Actually I have, but at this juncture it’s like following a dialog in a language you haven’t spoken in years. You can barely follow, let alone make coherent contributions.

I’m happy to see most of the earlier hostility seems to have substantially abated, and an interesting jocular rapport has developed between the antagonists. The humor is much more enjoyable minus the tension that would make me worry about the thread being locked. Kudos to all the regulars here-still the best show in town.

I'm thinking about putting together links to all the valid points others have conceded that humbert made, along with questions posed by him that no one answered. Has anyone else noticed a slight shift in the tilt of the 'learning curve treadmill' we are on here lately?

It is my impression, as I've mentioned, that humbert is actually quite aware of the many laws of physics. He knows where the bodies are buried. All the little QM/maths cul de sacs that lead to an unproven hypothesis or two and much uncertainty. I sense he also has some informed insights that he is slowly introducing here. Of course I could be way off, and perhaps he just enjoys challenging the self assured in a universe with so many unknowns. Either way seems like a worthy cause to a beta- like me. Carry on.

 

[Michael C]

Can you point me to an example of a "more stringent definition" on the web Michael. One that you would say is up to scratch. Or failing that, perhaps give me a more stringent definition in your own words because I'm not really sure I understand how you think a frame of reference is adding much unless you are always seeing "weight" as closer to what I called "apparent weight". In other words, basically what a set of scales would measure.

Weight is the mass of an object multiplied by the local gravitational acceleration measured in the chosen frame of reference. Since the gravitational acceleration is a vector quantity, weight is also a vector quantity.

You can find this definition, together with a discussion of the why and wherefore, on this page.

With regard to your questions about "weight" at different stages in a journey from Earth to moon, if I was answering that I would qualify it by saying something like the force from the combined gravitational effects of the Earth and the moon at point X is W newtons in such and such a direction. I would suggest "weight" by itself is always going to be open to misinterpretation unless the context is (as is usual) very clearly a scenario close to the earth or some other large and probably singular gravitational source (mass).

This is where your definition of "real weight" gets woolly. At what point above the earth does the problem of "misinterpretation" arise? If we specify the frame of reference, there is no ambiguity. If I choose the frame of reference defined by the spaceship, my weight is zero at any point along the voyage: using this frame of reference, I don't just feel "weightless", I really have no weight. At any point on the voyage I could theoretically choose a reference frame that makes my weight any value that takes my fancy, but it's obvious that some reference frames are more useful than others.

In any case, I'm not sure that "weight" is a particularly useful concept in physics as we're probably more interested in the total forces acting on a body from all sources and also understanding the separate components. I've nearly always understood discussions about "weight" as mainly being about giving some kind of more formal meaning to what the average person thinks of when they use that term and talk about how "heavy" something is, etc. So the W=mg definition turns up, but this doesn't gel too well when talking about "weightlessness" and so on. Am I missing something here? Are there times when "weight" in a formal physics setting is genuinely useful in a way that is quite unique and separate from just thinking about using mass and gravitational forces and so on?

You're probably right, in the sense that we can always replace "weight" by "the force due to gravity" (see the link I gave above). It's useful, however, if you want to get your head around general relativity, to reflect precisely on the implications of the fact that "Weight" is frame-dependent. Here's a quote from the author of that page:

It is worth emphasizing that all notions of weight (and weightlessness) are frame dependent.

As discussed in reference 2, a pencil is weightless in the frame of the space station because of the motion of the frame, not because of the motion of the pencil.

This is true, important, and often hard for students to grok. I use this as something of a litmus test: they understand weightlessness if-and-only-if they understand the frame dependence.

(see http://www.av8n.com/physics/gravity-perception.htm#sec-weightless)

 

[humber]

1. A treadmill reveals obviously loose linkages? Your example served...

How about another. Or is the treadmill unique?

 

[spork]

You have a cart and treadmill in your possession, yet you cannot come to terms with the idea that it is simply balancing.

You DON'T have a cart in your possession, yet you cannot come to terms with the idea that most of us that do actually understand what's going on.

 

[humber]

Weight is the mass of an object multiplied by the local gravitational acceleration measured in the chosen frame of reference. Since the gravitational acceleration is a vector quantity, weight is also a vector quantity.

You can find this definition, together with a discussion of the why and wherefore, on this page.

There is another good outline here:

http://homepages.wmich.edu/~korista/weightless.html

"Albert Einstein's theory of General Relativity finds that an astronaut falling freely toward or around (i.e., orbiting) a massive body and an astronaut completely isolated in space are completely equivalent (ignoring non-uniform gravitational fields, i.e., tidal effects) - indeed, both are weightless. Thus the effective ``force'' of gravity goes away when one surrenders completely to it!"

Exactly. That is the difference between free-fall and standing on the ground.
However, as soon as you stop doing that; do a push-up against the floor of the falling 'vomit comet' for example, or introduce drag, that force reasserts itself. In true free-fall, the force is always there, but as a "force".

If you should do such a push up against the floor, of your average velocity towards the ground will have (for the time of your motion) been reduced. This results in a displacement between you and the hull. At the point of impact of the hull with the ground, you would follow a little later, regaining that lost velocity, before also hitting the ground.

This is where your definition of "real weight" gets woolly. At what point above the earth does the problem of "misinterpretation" arise? If we specify the frame of reference, there is no ambiguity. If I choose the frame of reference defined by the spaceship, my weight is zero at any point along the voyage:

Certainly not while on the launchpad. There is a point between Earth and Moon where gravitational forces balance, Lagrangian L1, but that is not stable.

using this frame of reference, I don't just feel "weightless", I really have no weight. At any point on the voyage I could theoretically choose a reference frame that makes my weight any value that takes my fancy, but it's obvious that some reference frames are more useful than others.

To calculate the velocity of an object falling to Earth, you use its mass, not its weight.
There is no place in the Universe where there is no gravity, it may be small, but it is there. You can choose a reference, but it not any weight that takes your fancy. It is certainly quantifiable from your reference.
When in free-fall, you are effectively weightless, and this is so for satellites which remain in orbit by being in a constant state of free fall, and also true of the Earth around the Sun.

Here's a quote from the author of that page:

(see http://www.av8n.com/physics/gravity-perception.htm#sec-weightless)

It is worth emphasizing that all notions of weight (and weightlessness) are frame dependent.
As discussed in reference 2, a pencil is weightless in the frame of the space station because of the motion of the frame, not because of the motion of the pencil.

Surely the motion of both. They are both in that 'relative' position, because of a common coupling to the Earth's gravity

This is true, important, and often hard for students to grok. I use this as something of a litmus test: they understand weightlessness if-and-only-if they understand the frame dependence.

.

 

[humber]

You DON'T have a cart in your possession, yet you cannot come to terms with the idea that most of us that do actually understand what's going on.

Sounds like "seeing is believing".

 

[Clive]

Weight is the mass of an object multiplied by the local gravitational acceleration measured in the chosen frame of reference. Since the gravitational acceleration is a vector quantity, weight is also a vector quantity.

You can find this definition, together with a discussion of the why and wherefore, on this page.

And on the page you referenced we can read:

1 The Definition of Weight

For an object of mass m, its weight W is a vector and is given by

W = m g (1)
where g is the vector representing the local gravitational acceleration. This g can be determined by observing how a nearby freely-falling object accelerates relative to your chosen reference frame. See section 2 for more about the definition of “gravity”.

... <snip> ...

2 Various Definitions of “Gravity” and “g”

There is less-than-complete consensus as to what the word “gravity” means, and what the symbol “g” means.
To avoid confusion, it is helpful to define some more-specific terms. In increasing order of sophistication, we have:

• I-gravity is denoted gI and can be calculated using Newton’s law of universal gravitation, as follows:...

As I think John Freestone said a lot earlier, this thread is supposedly about classical physics, (whatever that is!) but you seem to have decided to move past using Newton's law of universal gravitation and move on to using one of the other versions of "g" given on that page - one that is more in keeping with the ideas of General Relativity. I would have thought that Newton's law was perfectly adequate for what we've been talking about. Anyway, apart from how you want to define "g", the definitions of "weight" that we've been arguing about are all essentially the same.

There's probably not much point in going around and around on this particular point any further, at least not from my point of view. I'm happy that I understand the basic equivalence between a uniform gravitational field and the effect of acceleration (and did so long before this discussion also). No doubt you do too! I think we all understand what "weightlessness" in space or in free-fall feels like also. We can try to "explain it" in it one way or another, and perhaps even try to claim that one explanation is "more correct" or even "totally correct", even if they will all probably turn out to be "wrong" when General Relatively is modified at some future date by superstring theory or whatever comes next!

Or should we just move right on to discussing a certain class of hypothetical massless particles with a spin of 2 and how they might affect a cart in a free-falling elevator? That would almost certainly lead me to completely new knowledge.
​

 

[Clive]

How about another. Or is the treadmill unique?

One is more than enough humber.

 

[John Freestone]

Humber, I'm sure my dictionary is just as proper as yours. It's just that I gave the definition of the word "most", while you give the definition of the word "majority". Not surprisingly, they aren't the same.

He regressed towards the meaning.

 

[John Freestone]

I do use the standard definition: in good textbooks it's more stringently defined than in the Wikipedia article or the Hyperphysics site. The definition "weight is a measurement of the gravitational force acting on an object" is imprecise if no reference frame is stated. In most situations the reference frame is implied, but it's important to know that the definition is incomplete without this specification.

Think of the spaceship on its way to the Moon from the Earth. What is your "real weight" in this spaceship just after it has left the Earth? When it is halfway between the Earth and the Moon? When it is in a spiralling orbit near the Moon? Just before it hits the Moon's surface?

But wouldn't that simply be the sum of the gravitational forces upon you? There is in fact no difference when you are on the earth's surface: there is still the moon's gravity taking part in your weight. You are in a gravitational field, and its various sources cannot be discerned necessarily. The same is true inside the earth - your weight is the result of all the gravity of the mass around you and the moon and sun, etc. What kind of reference frame do you mean weight should be specified in relation to, Michael? A rate of acceleration? A velocity vector? I don't understand. I thought maybe I did reading that jsd website, but it was just on the edge of my comprehension and a bit beyond.

ETA: I posted that before checking further on, which at least helped me clarify some of my problems! You say that you can choose the reference frame of the spaceship, where your weight will be zero. Can you explain? By extension, it seems I should be able to choose my frame of reference as this desk, and be weightless also. Unless you find yourself at that point where the gravities of moon, earth and all other masses around you sum to zero, I would have thought in both frames you are in a gravitational field, of a certain strength, pointing in a certain direction.

I'm getting the feeling that it's about GR, and I struggle with that. Also, as Clive notes, this classical/modern difference seems to be a large part of the different lingo and emphasis.

 

[humber]

http://www.av8n.com/physics/weight.htm#eq-def-weight
There seem to be many errors in this reference. It is not correct to say that the pencil is weightless because of the frame of reference. It has a certain weight from any given frame of reference. but it mustbe there. It is redundant.
Some points about scales are wrong. Manufacturers do not simply 'get away with considerable vagueness'
A scale may be calibrated under a particular local g. If that scale is moved to another location with a different local g, and a known mass is used to perform the calibration, then that result will be wrong. It will show say, 1kg, but then the reference mass is no longer the same 'weight'.
Scale manufacturers supply a list of known gravitational constants for a given area, and these may be entered by the user, at point of use.

 

[humber]

One is more than enough humber.

You agree. The treadmill is the only known example of its kind. It is uniquely false.

 

[John Freestone]

John, I suspect the source of your confusion about Sol's statement is that he probably didn't fully qualify it by noting that only accelerations relative to inertial frame of references were being considered. In other words, if you allow arbitrary frames of reference, then the acceleration measured for any given object is no longer guaranteed to be absolute - in fact you could get more or less any crazy kind of acceleration you wanted to to choosing equally "crazy" frames of reference. However, if you always use an inertial frame of reference, then any acceleration will turn out to be identical (across all such frames). This can be shown quite easily mathematically - it's actually done in one of the related wikipedia articles although I can't seem to find it again right now!

Thanks for that Clive. I'll have to sleep on it and maybe get more help on it later. I have heard so many different uses of "frame of reference", and associated phrases now (inertial frames, non-inertial frames, accelerating frames, Newtonian frames, non-Newtonian frames, pseudo-Newtonian frames,,,) that my brain of reference is fried. Oh, I missed "crazy frames of reference"!

 

[Furcifer]

I have mentioned many times, you are mixing post-modernist claims concerning the denial of reality, with physics. Meta-physics, is a word I used. Post modernists think that men are obsessed with the speed of light because it is "the fastest", and fluid dynamics remains unsolved because men have a bias towards stiff things. I am serious. I can find the references if you like.

Oh the irony of this statement. What then do you know of cart motion in a fluid? I mean if you want to argue "we" know nothing it stands to reason "you" know nothing.

I'm sure meta-physics is a word you use, as well as others around you do as well. You want to argue relativistic effects in Electordynamics feel free to do so, but please don't apply it to mechanics. It's the reason modern physics has moved towards Field Theory.

I'm interested in any fluid dynamics mysteries that need solving so please post links to them. I'm curious.

 

[Clive]

You agree. The treadmill is the only known example of its kind. It is uniquely false.

No no. I gave you, Humber, one example which is enough. I am not compelled to do more. Humb may produce (or constitute) a second. Improbable perhaps, although nothing's absolute and there's nothing unique to treadmills I'm sure.

 

[recursive prophet]

3bodyproblem: Just Google fluid dynamics mysteries. Lots of hits, like this one: Emphasis below mine.

While fluid dynamics may be considered a mature subject, since it has been pursued for several centuries and most of the great physicists, engineers and applied mathematicians have made contributions to it, it still holds many unresolved problems. The problem of turbulence is usually cited as one of the great unsolved mysteries of fluid dynamics and by extension of classical mechanics. While many properties of turbulent flows are understood, a deductive theory of turbulence from the basic equations of fluid dynamics has not been given. Indeed, the phenomenology of fluid flows is considerably better understood, through theory and experiments, than the basic mathematical properties of the governing equations. Physicists and engineers typically proceed with their analyses under the assumption that the basic equations have the necessary properties of smoothness and well-posedness, but some of these properties have not been proven and present major challenges.

 

[John Freestone]

Nuts, balls. Don't leave home without them. You don't "need to know"- it's not hide and seek, John.


I said that is is possible to detect the difference between the gravitational field of a planet (the case of free-fall) and a uniform field. If already in a uniform field, and you shut out all information, then that may be difficult to detect.
What is the point? Here, in the elevator, you shut out all means of velocity detection in order to make a claim, yet in the hobo-boxcar, all acceleration is shut out to preserve that case.

And yet lower down the page you seem to describe the principle of equivalence very well. Funny how you keep doing that, flatly deny something one minute, then write almost an erudite piece to the opposite effect a moment later.

http://en.wikipedia.org/wiki/Principle_of_locality
"Einstein liked to say that the Moon is "out there" even when no one is observing it.
Realism in the sense used by physicists does not directly equate to realism in metaphysics.[1] The latter is the claim that there is in some sense a mind-independent world."

I have mentioned many times, you are mixing post-modernist claims concerning the denial of reality, with physics. Meta-physics, is a word I used. Post modernists think that men are obsessed with the speed of light because it is "the fastest", and fluid dynamics remains unsolved because men have a bias towards stiff things. I am serious. I can find the references if you like.

Er, no thanks. Post that on Philosophy & Religion maybe.

It is the basis of you entire claim to relative motion, John. If you couldn't do under those circumstances, what you could do when stationary, your claim for equivalence would fail.
Free fall is like being on the surface, but without something under your feet to prevent motion. As you agree that motion changes nothing but your reference frame, all other Newtonian laws must apply.

Failed parsing.

Objects fall at the same rate in the Earth's gravitational feild because the force on them is proportional to mass, but the acceleration inversely proportional to mass. They "cancel out" to provide uniform acceleration, but differing masses are not subject to the same force. Same as on the surface.

You seem not to have understood, or at least acknowledged or spoken to, my argument. They do not cancel out, I believe, because the force on them is proportional to the combined mass:

F = GmM / r²
but the acceleration on each body is inversely proportional to its own mass:

a = F / m,
a' = F / M

Hence, for two mutually attracting masses of great difference, such as yourself and the Earth, when you fall, you accelerate considerably more than the Earth. For two different masses falling towards the Earth, the difference will be less obvious, since their masses are less obviously different in most experimental situations. Do you suppose a star will fall into a black hole at the same rate as an equidistant orange? I haven't been corrected on this yet, though I have to say I'm not absolutely sure of it. I have a feeling that if I google I'll find actual experimental data confirming this.

Acceleration is not relative, but that does not imply that it is "absolute".

Interesting.

How would you do that? Would you need to find some object that was universally understood not be in motion and measure from there?

It sounds like you don't know how you'd decide.

Should you do that, and accelerate and object, how long would it be before you lost that reference, and would it matter? Objects have a history.

Yes, it sounds very much like you don't know what, in between "relative" and "absolute" acceleration might be, if you could "do that".

Acceleration is present or not. Leave it at that.

Brilliant.

ETA:
<long spoiler hidden piece about planetary motion being round shared CM>

I wrote about this myself not so long ago. Not particularly relevant here, but fine, we agree.

ETA: Oh no, it's actually a very good example of the different accelerations due to gravity, as I argue above. The big mass of the sun doesn't move as much as the Earth orbiting it.

Same goes for free-fall and gravitational gradients. Absolute acceleration may not (perhaps not yet) have the meaning it may suggest, but it does have meaning.

The "same goes for" it? What, acceleration orbits round its shared centre of mass? Absolute acceleration may be about to gain meaning, perhaps? Yea Dude, pass the Rizlas. I think you might be on to something.

Under the same conditions? No.

Shall I wikki wikki, or will you?

You would make a good passenger for the Captain; plummeting to Earth is the same as zero-g.
If you are stationary in a gravitational field, then you will do work against that field should you raise your arm. That does not change if you jump from a table, or fall in an enclosed elevator.
How can it be that a situation where you gain KE and velocity, can be the same as not being accelerated?

This is all rather garbled, but seems to be refuting the P of Equivalence, y'know, that Einstein thing you described almost perfectly later? You even pose the wrong conditions in this - "stationary in a gravitational field" is not the condition, it is accelerating in a gravitational field that is indestinguishable from zero g. Yes, plummeting towards the Earth. I criticised the idea that we can't tell the difference earlier, but by considering visual reference outside. You can't tell if the force on your arm is due to being in a vomit comet or out in the depths of space with no gravity, except by knowing that some other way. But as you have shown, you don't understand the importance or purpose of taking away information and asking what can be discerned. To you it just seems a cruel trick, I suppose.

Pour a glass of water, and move the glass. There will be a detectable difference between zero G and free-fall.

That's not what more reliable sources are telling me. When I say "more reliable", I mean ones who wouldn't suggest you "pour a glass of water" in either scenario. How? How do you even set that experiment up?!

Two force balance gauges can show the gradient.

Acknowledged already. Nil points. That's like saying that the cart can tell the difference when it's on the treadmill because it can see the television.

There are many such means. The spinning pair of three-axis accelerometers, gives a complete picture.

 

[humber]

Oh the irony of this statement. What then do you know of cart motion in a fluid? I mean if you want to argue "we" know nothing it stands to reason "you" know nothing.

I'm sure meta-physics is a word you use, as well as others around you do as well. You want to argue relativistic effects in Electordynamics feel free to do so, but please don't apply it to mechanics. It's the reason modern physics has moved towards Field Theory.

I'm interested in any fluid dynamics mysteries that need solving so please post links to them. I'm curious.

They are from Luce Irigaray;
' E=mc^2 is a sexed equation...it privileges the speed of light over other (less masculine) speeds that are vitally necessary to us '.

'...the privileging of solid over fluid mechanics, and indeed the inability of science to deal with turbulent flow at all '

Now, she is a cast-iron idiot and a feminist. What is your excuse for showing your petty-coat?

 

[John Freestone]

While fluid dynamics may be considered a mature subject, ...

fluid dynamicists regularly bend over and set fire to their gas.

What, you mean this area of science still has things yet to be understood?

 

[Dan O.]

Or should we just move right on to discussing a certain class of hypothetical massless particles with a spin of 2 and how they might affect a cart in a free-falling elevator? That would almost certainly lead me to completely new knowledge.

This is supposed to be a family oriented forum so I don't think discussions of such skimpy attire would be appropriate.