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

[Michael C]

Now comes the question: in which reference frame are the laws of motion simpler?

a) the surface of the earth
b) the elevator in free fall

The laws of motion that we can use are equally simple in both cases. Both of your examples are reasonably close approximations to inertial frames of reference for suitably "small scale" scenarios, and so Newton's Laws should be adequate. After all, one of the main points of using inertial reference frames is that the same (and also simplest) set of laws apply in all such frames.

If we call both frames inertial things can get confusing, since one frame is clearly accelerating with respect to the other. There's a good presentation of the conflict in terminology here: the author suggests that we should avoid the term "inertial frame" altogether and use the terms "freely-falling frame" and "Newtonian frame".

The laws of physics really are simpler in the freely-falling frame, since it gets rid of the pesky problem of the weird force of gravity acting at a distance. This doesn't mean that it's always simpler to analyse a problem using a freely-falling frame, though! In particular, we're better off using the Newtonian frame (well, the almost Newtonian frame...) of the surface of the earth to analyse the antics of the DDWFTTW cart: no need to complicate matters by looking at it from a freely falling elevator.

There are definitely some very ambiguous and potentially confusing terms "floating around" this whole area. Taken being "weightless" for example. In terms of pure physics, we aren't actually without weight when we say we are experiencing "weightlessness" (because we're in free-fall, etc.). Our "apparent weight" is zero, simply because we are experiencing the state where there are essentially no internal stresses on body organs, etc. In other words our brains think we are without weight because the usual stresses are suddenly removed. But our "real weight" is still given by mass times "acceleration due to gravity". In other words, our "real weight" is the force exerted on our body by gravity, and that force doesn't disappear when we start to free-fall, or start orbiting the Earth.

"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.

 

[humber]

High Chris, welcome to our little thread where we ridicule arguments...

Ridicule not witnessed since the courts of Versailles

...that contradict what's been known about classical physics for over 300 years.

If you ignore all advances made in that time, and assume that Galileo, Newton and Einstein, would have learned nothing since their demise.

If you want to discuss the curious little cart, we have a whole other thread

Where a cat can hide from the dog.

There has been little talk of the treadmill or cart here.

Beware of the dog.

The rotating parts of the treadmill are usually covered to prevent liability...

Treadmill 2 solves this problem by eliminating the belt. It's a real brainteaser.

That's funnier than all your previous posts, Dan_O.

 

[RossFW]

The belt is simply a moving surface with harmonic motion.

HARMONIC motion now!

Wow!

And tell me Humber, what key is the treadmill in?????

 

[humber]

HARMONIC motion now!

Not simple harmonic motion, but harmonic nonetheless.
http://en.wikipedia.org/wiki/Simple_harmonic_motion

Wow!

And tell me Humber, what key is the treadmill in?????

Keystone Cops.

 

[RossFW]

Not simple harmonic motion, but harmonic nonetheless.

Not even close!!

Dan, Nomination time,

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

 

[fredriks]

Whatever is happening on the treadmill may very well be faster than the treadmill, but it may also be attributed only to the fact that the surface is rotating at the same time the wheels are rotating.

It is always nice when people suggest new physical laws without given any explanations. How is it possibly that the wheel belt interaction is going to depend on the motion of the belt far from the place of the wheel belt interaction? I thought the only relevance was motion of the belt under and close to the wheels.

I would have expected that the ones making this claim, which is unsubstantiated by either experiment or theory, to be under attack.

This is such a nice opening post. "You are all wrong and you really should be attacked because you are all wrong".

You said you have done the research. Why don't you show the errors in for example Drela's analysis? Why don't you explain why the treadmill is not a valid test using well known physics?

 

[John Freestone]

If we call both frames inertial things can get confusing, since one frame is clearly accelerating with respect to the other. There's a good presentation of the conflict in terminology here: the author suggests that we should avoid the term "inertial frame" altogether and use the terms "freely-falling frame" and "Newtonian frame".

The laws of physics really are simpler in the freely-falling frame, since it gets rid of the pesky problem of the weird force of gravity acting at a distance. This doesn't mean that it's always simpler to analyse a problem using a freely-falling frame, though! In particular, we're better off using the Newtonian frame (well, the almost Newtonian frame...) of the surface of the earth to analyse the antics of the DDWFTTW cart: no need to complicate matters by looking at it from a freely falling elevator.

Nor is it much better to look at the freely falling elevator from the frame of the freely falling elevator, IMO! The cart obviously poses an easier choice, at least without treadmills at angles. We are used to dealing with a ball's motion through the air from the approximate frame of the Earth, knowing that if we look too closely we'll find all sorts of errors or peculiarites, depending on the time of month (and not just due to female experimenter error), lattitude, and...well, who knows where the list ends? Sun cycle? Precession of all the planets? Galactic rotation? ...

I'm not sure I've understood that article, but it seems to be implying that the freely falling elevator is not a Newtonian frame either. I'm not saying that you're saying it is, but you seem to be implying that. If my reading of it is right, the frame of the falling lift is quasi-Newtonian. "jsd" defines Newtonian frames as "frames that are unaccelerated relative to the CM of the whole system". While the lift may seem to be freely falling in common parlance, it isn't unaccelerated relative to the CM of the whole system, just as the surface of the Earth is not. It may be a negligible difference depending on the problem. Have I misunderstood? Similarly, it would be a criticism of the view that "in reality" a thrown ball is free floating and the Earth is accelerating upwards (Dan O's proposition, IIRC).

The little riddle on that page is interesting, and once again overturns a truism of school physics, that a lead ball and a rubber ball will fall to Earth at the same rate (in a vacuum), whereupon the kids all gasp at how "counter-intuitive" it is and teacher goes home feeling smug. It's simply wrong, but only by a small amount, and for a different reason than the usual intuitive one (which is to do with our experience of such things in air).

So my problem (revisited) with switching frames all of a sudden when someone mentions elevators changes subtly, from "How accurate do you really want to get your Newtonian frame?", to "Is there any way of knowing what needs to be included in 'the system', such that we find it's CM?" It's a bit like the relative velocity point I made to humber: is there a way of knowing whether anything in the universe is motionless, so as to decide what's absolute velocity? If you want a truly Newtonian frame according to jsd's definition, does it have to include the whole universe? That's a pedantic sized lump of "truly", I know, but see the point?

Cosmologically, of course, these questions are massively (ouch) important. Elevator engineers don't need to worry.

"Weight" only has meaning relative to a particular frame of reference.

"Weight" only has meaning relative to a particular definition.

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".

Again, this is a semantic problem, I think. I don't consider weight quite as Clive does. I don't talk about 'apparent weight'. I assume that weight is relative to a frame and a function of gravity, and is actual or not there, and yet some of the ways I have described this make more sense from his terminology. He says, for instance,

In other words, our "real weight" is the force exerted on our body by gravity, and that force doesn't disappear when we start to free-fall, or start orbiting the Earth.

Again, it seems we're all saying the same things in different words. Do we "go weightless because" we are accelerating (rather as I said it earlier), or is our weight a measure relative to a frame, as you say, or is our weight unchanging, and must be there because it is what is causing us to accelerate? Clive's version makes a lot of sense. The force remains G.m₁.m₂/d², none of the masses or the gravitational constant change, and in fact, the distance is getting less: you get heavier as you fall in an elevator, then, in the Clive-ian sense!

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.

Yes, that makes sense to me.

These are just some of the reasons I absolutely hate and detest multiple-choice questions, BTW! Not allowing the student to qualify the assumptions that every exam question makes is awful. We got a taste of that doing those little physics tests earlier!

 

[John Freestone]

The Earth has a gravitational gradient, so it is always possible to detect that. I know of a small, simple and commercially available force-balance device that can detect a difference in the gravitational field over a difference in height as little as 1 meter.

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

To argue the case for a uniform gravitational field is pointless, because then the claim becomes the obvious statement that "absolutely uniform acceleration, cannot be distinguished from absolutely uniform acceleration."

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.

You all seem to be making a lot of fuss about a simple fact. Objects fall at the same rate, regardless of mass. (But the force upon the more massive object is greater.)

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. 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.

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...

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.

If in free-fall inside an aircraft, the simple act of lifting your arm and measuring the force, will tell you that you are in a gravitational field.

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.

A few more simple tests should allow you to conclude that you are in free-fall in an aircraft, and not in zero G. Spinning an accelerometer, will allow a pair of three-axis devices to provide conclusive proof.

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

 

[Michael C]

I'm not sure I've understood that article, but it seems to be implying that the freely falling elevator is not a Newtonian frame either. I'm not saying that you're saying it is, but you seem to be implying that.

I certainly didn't want to imply that. A freely-falling elevator near the surface of the earth is definitely not a Newtonian frame. The frame of a freely-floating elevator somewhere in "deep space" far from any gravitational influences will be a Newtonian frame: the tricky question here is how far away from gravitational influences do we have to be...

If my reading of it is right, the frame of the falling lift is quasi-Newtonian. "jsd" defines Newtonian frames as "frames that are unaccelerated relative to the CM of the whole system". While the lift may seem to be freely falling in common parlance, it isn't unaccelerated relative to the CM of the whole system, just as the surface of the Earth is not. It may be a negligible difference depending on the problem. Have I misunderstood?

What's the "whole system"? It's important to remember that the freely-falling elevator defines a local inertial frame: unlike a Newtonian frame, which would theoretically extend uniformly over the whole universe, a freely-falling frame is only inertial if we cannot detect a gravity gradient within it. The frame is local with respect to both space and time. The elevator falling through the tunnel drilled north-south through the earth represents an inertial reference frame over a short distance of time, but an elevator that starts falling through the south end of the tunnel when the first one starts from the north end clearly doesn't define the same reference frame.

So my problem (revisited) with switching frames all of a sudden when someone mentions elevators changes subtly, from "How accurate do you really want to get your Newtonian frame?", to "Is there any way of knowing what needs to be included in 'the system', such that we find it's CM?" It's a bit like the relative velocity point I made to humber: is there a way of knowing whether anything in the universe is motionless, so as to decide what's absolute velocity? If you want a truly Newtonian frame according to jsd's definition, does it have to include the whole universe? That's a pedantic sized lump of "truly", I know, but see the point?

That is indeed a problem with a "truly" Newtonian frame. There's no real answer to this, since absolute position doesn't exist any more than absolute velocity does.

"Weight" only has meaning relative to a particular definition.

Again, this is a semantic problem, I think.

Agreed. That's precisely why we need to be very careful that people are using the same definitions, or understanding them the same way. For scientific use, we need to agree on definitions that don't give rise to confusion or contradiction. On Earth your weight is x kilos. On the Moon your weight is about x/6 kilos. If you are in an unpowered space vessel (such as the one Jules Verne imagined) on a journey between the Earth and the Moon and somebody asks the question "what is your real weight?", what should the answer be? If you say that you always have a "real" weight, which is your mass multiplied by the acceleration due to gravity, what gravitational influence do you choose: the Earth's or the Moon's? At which point does your "real weight" swap over from being defined by the Earth to being defined by the Moon? As far as I can tell, the only consistent definition of an object's "weight" that is acceptable for universal scientific purposes is that it is the object's mass multiplied by the local gravitational acceleration as measured in the chosen reference frame. Depending on where you are, one reference frame may be more useful than another, but this is not an absolute choice.

(As a side-note it's interesting that Jules Verne got it wrong: he imagined that the passenger in the spaceship would stay pressed against the Earth side of the ship for the first part of the journey, then at some point the influence of the Moon's gravity would become more important and the passenger would then be pressed against the Moon side of the ship. Of course 20th-century space flight has proven him wrong, but it's funny he didn't see the contradiction with the state of the dead dog. The body of the dog floats next to the spaceship during the whole voyage, so why doesn't the man in the spaceship float within it?)

 

[mender]

These are just some of the reasons I absolutely hate and detest multiple-choice questions, BTW! Not allowing the student to qualify the assumptions that every exam question makes is awful. We got a taste of that doing those little physics tests earlier!

John, that's why they have multiple choice questions, to see if the assumptions that the student uses to answer the questions are correct. The foundation must be correct and solid and the thought process logical before applying the math.

The assumptions are the curriculum and aren't open for debate, because they are physical laws.

That's why humber doesn't answer anything with a clear statement. His physics "foundation" is flawed and self centered. Answering a question in an unambiguous way exposes that.

 

[mender]

So in light of humber's most recent post (I keep forgetting to log in so I don't get exposed to humber's madness!) in reference to the cart moving up the belt in still air:

That means that it moves just a little faster than zero.

I'd say that this

Wind speed is stand still, in still air.

needs to be scratched from the list of things that humber has learned here. He no longer (likely never did) accept still air as being the actual speed of the wind when it is at a standstill. Amazing.

I believe that brings the total of things that humber has learned in 5000 posts to zero.

Dan, you wanted something to scratch out ...

 

[CNY_Dave]

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


Dave

 

[Dan O.]

Not even close!!

Dan, Nomination time,

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

I wouldn't use that because the belt's motion can be described by a series of harmonic waves (as can any motion that repeats on a uniform cycle in a bounded space).

The link provided for "Simple harmonic motion" was probably intended to mislead. A more apropriate link would have been Complex harmonic motion^WP.

 

[RossFW]

I wouldn't use that because the belt's motion can be described by a series of harmonic waves (as can any motion that repeats on a uniform cycle in a bounded space).

The link provided for "Simple harmonic motion" was probably intended to mislead. A more apropriate link would have been Complex harmonic motion^WP.

That's interesting, Dan.

My understanding was that simple Harmonic motion consisted of a regular, recipricating motion that could be defined by a sine wave, and complex harmonic motion was simply a series of simple harmonic motions superimposed.

I'm not sure how a rotating structure could be described as harmonic. Can you expand?

 

[John Freestone]

John, that's why they have multiple choice questions, to see if the assumptions that the student uses to answer the questions are correct. The foundation must be correct and solid and the thought process logical before applying the math.

The assumptions are the curriculum and aren't open for debate, because they are physical laws.

That's why humber doesn't answer anything with a clear statement. His physics "foundation" is flawed and self centered. Answering a question in an unambiguous way exposes that.

That isn't my experience, nor that of many of the people I did such exams with. Of course, you could argue that we just weren't applying the right logical thinking and physical laws, and this is griping about it, but often there are different interpretations of the question, many things are assumed without being stated, often there are ignored complications of the kinds we have been discussing here, which can make "none of the above" or "we can't tell" the correct answer when (without the complications) the examiner is expecting a specific figure for something. Maybe your curriculum was more closely tied to the range of questions. A great many of mine weren't, and my true answer, had I been able to write it down, would often have started something like "This doesn't say whether this is in a vacuum or not...If you mean...then...". I haven't done any in 25 years, but I noted quite a number of people discuss the physics tests we linked to from here, some making similar comments about what was assumed in the question. Of course, some of that was a mismatch, since we hadn't just done the specific curriculum leading to the questions, and some said that the theory and terms had moved on since they did it. But I'm convinced some of it is just sloppy question setting...or was way back when.

Humber's answers are a slightly different category, but closely related. We are all making silly human errors and assumptions, which he can see beyond. Balloons floating at windspeed! Gah!

 

[H'ethetheth]

That's interesting, Dan.

My understanding was that simple Harmonic motion consisted of a regular, recipricating motion that could be defined by a sine wave, and complex harmonic motion was simply a series of simple harmonic motions superimposed.

I'm not sure how a rotating structure could be described as harmonic. Can you expand?

Check out Fourier series. It's basically that; an infinite number of harmonic functions superimposed.

 

[spork]

I wouldn't use that because the belt's motion can be described by a series of harmonic waves (as can any motion that repeats on a uniform cycle in a bounded space).

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.

 

[ThinAirDesigns]

Christoph:

Certainly, there is no evidence of a cart actually moving directly downwind, faster than the wind, whilst being powered by the wind, and I have done some research on this.

Hi Cristoph.

If you are looking to talk about the cart/treadmill specifically, here's you're thread:

http://www.internationalskeptics.com/forums/showthread.php?t=128483&page=81

... but be prepared to defend a statement such as the one you make above as the number of folks who have independently built, tested and demonstrated DDWFTTW devices is fast approaching double digits.

JB

 

[Michael C]

John, that's why they have multiple choice questions, to see if the assumptions that the student uses to answer the questions are correct. The foundation must be correct and solid and the thought process logical before applying the math.

That isn't my experience, nor that of many of the people I did such exams with. Of course, you could argue that we just weren't applying the right logical thinking and physical laws, and this is griping about it, but often there are different interpretations of the question, many things are assumed without being stated, often there are ignored complications of the kinds we have been discussing here, which can make "none of the above" or "we can't tell" the correct answer when (without the complications) the examiner is expecting a specific figure for something.

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. 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.

Here is the multiple-choice physics question for today:

Augusta and Bertha have a tug-of-war. Bertha wins the game, pulling Augusta into the mud. Why did she win?

a) Bertha exerted a greater force on the rope than Augusta did.
b) The rope exerted more force on Augusta than it did on Bertha.
c) Bertha exerted more force on the ground than Augusta exerted on the ground.
d) All of the above.
e) None of the above.

Please solve with Newtonian mechanics

 

[humber]

So in light of humber's most recent post (I keep forgetting to log in so I don't get exposed to humber's madness!) in reference to the cart moving up the belt in still air:



I'd say that this

needs to be scratched from the list of things that humber has learned here. He no longer (likely never did) accept still air as being the actual speed of the wind when it is at a standstill. Amazing.

I believe that brings the total of things that humber has learned in 5000 posts to zero.

Dan, you wanted something to scratch out ...

You really didn't expect me to believe that did you. Mender?
Still air is wind? You have a cart and treadmill in your possession, yet you cannot come to terms with the idea that it is simply balancing.