The Naked Scientists

The Naked Scientists Forum

Author Topic: Are frames of reference even more misunderstood than centripetal force?  (Read 30590 times)

Offline wolfekeeper

  • Neilep Level Member
  • ******
  • Posts: 1092
  • Thanked: 11 times
    • View Profile
Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.

But I'd also like to point out that you can't directly feel gravity either; if you fall off a roof, you can not feel anything, yo cannot feel gravity, but the deceleration hurts a lot at the bottom. So you indirectly feel gravity that way. Likewise you can feel centrifugal or coriolis force when something prevents it from acting.
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.

But I'd also like to point out that you can't directly feel gravity either; if you fall off a roof, you can not feel anything, yo cannot feel gravity, but the deceleration hurts a lot at the bottom. So you indirectly feel gravity that way. Likewise you can feel centrifugal or coriolis force when something prevents it from acting.

Agreed, but gravity has little direct bearing on the particular example.

My only purpose in making such a big fuss about this is, where possible, to encourage the use of the most straightforward explanations of physical phenomena. Sometimes things do get very complicated and it's necessary to introduce things like "coriolis force" because they can greatly simplify the problem at hand. That's fine. All I'm trying to do is make sure we don't forget that the apparent coriolis force is really just a consequence of Newton's first law.

More importantly, I don't want a lot of casual scientists who visit TNS to get the idea that there are any more mysterious forces known to science than the existing set of mysterious forces.
 

Offline wolfekeeper

  • Neilep Level Member
  • ******
  • Posts: 1092
  • Thanked: 11 times
    • View Profile
Well, in that case, magnetism doesn't really exist either; it's just moving charged particles and relativity.

(At least it is unless or until somebody finds a magnetic monopole, if that ever actually happens, in the meantime all known magnetic fields are just electric fields.)
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
Well, in that case, magnetism doesn't really exist either; it's just moving charged particles and relativity.

(At least it is unless or until somebody finds a magnetic monopole, if that ever actually happens, in the meantime all known magnetic fields are just electric fields.)

I'm sorry. Why does this help to explain why our subject rotates? Are you saying he is a particle with no mass, or are you just being obtuse?
 

Offline wolfekeeper

  • Neilep Level Member
  • ******
  • Posts: 1092
  • Thanked: 11 times
    • View Profile
It's often convenient to choose a rotating reference frame in which the subject doesn't rotate; that's the main point.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.
The coriolis field cannot be felt. Regarding the gravitation field, that's only true under special cases, i.e. when the gravitation tidal forces are too week to feel.

As I recall it's the gravitational tidal force which is responsible for reorienting artificial satelites.
This is also wrong.
Likewise you can feel centrifugal or coriolis force when something prevents it from acting.
In those cases its the electric force between you and the rotating  is what you feel.
« Last Edit: 03/05/2012 14:55:11 by Pmb »
 

Offline Geezer

  • Neilep Level Member
  • ******
  • Posts: 8328
  • "Vive la résistance!"
    • View Profile
It's often convenient to choose a rotating reference frame in which the subject doesn't rotate; that's the main point.

Yes, it can be convenient, but we should not forget that it does not overturn the fundamental physics. I'm pretty sure a significant number of TNS subscribers are now convinced that, because they can "feel" a centrifugal force, it's a real force. I think that's unfortunate.

Essentially, Newton said that, in the absence of a force, things stay put, or travel in straight lines. This is just as valid today as it was when Newton figured it out. The only refinement in the meantime is that, in certain cases, "straight lines" are not quite what he might have thought they were.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
So... we all know forces (except gravity) are mediated by fields which are made up of virtual particles.  We have photons for electromagnetism, gluons for the strong force, W and Z bosons for the weak force...

So are there any centrifugons?
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
I'm pretty sure a significant number of TNS subscribers are now convinced that, because they can "feel" a centrifugal force, it's a real force. I think that's unfortunate.
What I think is unfortunate is that people think that inertial forces are not real forces and that real forces are different from non-inertial force in some significant way.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Your page proves exactly why...
To assert that my page proves something cannot be determined unless s msjority of readers step up and vote on that point, and that never happens, especially since there
is no way to determine who is reading it but not posting in it.
you can't group inertial forces together with "real" forces (or whatever other term you want to use for them.)
I disagree. Why do you belive that they can't be so grouped?
They're caused by different things and behave differently under transformations of reference frames (in particular, inertial forces vanish in inertial reference frames).  Grouping them all together is misleading. 
That is not true. One can't say "This kind of force transforms in this way while this other force transforms in this other way." How something transforms is determined only by the theory being used.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Centrifugal force (and coriolis force) are pseudo forces due to a combination of rotation of the reference frame and momentum; and they are needed when you are analysing a situation in a rotating reference frame to get the correct movements; together they explain why the stars spin around the Earth (for example).
The Centrifugal force and Coriolis force are found in the same frame that you re sitting in right at this minute. During WWII the deck guns used to fire shots over 20 miles away to plument a beach has to be calibrated to take into account the  Coriolis force, otherwise the shells won't hit their target.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.

But I'd also like to point out that you can't directly feel gravity either;
coriolis forces and centrifugal forces are considered gravitational forces in general relativty.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
That is not true. One can't say "This kind of force transforms in this way while this other force transforms in this other way." How something transforms is determined only by the theory being used.

To be clearer, inertial forces are proportional to mass, and can be transformed away by an appropriate reference frame choice, since changes in reference frames introduce terms proportional to mass.  Non-inertial forces need not be proportional to mass (electromagnetism, for example) and cannot be transformed away in general, since they appear in Newton's equations as proportional to charge, so subtracting a term that depends on mass will not, in general, cancel them out. 
« Last Edit: 03/05/2012 19:45:00 by JP »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
By the way, what model are we assuming for this debate?  The mantra centrifugal force is not a force is usually used when teaching Newtonian mechanics in inertial reference frames.  In that case, it's absolutely correct.  Not only is centrifugal force not a force, it doesn't even enter the equations! 

It only becomes an issue when you move to non-inertial reference frames, in which case a proper frame-independent treatment of Newton's equations shows what I said above: some terms appear proportional to mass and others not, so there's still a distinction. You could also make a distinction based on which forces are still there in an inertial reference frame, since this makes it clear how this model ties into the previously learned inertial-frame models.

My position is that we should teach Newton's laws in inertial reference frames first, since they're hard enough to understand for beginning students.  Since these students have heard the term "centrifugal force" before and it's something that intuitively makes sense, its important to stress that it is not a force in this model.  At this point, they know F=ma and they're ready to change reference frames (when they have some fancy calculus to back it up.)  Then it's still important to start from what they know, F=ma and show how changing reference frames introduces extra terms to that equation.  Since you're building on F=ma, it makes sense to still treat the F's in inertial reference frames as one type of thing, and those terms which appear due to a non-inertial reference frame as another type of thing.  It also makes sense to draw attention to the fact that transforming away from an inertial reference frame gets you new terms that are proportional to mass (because the "a" spits out extra terms, which are multiplied by m).  This has the extra benefit of getting them ready to notice that gravity is also proportional to mass, which leads to general relativity...

You can, of course, argue that that's not an effective way to teach the material, but its widely used these days and having taken and taught it, it seems effective.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
To be clearer, inertial forces are proportional to mass, ...
I thought I had stated that already. Sorry if I missed that. :(
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
To be clearer, inertial forces are proportional to mass, ...
I thought I had stated that already. Sorry if I missed that. :(

You probably did.  :)  I don't think my statements are all 100% clear, so I wanted to reiterate that I'm talking about mass-dependent terms as opposed to forces which aren't proportional to mass.

I assume we all agree on the physics and the math involved, so the whole argument here is about coming up with physical/mathematical reasons for grouping terms in the equations under different labels (or not).
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
By the way, what model are we assuming for this debate?  The mantra centrifugal force is not a force is usually used when teaching Newtonian mechanics in inertial reference frames. 
I don't know where you got that idea. Typically it's undergrad courses which some teach that, i.e. Physics I, II, III. Not in more advanced courses. See

More modern teachings give the following
http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm

Most notably,

From Gravitation, by Misner, Thorne and Wheeler, Box 6.1, page 164
Quote
A tourist in a powered interplanetary rocket feels "gravity." Can a physicist by local effects convince him that this "gravity" is bogus? Never, says Einstein's principle of the local equivalence of gravity and accelerations. But then the physicist will make no errors if he deludes himself treating true gravity as a local illusion caused by acceleration. Under this delusion, he barges ahead and solves gravitational problems by using special relativity: if he is clever enough to divide every problem into a network of local questions, each solvable under such a delusion, then he can work out all influences of any gravitational field. Only three basic principles are invoked: special-relativity physics, the equivalence principle, and the local nature of physics. They are simple and clear. To apply them, however, imposes a double task: (1) take spacetime apart into locally flat pieces (where the principles are valid), and (2) put the pieces together into a comprehensible picture. To undertake this dissection and reconstruction, to see curved dynamic spacetime inescapably take form, and to see the consequences for physics: that is general relativity.

From Introducing Einstein's Relativity, by Ray D'Inverno, Oxord/Clarendon Press, (1992) page 122 
Quote
Notice that all inertial forces have the mass as a constant of proportionality in them. The status of inertial forces is again a controversial one. One school of thought describes them as apparent or fictitious which arise in non-inertial frames of reference (and which can be eliminated mathematically by putting the terms back on the right hand side). We shall adopt the attitude that if you judge them by their effects then they are very real forces.

From Nature, Albert Einstein, February 17, 1921 issue
Quote
Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.

A.P. French - Inertial force is defined as the force on a body that results solely from observing the motion of the body from a non-inertial frame of reference. This in addressed in Newtonian Mechanics, A.P. French, The M.I.T. Introductory Physics Series, W.W. Norton Pub. , (1971) , page 499. After describing the inertial force as seen from an accelerating frame of reference French writes
Quote
From the standpoint of an observer in the accelerating frame, the inertial force is actually present. If one took steps to keep an object "at rest" in S', by tying it down with springs, these springs would be observed to elongate or contract in such a way as to provide a counteracting force to balance the inertial force. To describe such force as "fictitious" is therefore somewhat misleading. One would like to have some convenient label that distinguishes inertial forces from forces that arise from true physical interactions, and the term "psuedo-force" is often used. Even this, however, does not do justice to such forces experienced by someone who is actually in the accelerating frame of reference. Probably the original, strictly technical name, "inertial force," which is free of any questionable overtones, remains the best description.

Cornelius Lanczos - The subject of inertial force is also addressed in The Variational Principles of Mechanics - 4th Ed., Cornelius Lanczos, Dover Pub., page 98.
Quote
Whenever the motion of the reference system generates a force which has to be added to the relative force of inertia I’, measured in that system, we call that force an “apparent force.” The name is well chosen, inasmuch as that force does not exist in the absolute system. The name is misleading, however, if it is interpreted as a force which is not as “real” as any given physical force. In the moving reference system the apparent force is a perfectly real force, which is not distinguishable in its nature from any other impressed force. Let us suppose that the observer is not aware of the fact that his reference system is in accelerated motion. Then purely mechanical observations cannot reveal to him that fact.

From Cosmological Physics, John A. Peacock, Cambridge University Press, (1999), page 6-7 (See URL, last quote at bottom)

One last comment - It seems like a very common thing that is done on discussion forums that when someone uses the term maths (not the "s" at the end of math) it seems to come from someone who doesn't know math very little or at all. If you want to give the impression that you know math then I recommend that you don't use the term "maths" or if you need to then leave the "s" off the end of maths.

Pete
« Last Edit: 03/05/2012 20:40:29 by Pmb »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
By the way, what model are we assuming for this debate?  The mantra centrifugal force is not a force is usually used when teaching Newtonian mechanics in inertial reference frames. 
I don't know where you got that idea. Typically it's undergrad courses which some teach that, i.e. Physics I, II, III. Not in more advanced courses. See

More modern teachings give the following
http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm

Most notably,

From Gravitation, by Misner, Thorne and Wheeler, Box 6.1, page 164
. . .



Yes, I'm well aware of general relativity as well as non-relativistic formulations of classical mechanics in arbitrary reference frames.  My point still holds that when we Newtonian mechanics in inertial reference frames (usually in the first course of an undergraduate physics education), centrifugal force is not a force in that model.  It doesn't matter what models they'll learn later--it doesn't make centrifugal a force magically a force in inertial Newtonian mechanics simply because general relativity exists.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
My point still holds that when we Newtonian mechanics in inertial reference frames (usually in the first course of an undergraduate physics education), centrifugal force is not a force in that model.  It doesn't matter what models they'll learn later--it doesn't make centrifugal a force magically a force in inertial Newtonian mechanics simply because general relativity exists.
In Notonian physics that is not always true. That's why I posted examples from Newtonian texts. It appears that you skipped through post you're quoting and cut out all the Newtonian texts/examples. The ones you skipped over are very important physics texts. Please look over the examples I used before you skip over them
http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm

A.P. French - Inertial force is defined as the force on a body that results solely from observing the motion of the body from a non-inertial frame of reference. This in addressed in Newtonian Mechanics, A.P. French, The M.I.T. Introductory Physics Series, W.W. Norton Pub. , (1971) , page 499. After describing the inertial force as seen from an accelerating frame of reference French writes
Quote
From the standpoint of an observer in the accelerating frame, the inertial force is actually present. If one took steps to keep an object "at rest" in S', by tying it down with springs, these springs would be observed to elongate or contract in such a way as to provide a counteracting force to balance the inertial force. To describe such force as "fictitious" is therefore somewhat misleading. One would like to have some convenient label that distinguishes inertial forces from forces that arise from true physical interactions, and the term "psuedo-force" is often used. Even this, however, does not do justice to such forces experienced by someone who is actually in the accelerating frame of reference. Probably the original, strictly technical name, "inertial force," which is free of any questionable overtones, remains the best description.

 The subject of inertial force is also addressed in The Variational Principles of Mechanics - 4th Ed., Cornelius Lanczos, Dover Pub., page 98.
Quote
Whenever the motion of the reference system generates a force which has to be added to the relative force of inertia I’, measured in that system, we call that force an “apparent force.” The name is well chosen, inasmuch as that force does not exist in the absolute system. The name is misleading, however, if it is interpreted as a force which is not as “real” as any given physical force. In the moving reference system the apparent force is a perfectly real force, which is not distinguishable in its nature from any other impressed force. Let us suppose that the observer is not aware of the fact that his reference system is in accelerated motion. Then purely mechanical observations cannot reveal to him that fact.
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Please read what I said above.  I specifically stated (twice) that it's Newtonian mechanics in inertial reference frames where centrifugal force doesn't exist.  This is 100% true. 

You keep citing things on non-inertial reference frames and general relativity, which are different models.  I agree with the French quote you give above, but its irrelevant to Newtonian mechanics in inertial reference frames.

By the way, "math" is the standard American English spelling...
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
The correct spelling for the short term of mathematics is "math" and not "maths". In any case I never seen a mathematician of physicist use the term "maths" In the last 30 years.
« Last Edit: 03/05/2012 21:20:46 by Pmb »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
The correct spelling for the short term of mathematics is "math" and not "maths". In any case I never seen a mathematician of physicist use the term "maths" In the last 30 years.

Being a Yank, I use the American spelling for that one.  However, I did spend a year in England when I was six, and learned to read there.  I tend to use "grey" and "theatre" instead of the preferred American English "gray" and "theater" as a result.
 

Offline wolfekeeper

  • Neilep Level Member
  • ******
  • Posts: 1092
  • Thanked: 11 times
    • View Profile
I think 'math' is an Americanism, and this is a UK website and show. But that aside...
So... we all know forces (except gravity) are mediated by fields which are made up of virtual particles.  We have photons for electromagnetism, gluons for the strong force, W and Z bosons for the weak force...

So are there any centrifugons?
No. But science certainly isn't restricted to fundamental particles. Topics like chaos theory involve no fundamental particles, but is certainly part of science.
« Last Edit: 04/05/2012 01:41:25 by wolfekeeper »
 

Offline JP

  • Neilep Level Member
  • ******
  • Posts: 3366
  • Thanked: 2 times
    • View Profile
Well that's part of my point that this entire argument hinges on which model you choose to use. 

Anyway, regarding the fundamental particles, I wasn't trying to prove or disprove anything.  I'm legitimately curious.  I suspect there might actually be "centrifugons."  After all, we suspect that gravity is governed by gravitons, and "centrifugal force" is an inertial force like gravity.  Would inertial forces also be governed by gravitons?  I don't know, but its interesting.
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
I find the following comments confusing:
My point still holds that when we Newtonian mechanics in inertial reference frames ... centrifugal force is not a force in that model.
What you've said here, literally, is "In inertial frames of reference, inertial forces don't exist and when they don't exist they are not forces?"

See my confusion? Sorry I didn't cacth myself earlier. :(
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums