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

[humber]

I would say don't get sucked in, either humber has developed another personality, or someone has decided to out-humb humb.


Dave

And which personality do you have?

 

[humber]

Yes, but I consider that an obfuscation, and perhaps an intentional one on humber's part. While a given element of the belt does in fact go round and round, that's not what the cart experiences. What the cart sees is a continuous surface moving at constant speed and direction.

Like the cart, the average displacement of the belt is zero.

 

[John Freestone]

I think that for most multiple choice questions, someone with a good grasp of the subject can work out what the expected answer is, even if the question is sloppily posed.

By "most" I assume you mean "a satisfactory proportion", rather than "over fifty percent".

Of course, then it's up to the student if they want to be "good" and give the expected answer, or if they want to be smart and show up the weaknesses of the question.

Well, that is a different meaning from the one I intended, but demonstrates the problem adequately.

Here is the multiple-choice physics question for today:

Cheers then.

 

[humber]

Not even close!!

Dan, Nomination time,

"The belt is simply a moving surface with harmonic motion. " #3299

No, he's busy with a dead bird.

If you don't understand what harmonic motion means, and how it relates to the belt, it is no surprise that you didn't understand the way that superposition can be used in the case of the boundary layer.

As an aside, Captain, did you know that Fourier analysis can be used to decompose waves into harmonics? Radar does that. I though it might be useful for you for you to know how one of your primary instruments works.

 

[humber]

I

You said you have done the research. Why don't you show the errors in for example Drela's analysis?

Why don't you show us that you understand it enough to find them?

 

[Michael C]

By "most" I assume you mean "a satisfactory proportion", rather than "over fifty percent".

By "most" I mean, as defined in my Collins English Dictionary, "a great majority of, nearly all"

Well, that is a different meaning from the one I intended, but demonstrates the problem adequately.

What I intended to imply is that a smart student has the power in their hands: depending on the importance of the test for them, and their possible knowledge of who may be marking it, they may themselves decide how well they will be marked. If it's some state exam where the answers are probably marked electronically, they may go for the "expected answer" route. If they know that the papers are being marked by their quirky physics professor who wants people to question everything, they may choose a different strategy. At the other end, it's up to the professors to sort out the sheep from the goats! I agree that multiple-choice tests can be problematic, and are certainly not alone sufficient for assessing the level of students, but they have their uses.

 

[humber]

I agree, but we need to be careful throwing "always" around. You couldn't detect it if you left your gravity gradiometer at home.

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

Except that the point was to connect an absolutely uniform gravtitational field with an acceleration. You've just passed that bit by calling one the other to start with.

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.

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.

This is a very, very, very interesting statement. I think the general argument would make it false, like so: If you were right, then presumably we can take these different masses as falling towards each other, to test the idea.

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.

The gravitational force on two objects is the same on each, given by Newton's Law of Universal Gravitation. It is proportional to the product of their masses, and does not distinguish those masses from each other. The acceleration on each is F/m, however, where m is the mass of the local object. So, from the perspective I have used here, your statement is wrong. That is the perspective of, say, someone floating in space between the masses. The low mass would accelerate and "fall" towards the other faster than the other fell towards the first.

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.

However, it might be possible to see this from another perspective, in which your version is right. I'm not clear about it, but it sort of starts from the point of view that the masses move towards each other at the same rate relative to either of them (a trivial-sounding idea, but true nevertheless). I'm not sure, however, if they accelerate towards each other at the same rate (by which I mean in some sense that does not immediately put us back in a "neutral" position watching). Is there not an increasing rate of approach that is singular, just as there is a singular momentary rate of approach - my hunch is yes, but I'd have to learn more. I hardly understand accelerating frames of reference at all. This makes me puzzle again about Sol's brief and unqualified statement that acceleration is not relative. Oh well...

Acceleration is not relative, but that does not imply that it is "absolute".
How would you do that? Would you need to find some object that was universally understood not be in motion and measure from there?
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.
Acceleration is present or not. Leave it at that.

ETA:

http://www.physlink.com/education/askexperts/ae118.cfm
Question
If the term 'absolute motion' has no meaning, then why do we say that the earth moves around the sun and not vice versa?
Asked by: Akhilesh
Answer
The term 'absolute motion' has no meaning in the following sense; if you were moving in a straight line, with constant velocity, and there were no windows to see the outside, there is no way you can tell what speed you are moving at (or, for that matter, whether you are moving at all) with any measurement. Thus, speed has only meaning relative to something else.

However, the term 'absolute acceleration' _does_ have a meaning. If you were on a roller coaster, even on one which has closed cars with no windows, you would still be able to tell you were moving -- you'd be thrown every which way, and you would even be able to feel the motion in your guts, given you were securely fastened in your seat. Now, when riding a roller-coaster, you definitely know it is YOU that is moving, and not your friend standing on the ground waving to you.

Now, another thing to be pointed out is that circular (or elliptic) motion inherently has an acceleration associated with it. Now, given the magnitude of the force between the earth and the sun is equal, the earth being much lighter, accelerates much more than the sun does, so that's why we can say the earth moves around the sun, and not vice versa.

On a side note, the sun is not stationary either. If there were no other planets but the earth (they make the overall motion of the sun pretty complicated) the sun would also be rotating around the center-of-mass of the sun-earth system. But, since the sun is way massive than the earth, the center of mass would (probably, I haven't calculated it) fall inside the sun.

Similarly the same kind of situation exists between the earth and the moon -- the two rotate around the center of mass of the earth-moon system, but this point lies well within the earth, so it makes sense to say the moon is moving around the earth, and not vice versa.

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.

But in the usual sense, your description is not true, as I said earlier - large masses on the Earth actually fall faster in a vacuum in the lab than small masses do, relative to the Earth.

Under the same conditions? No.

I disagree. So does Einstein. Without some clever kit, you won't be able to tell the difference between an acceleration due to freefall in a gravitational field and zero net gravitation. The force of lifting your arm in these circumstances will be the same. Unless you're bionic and have a gravity gradiometer built in.

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?

We can posit any number of hypothetical methods if we describe them vaguely enough.

Pour a glass of water, and move the glass. There will be a detectable difference between zero G and free-fall.
Two force balance gauges can show the gradient. There are many such means. The spinning pair of three-axis accelerometers, gives a complete picture.

 

[humber]

Why don't you show us that you understand it enough to find them?

You made the challenge, you answer it.
Is the paper correct or not?

 

[ThinAirDesigns]

Humber now takes both sides. (directly above).

LOL

JB

 

[humber]

Humber now takes both sides. (directly above).

LOL

JB

Actually JB, you will not find me saying that I think Drela is correct...

 

[humber]

By "most" I mean, as defined in my Collins English Dictionary, "a great majority of, nearly all"

Use a proper dictionary, Michael_C.

majority /m<schwa>"dZQrIti/ n.M16. [Fr. majorité, in branch I f. med.L majoritas, f. as MAJOR a., in branch II f. as MAJOR n.1: see -ITY.]I <unknown>1 The state or fact of being greater; superiority; pre-eminence. M16–E18.2 The state of being of full age. M16. 3 The greater number or part; a number which is more than half the whole number; spec. the larger party voting together in a deliberative assembly or electoral body. L17.4 The number by which the votes cast for one party etc. exceed those for the next in rank. M18.2 L. STRACHEY A few days before her eighteenth birthday—the date of her majority.Listener It is sad that, as it approaches its majority, this organisation should have run into deep waters.3 BYRON The majority In council were against you.F. H. A. SCRIVENER Nor in the vast majority of instances does it exist.N. CHOMSKY The large majority of its population..is Khmer..but there are substantial Chinese and Vietnamese minorities.J. NAGENDA These friends, the majority of whom had been at school with him.4 J. MCCARTHY A majority of forty-six was given for the resolution.V. BRITTAIN Mr. Harris won the election with a comfortable majority.II 5 The rank or office of a major. L18.5 R. CAPELL This redoubtable sapper, risen from the ranks to a majority, is a type such as makes empires.Phrases: absolute majority: see ABSOLUTE a. 8. in the majority belonging to or constituting the majority. silent majority: see SILENT a. the great majority: see GREAT a. the majority spec. the dead; join the majority, die. the vast majority: see VAST a. 5.Comb.: majority carrier Electronics in a semiconductor, a charge carrier (electron or hole) of the kind carrying the greater proportion of the current; majority rule the principle that the greater number should exercise greater power; majority verdict a verdict given by more than half of a jury, but not unanimous.

What I intended to imply is that a smart student has the power in their hands: depending on the importance of the test for them, and their possible knowledge of who may be marking it, they may themselves decide how well they will be marked. If it's some state exam where the answers are probably marked electronically, they may go for the "expected answer" route. If they know that the papers are being marked by their quirky physics professor who wants people to question everything, they may choose a different strategy. At the other end, it's up to the professors to sort out the sheep from the goats! I agree that multiple-choice tests can be problematic, and are certainly not alone sufficient for assessing the level of students, but they have their uses.

Human markers have been tested. The same paper may get an A or an F
As for multiple choice, see 'regression towards the mean'

 

[mender]

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

Balancing what?

 

[Michael C]

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.

 

[Clive]

"Weight" only has meaning relative to a particular frame of reference. With respect to the freely-falling frame of reference, our weight is zero. With respect to the frame of reference of the Earth, our weight is our mass multiplied by g. If we want to be pedantic, instead of talking about "real weight" and "apparent weight", we should talk about "weight as measured in such-and-such frame of reference". I can happily decide that I weigh nothing at all, even though I'm not in free fall, by considering my position from the point of view of a freely-falling reference frame. In this case, the upward force I'm feeling from my chair comes from the fact that it is accelerating my mass upwards through this reference frame at a rate of 9.8 m/s².

You may choose to look at weight in that way Michael but it seems to me you're going to quite a lot of trouble to sidestep the standard definition commonly used in physics contexts. Possibly though it is just that you are locked into the idea of replacing the gravitational effects of mass with accelerating frames of reference. If so, I suspect things will get tricky when you start dealing with systems with multiple objects large enough to have significant gravitational effects.

In any case, perhaps you want to read these links to see where I am coming from:

The wikipedia article for weight^WP says:

In the physical sciences, weight is a measurement of the gravitational force acting on an object.

This definition of weight is exactly what I was taught at high school (not recently!) but also what I've seen taught more recently to my own kids.

The hyperphysics site has essentially the same definition for 'weight' (halfway down the page) but talks about 'perceived' or 'effective' weight when discussing 'weightlessness' (bottom of page) instead of wikipedia's use of 'apparent weight^WP' but once again there doesn't seem to be much to debate in terms of the standard definition(s).

From the wikipedia article for apparent weight^WP:

An object's weight, henceforth called "actual weight", is the force exerted upon it by a gravity field. By contrast, an object's apparent weight is the weight that a weighing scale measures.
An object's apparent weight is equal to its actual weight, unless:

• The object has an acceleration, as in a lift, a rocket, or a rollercoaster.
• Some force other than gravity and the associated normal force is acting on the object. This may, for example, be buoyancy, centripetal force due to the Earth's rotation, magnetic force.

Apparent weight is responsible for our sensation of the weight of our own bodies. A greater apparent weight results in a heavier or greater sensation of our weight, and vice-versa.

 

[Michael C]

You may choose to look at weight in that way Michael but it seems to me you're going to quite a lot of trouble to sidestep the standard definition commonly used in physics contexts.

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?

 

[Clive]

I hardly understand accelerating frames of reference at all. This makes me puzzle again about Sol's brief and unqualified statement that acceleration is not relative. Oh well...

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!

 

[humber]

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!

Give one example, Clive.

 

[humber]

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.

Perhaps that is the point. Words.

 

[Clive]

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?

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.

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).

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?

ETA: I've been googling for more strict/stringent/precise definitions of "weight" Michael, but everything I find seems to be much the same as what I've already given. Even the 3rd Conférence Générale des Poids et Mesures seems to agree according to a reference in National Institute and Standards and Technology (NIST) document that I found. See page 52 in http://physics.nist.gov/Pubs/SP330/sp330.pdf

 

[Clive]

Give one example, Clive.

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