Talk:Fictitious force

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The physical use for taking account of inertia

I can't think of any physical use for the concept of "fictitious force", or any problem which is simplifies or makes possible. Can you? Does the page provide any? (William M. Connolley 17:20, 11 Feb 2005 (UTC))

The current article on centrifugal force describes the usefulness of the formula for centrifugal manifestation of inertia in a rotating reference frame. The current article on the coriolis effect describes the usefullness of the formula for the coriolis manifestation of inertia in a rotating reference frame. The formulas given in these articels are used by physicists and engineers all over the world.

(William M. Connolley 12:12, 12 Feb 2005 (UTC)) Well sure - I'm a meteorologist, I use a formula with the coriolis force in it, and so do the climate models. But thats not the question: the question is, is the use of "fictitious force" a useful concept? In metorology, not at all. Its just in the equations as a force like any other.

In corresponding with you I will no longer use the expression 'fictitious force'. In dynamics there is no necessity for the use of the expression 'fictitious force'. From now on I will consistently use the expression 'manifestation of inertia'. I prefer to call it manifestation of inertia, because that simplifies thinking about the dynamics, whereas the expression 'fictitious force' complicates thinking about the dynamics. --Cleon Teunissen 09:29, 12 Feb 2005 (UTC)

(William M. Connolley 12:12, 12 Feb 2005 (UTC)) Well... that sounds like you are coming rather close to my POV that the concept "fictiitious force" isn't very useful.

Well, by rephrasing I have done away with some apparent distance. My paradigm here is the example of the electric car that recharges its battery system when switched to braking. At first the car and planet Earth have a relative velocity with respect to each other, and afterwards their relative velocity is zero. The cars battery system has been recharged; the manifestation of inertia has been doing work.

So why not simply call manifestation of inertia a force? It's doing work, isn't it?

I am reluctant to put manifestation of inertia in the category of forces, because inertia can only manifest itself in response to a force; manifestation of inertia is never present independently. As long as no force is acting on an object, the object's velocity remains the same, the inertia is there, ready to manifest itself should some force start pushing in some direction.
Manifestation of inertia has unique properties that justify, in my opinion, that manifestation of inertia is seen as a category in itself. --Cleon Teunissen 14:18, 12 Feb 2005 (UTC)

Well sure - I'm a meteorologist, I use a formula with the coriolis force in it, [...] Its just in the equations as a force like any other.(William M. Connolley 12:12, 12 Feb 2005 (UTC))

Yes, in those equations it is a force like any other. The model that is used works with a stationary Earth, so the model needs to be tweaked in order to produce exactly the same outcomes as a model that uses a rotating Earth. In the rotating Earth model no tweaking is necessary, for in the rotating Earth model the inertial effects that will occur flow automatically from the standard laws of motion.
The stationary Earth model must be tweaked by introducing a coriolis force.

The stationary Earth model is a fictitious model, and just by itself it is wrong. By introducing exactly the right fictitious force, this wrongness is cancelled completely. This is analogous to the repair job on the Hubble space telescope. The main mirror was wrong. They discovered exactly how it was wrong, and then they replaced the perfect secondary mirror with a mirror with a wrongness that exactly cancelled the main mirror's wrongness. --Cleon Teunissen 14:18, 12 Feb 2005 (UTC)

All force laws refer to the inertial reference frame

In physics, it is recognized that the inertial reference frames have something in common that the rotating reference frames do not share. It is possible to formulate a set of laws of motion that is valid in all inertial references frames. For example, the force law for centripetal force is valid in all inertial refecence frames. To perform a calculation in a rotating reference frame, corrective laws must be introduced to get correct calculation results. The coriolis force law is an example of such an corrective force law.
Each rotating reference frame needs its own specific correction.
This frame-specific correction refers to the inertial reference frame.
That is why the formula for the coriolis force has the factor ${\vec {\omega }}$ in it: the corrective force law refers to the inertial reference frame.

Non-physicists sometimes claim that rotating reference frames are indistinguishable, that among the rotating reference frames there is no preferred frame. Yet each time these non-physicists perform a calculation involving a coriolis effect, they are referring to the non-rotating reference frame.

In relativistic phyisics, it is recognized that it is not possible to formulate laws of motion that are valid in rotating reference frames. Instead, algorithms are devised that specify exactly what corrective laws need to be introduced in each specific rotating frame in order to get correct calculation results. --Cleon Teunissen 01:08, 13 Feb 2005 (UTC)

Earlier I wrote:

According to relativistic phyiscs, the same physics is going on in all references frames. Or, phrased in another way: according to relativistic physics, laws exist (and they are found by physicists) that are invariant under transformation, they are valid in all reference frames.
--Cleon Teunissen 08:01, 12 Feb 2005 (UTC)

I retract the 'phrased another way' part. Theoretically it is correct, but it refers to physics at relativistic rotation rates. The physics inside a hollow cylinder rotating at a speed very close to the speed of light will display gravitomagnetic forces. The mathematics of general relativity can handle hollow cylinders rotating at a speed very close to the speed of light, but they do not occur in nature. I feel a discussion of laws of motion should be limited to what occurs in nature. --Cleon Teunissen 02:59, 13 Feb 2005 (UTC)

Misplaced?

This paragraph seems out of place; is there a good reason for it to be in the article? ‣ᓛᖁ♀ᑐ 05:25, 21 September 2005 (UTC)

Within physics, there is no obvious use for the term "fictional", or even any precise definition. It is not clear that this characterisation is particularly useful, and many deny that forces are "fictitious" or "imaginary" in any real sense.

The request for comment on this article drew my attention here. I don't like this statement for several reasons, primarily because its too short and gives an impression of concensus. I suspect it might be an attempt to point out that the use of the term fictitious force is debated among scientists. A fictitious force, however, is well-defined as an apparent force. Whether the concept is useful in getting across the properties of forces and Newton's law is a subject of debate, although my impression is that most of us use it in teaching introductory physics, whether we use it afterwards varies . Serway and Jewett's, Physics for Scientists and Engineers, has a wonderful explanation of ficticious forces, and puts "centrifugual force" . Halliday, Resnick and Walker, Fundamentals of Physics {the other very popular, at least in the US, intro text}, on the other hand, doesn't use the term at all, and uses centripical force to refer to the external force keeping whatever object in circular motion. Salsb 11:42, 21 September 2005 (UTC)

Validity

William M. Connolley, your first edit to this article included the statement "Simply characterising a force as fictitious because it arises from a change of reference is not reasonable." What is your basis for this claim? ‣ᓛᖁ♀ᑐ 06:10, 21 September 2005 (UTC)

Because that is an entirely arbitrary choice of definition. William M. Connolley 08:17, 21 September 2005 (UTC)

Do you have a source that supports this? ‣ᓛᖁ♀ᑐ 08:20, 21 September 2005 (UTC)

I disagree with the idea that it is arbitrary; a real force in classical mechanics requires an interaction. On the other hand since not everyone likes using the idea of a fictitious force, do you have any source that claims it is not reasonable? Salsb 11:47, 21 September 2005 (UTC)

From PNA/Physics

• Fictitious force - eight months of editing have been reverted; this page needs attention from someone familiar with general relativity. — Preceding unsigned comment added by Eequor (talk • contribs) 21 September 2005

Revert (2)

Eequot reverted the article, sans discussion. I've reverted it back. The Eequor/CT version is very wordy but not very useful. It is very fuzzy over something rather basic: whether gravity is to be considered fictitious. There is, as far as I can see, no real use for the concept fictitious force in physics: arguing about whether forces are fictitious or not seems to belong to not-physics. William M. Connolley 08:17, 21 September 2005 (UTC).

I'm very concerned that you feel comfortable reverting eight months of editing, especially when so much detail is removed. It is also not basic at all whether gravity is fictitious, which may be why it appears fuzzy. How solid do you consider your understanding of general relativity? ‣ᓛᖁ♀ᑐ 08:27, 21 September 2005 (UTC)

I actually think the reverted version is a better starting point. However, I disagree with your assesment of the concept of fictitious force, since it is used often in both Newtonian physics and in gravity. But I question the claim that gravity is considered fictitious in Newtonian physics (yes, even with as little GR as I know, I know that in GR it is considered fictitious) . I have never heard this before, nor could I find it after a quick glance through a few intro and mechanics books. In Newtonian physics, the key point in making a force fictitious is not the mass-dependance, but that it apparently comes without an interaction with another body. I think the claim about gravity being fictitious in Newtonian physics should be removed, since it is erroneous, and some short exposition on why gravity is fictitious in GR would be good. Salsb 11:57, 21 September 2005 (UTC)

I agree. One could mention the radiation emitted by an accelerated charge as an example here. An electrical charge falling in the gravitational field of a planet will radiate em-waves. But a charge won't radiate just because you are observing it from an accelerating frame. The equivalence principle isn't violated here, because the emission of em radiation depends on how you impose the boundary conditions at infinity. Count Iblis 12:46, 21 September 2005 (UTC)

I'm not aware of any physics that fruitfully uses "fictitious force" as a useful concept (except, arguably, in GR. But calling gravity fictitious is against most peoples intuition and not what they are thinking of when they hear "fictitious force"). There are however numerous examples - meterology being obvious - where treating "fictitious" forces such as coriolis as real forces is useful.

As to my understanding of GR, I don't consider it solid at all. But the GR bit was put in there by EMS, whose understanding *is* good, as far as I'm able to tell.

As to reverting 8 months editing... the point was that the article had developed into a mess of words and unclarity. More isn't necessarily better. If you (or anyone) can indeed find some useful physics being done around the "fictitious" idea, then do please include it.William M. Connolley 13:07, 21 September 2005 (UTC).

I agree completely with reverting as the article did look like a mess. However, as for useful physics being done with fictitious forces, the concept is usefully in distinguishing between force from interactions, versus from accelerating reference frames. For some introductory references, I can refer you to Ch 6 of Physics for Scientists and Engineers, by Serway and Jewett, or Ch 8 of Kleppner and Kolenkow's An Introduction to Mechanics, or in Landau and Lifshitz's Mechanics, where the term fictitious isn't used as such, rather (in the translation I have) "inertia forces" is in quotes when refering to the effect of rotating frames, and there's exposition discussing the rotation as having the same effect as a force, as opposed to being a force. Then in derivations of brownian motion {including if I recall correctly Einstein's original work}, one often refers to ficticious forces, and in a different context, the radial quantum solution to an S-wave has a potential which is often referred to as a quantum fictitious potential, leading to a quantum fictitious force. {There probably are also examples beyond those that pop into my head right now and references beyond those within a couple steps from my desk } So the idea that fictitious forces are somehow not used in physics to distinguish from effects do to changing reference frames from real forces is incorrect Salsb 13:30, 21 September 2005 (UTC)

It's not just for meterologists. Afterall, how else are stranded astronauts supposed to decide which direction to throw their wrench to get back to the shuttle? Cause I hear that NASA lets that happen a lot.

Also, I agree that saying Newtonian gravity is a fictitious force is totally sketchy. If rewritten, the normal force discussion might be helpful to this article, but Eequor's version danced around too much. — Laura Scudder | Talk 14:16, 21 September 2005 (UTC)

Let me state some points very clearly:

1. Fictitious forces appear in many basic physics textbooks as a calculational method, and they are useful in many cases. See, for example:
   • Daniel Kleppner, Robert Kolenkow (1973). An Introduction To Mechanics. McGraw-Hill Science/Engineering/Math. ISBN 0070350485.
2. "Fictitious force" is absolutely not a philisophical label, it is rather a well-defined term that is dependent on the particular model being used. Thus:
   • In Newtonian Mechanics, gravity in a force.
   • In General Relativity, it is not a force, but it is sometimes useful to treat it as one in certain reference frames; it is then a "fictitious force."

We need a complete article, and the long version was a better starting point than this crappy little stub. I'm going to revert, and then we can work on it. -- SCZenz 15:16, 21 September 2005 (UTC)

I totally agree with SCZenz's points above with respect to gravity and general relativity. Those are the points that must be made in the article.

At first glance, the new edits look fairly good. The GR part needs work however, since the new edit keeps refering to "gravity" when it should be refering to gravitation. (See talk:gravitation.) --EMS | Talk 16:53, 21 September 2005 (UTC)

I totally disagree with the intro to the current article, and indeed most of the article. "The two most common fictitious forces are the Coriolis force and the centrifugal force," - is this true? Isn't gravity a fictitious force? Or... is it? The current version of the article completely wimps out of clarifying this, and instead goes in for vast amounts of vague wurble. Its also quite unsatisfactory to assert that gravity's fictitiousness varies according to theory. Gravity is a real thing in the world, whatever its nature. SCZ asserts that FF is "well defined" and his version of the article defines it as "The term fictitious force refers to a calculational tool in physics". Spiffy. Does gravity fit that definition? I don't think so. As for "its a useful concept in physics" I see that asserted in the talk pages but its notably absent from the article itself. William M. Connolley 21:31, 21 September 2005 (UTC).

William, I have been absolutely clear. I'll try saying everything a different way.

1. "Ficitious" doesn't mean that the action of an apparent force isn't really there, it just means it isn't really a force. Maybe it's a confusion between the definition of the term "fictitious force" and the usual English meaning of "fictitious" that is giving you trouble...?

   The confusion between the two meanings is indeed unhelpful. I don't think "it isn't really a force" has any meaning.

2. A "fictitious force" is "a term in Newton's Second Law, in a particular non-inertial reference frame, that is not actually a force but is treated as one to make a calculation easier." That is a precise definition. Yes, it is model-dependent. That's why it's a calculational tool and not a statement about the true nature of a force.

   If that is indeed the precise definition, then it should be in the article, not languishing on the talk page. But... have you just made that up or can you source it to somewhere reliable?

3. Gravity is a force in Newtonian mechanics. It is not a force in General Relativity. I'm sorry, that's just how it is. It's how those theories work. That is not a statement that gravitation exists in one theory and not the other; rather, in General Relativity, gravitation arises due to the curvature of spacetime, and affects the equivalent of a "straight line" (a geodesic) that particle will follow. (I know that's heavy--if anyone hasn't seen the idea before I can recommend any number of good general-info books on relativity.)

   This won't do. Gravity is a real thing in the real world. The thing called gravity in the models called NM or GR is another thing.

4. It sometimes happens in GR problems that you pick a non-inertial frame of reference and do a problem there. Strange as it is, in GR, standing on the surface of the earth is non-inertial (even neglecting rotation)--and, in that frame in GR, gravity appears as a fictitious force.

   Fine. I have no problem with that. Note that your rephrased version is still a bit iffy over whether gravity is fictitious in GR or not.

5. You are right that more examples could be given in the article, and more references. I will work to provide these. But they exist, I promise.

   What do you mean, more examples? There are none. I'll believe them when I see them.

6. The article has examples, which I think would be helpful to introductory students working with fictitious forces. It is not so well-geared to explaining the idea to laypeople, and this should be fixed also.

By all means, keep asking questions and I'll keep answering. But there's nothing fundamentally incorrect about what's there. You can't change the definition of a scientific term on Wikipedia because you don't like it. -- SCZenz 22:01, 21 September 2005 (UTC)

I'd be much happier if you could provide a reliable, sourced, definition of FF. You haven't done that yet. Maybe you can. William M. Connolley 22:37, 21 September 2005 (UTC).

William, I do not appreciate the tone of your response.

Really? How interesting. I didn't much like your "crappy little stub" either. William M. Connolley 08:40, 22 September 2005 (UTC).

Although Wikipedia doesn't grant special status to "experts" in a field, and requires them to cite sources like anyone else, I am in fact an expert in Physics (albeit of the lowliest kind of expert). Thus when I write about an elementary physics topic from memory, it is quite likely that I am (approximately) correct. I am not citing details because I have been at work the entire time since I became aware of the debate on this article, and my mechanics textbooks are at home--plus I'm busy.

Well if you're too busy to do a decent job, don't produce a botch and then feel the need for a clean-up tag. But do please produce a source for your clear definition. William M. Connolley 08:40, 22 September 2005 (UTC).

I have been clarifying the definition, from memory, based on details I remember when you raise objections. But if I were to go through and cite every sentence, with ample quotes from sources, it would still say more or less what it says now. -- SCZenz 00:35, 22 September 2005 (UTC)

Your difficulties with gravity in GR seem to come from an incomplete understanding of GR--which is perfectly reasonable, as it's really quite an obscure subject. But if you don't completely understand it, wouldn't it be better to give others who have studied the subject the benefit of the doubt?

And you can cut the condescending crap too. This article isn't about GR, its about "fictitious force". My intuition was that calling gravity fictitious was odd. But I defered to EMS, who inserted (into the short version, which unlike your version wasn't stuffed full of wurble) the fact that under GR gravity is fictitious. That was fine by me. Your version however lacks clarity: it can't decide whether gravity-in-GR is fictitious or not. I lack kmowledge of GR, so can't fix that: but you and EMS (and anyone else with your "complete" understanding of the subject) ought to decide whether gravity-in-GR is fictitious sometimes or always. And you ought to provide a reliable citation for that.

Yes, acceleration due to gravitation is a real thing in the real world--but since we don't actually measure forces (only accelerations), we do not have to explain gravity as a force. Sometimes we do (Newton) and sometimes we don't (GR)--if we don't, then it may show up as a fictitious force. To have gravity appear as a fictitious force in a calculation is not a claim that gravity is fictitious, and this debate can't get anywhere if you insist that it is. -- SCZenz 00:35, 22 September 2005 (UTC)

Oh, and one more question... What do you mean there aren't any examples in the article? The examples include:

• The coriolis force
• The centrifugal force
• Acceleration/deceleration of a car from the perspective of the car's occupant

I admit they are currently poorly written, and I will contribute to fixing them... but there are examples. -- SCZenz 01:26, 22 September 2005 (UTC)

All they are examples of, are examples of "fictitious" forces. You've completely missed the point. They aren't examples of where *using* the concept of fictitious force is of any use at all. William M. Connolley 08:40, 22 September 2005 (UTC).

William, this is likely my last try at some sort of dialogue. You're being rude,

Oh really? So what was "crappy little stub"? Was that polite? I suggest you take the beam out of your own eye before looking at other peoples motes. William M. Connolley 15:41, 22 September 2005 (UTC).

and my obligation to address your legitimate concerns while being snapped at is pretty small, don't you think? I'm not going to speed up the usual pace of fixing problems in physics articles just because you're insistent that you don't like what's here now. We have very few physics editors, and we do our best.

This version (which I didn't write) is badly written, and I may have picked the wrong one to revert to.

I know you didn't write it. You reverted to it, so you're responsible for it being there. Yes you did pick the wrong one to revert to, so the obvious thing would be to fix that mistake. William M. Connolley 15:41, 22 September 2005 (UTC).

However, I think examples are useful to have, so I plan on going through and fixing up what's here. It's true that I have finite time, so the article may be a work-in-progress for a few days. That's not unusual, in my experience. What I don't want to do is throw everything out, or have an article that claims this is all nonsense just because nobody can instantaneously cite every sentence.

As for an example of "applying" a fictitious force, warship gun tables routinely correct for the coriolis force. If you fire a projectile for miles, it can miss by (I think) something on the order of 10's of meters. Yes, I can provide a citation for that fact (except the amount you miss by, which I'm not sure of, but I know it's substantial). And yes, I will put it in the article, along with other examples (some of which, like modleing gases, are already on this talk page). The fact that I have not done so yet is not a useful thing to point out, so please don't. -- SCZenz 13:15, 22 September 2005 (UTC)

You are still missing the point. The warship gun tables use the coriolis force, but make no use of it being fictional. I know of no examples where the idea that the force is *fictional* is of the slightest use. And clearly you don't either. William M. Connolley 15:41, 22 September 2005 (UTC).

See [1]. ‣ᓛᖁ♀ᑐ 15:50, 22 September 2005 (UTC)

I'm not quite sure how I'm making my point quite so unintelligible when it seems clear enough to me. To try again: your link is irrelevant. It merely asserts that the coriolis force is fictional. It does not *use* the "fictional-ness" for any purpose. If you dropped fictional-ness entirely from that link your understanding of the coriolis force, or physics in general, would not be changed. A better scienceworld link is this one: http://scienceworld.wolfram.com/physics/FictitiousForce.html. Note how it totally fails to define fictitious force, and simply gives examples. Note that gravity is not one of those examples. This is why my version said Within physics, there is no obvious use for the term "fictional", or even any precise definition. It is not clear that this characterisation is particularly useful, and many deny that forces are "fictitious" or "imaginary" in any real sense.. The challenge (so far un-taken-up) is for someone to find an example where using the concept "fictional" actually helps. In the case of the coriolis force, precisely the reverse is true: forgetting entirely about its fictionality and treating it as a force like any other is invariably the most useful way of thinking of it. William M. Connolley 17:02, 22 September 2005 (UTC).

The difficulty is that the coriolis force is a NOT a force, and I have provided several references in this talk, and in the article, where this fact is discussed.

Fair enough. Unfortunately they are all off-line and I don't have access to them. For myself, I still don't accept this viewpoint. William M. Connolley 18:08, 22 September 2005 (UTC).

The utility of the concept of a fictitious force -- in Newtonian physics, as opposed to GR where gravity is due to curvature issue -- is that we can be in a noninertial reference frame {typically a rotating frame but not necessarily} , and yet use Newton's laws by treating the effects of rotation exactly as a force. Hence, fictitious because it does not arise from an interaction, but behaves like a force in Newton's laws Salsb 17:37, 22 September 2005 (UTC)

OK, but I still think you're missing my point. Take the coriolis force. We can be in a rotating frame - say on the sfc of the earth - and yes the equations balance when you add in the coriolis force. Thats all agreed. But what isn't obvious at all is that calling it fictitious helps at all. What would go wrong if you called it a real force?

The problem is that you'd have to redefine what a force is. If you were to say that a force is anything that causes an acceleration regardless of reference frame {as opposed to in noninertial frames only}, then you wouldn't use a fictitious force at all, unfortunately, you would then be considering forces from interactions and "forces" due to coordinate transforms identically. This would be both bizarre and confusing when you deal with different problems. A;lthough if you stay in the same rotating reference frame perpetually {probably you do in climate modelling} this wouldn't be a big deal, except for the lose of physical intution, as a force could seem to spring up magically without an interaction. Not that this is particularly relevant to the article, since we're supposed to be reflecting how things are used, not how we think they should be used. Salsb 18:59, 22 September 2005 (UTC)

The distinction is important because fictitious forces "act" differently from real forces on objects of different masses. Suppose we have a large, frictionless box containing a number of balls of various masses. If the box is accelerated, the balls will remain in their locations until the end of the box catches up to them. To an observer inside the box, it will appear that the balls are all accelerating at the same rate. Since F=ma (Newton's second law), this would imply a different force is acting on each ball. If the balls were all acted upon by a single, real force, they would accelerate at rates inversely proportional to their masses. ‣ᓛᖁ♀ᑐ 17:33, 22 September 2005 (UTC)

Yes yes, this is splendid, the point you are missing here is that this is exactly like gravity. Since we appear to be agreed that gravity-in-newtonian *isn't* fictitious, your attempted definition fails. William M. Connolley 18:08, 22 September 2005 (UTC).

That there is an acceleration which is mass-dependent is a consequence, not a definition, i.e. the examples all have mass-dependent accelerations, but not all forces with mass-dependent accelerations are fictitious. Salsb 18:59, 22 September 2005 (UTC)

The Scienceworld link is "under construction," so I don't think that we can draw any conclusions from the fact that they don't define the term. It has no text at all! -- SCZenz 17:34, 22 September 2005 (UTC)

Yes. Precisely. Scienceworld came to write that article and realised (this is my interpreation of course) that they could find no source or definition of fictitious force, and so wisely they didn't define it.

William, "fictional" is a precisely defined notion based on the mathematics of Newtonian mechanics, and the concept of an inertial reference frame. I wrote a section on this, which is in the article--do you have any comment on it? The whole point of fictitious forces is that you do treat them as real forces, in the reference frame you're using, for the problem you're doing. But they do, in fact, arise from the fact that the reference frame is non-inertial, and the term that physicists use for this is fictitious force. Maybe it isn't the best term, but that's not something to be decided on Wikipedia. The article can only explain how the term is actually used. -- SCZenz 17:34, 22 September 2005 (UTC)

If you mean the section maths/general definition *this isn't a definition*. Its a derivation, which is quite different.

Actually, to quote myself paraphrasing Kleppner and Kolenkow:

"Now we define

${\mathbf {F}}_{fictitious}=-m{\frac {d^{2}{\mathbf {X}}}{dt^{2}}}$"

The rest of the section is providing context for that definition. -- SCZenz 18:17, 22 September 2005 (UTC)

OK, you're right there - I read through too quickly. I agree: within newtonian mechanics this provides a consistent definition. I maintain my other objections. William M. Connolley 18:34, 22 September 2005 (UTC).

I also apologize for referring to previous version of the article as "crappy." It was a shorthand for the POV issues and lack of content, which I didn't think of as a big deal because I didn't realize the debate was going to become so heated. But I still shouldn't have said it. If you would reciprocate by asking for clarification when something is unclear, rather than assuming that I am wrong/lying about the content of my education in physics, I would be appreciative. -- SCZenz 17:34, 22 September 2005 (UTC)

OK, then I too apologise for being brusque and snappish. I have no issues with your education. If I've said stuff that was interpretable that way, I apologise again.

Cleanup

William has raised one very good point. This article is poorly-written, and it may be confusing to people who don't know already what it's talking about. I've worked on the intro, but that's it so far. I'll do more, but hopefully I can get some help. -- SCZenz 01:54, 22 September 2005 (UTC)

Yes. The article is very poorly written. Why was why reverting to this version was a very bad idea. William M. Connolley 08:40, 22 September 2005 (UTC).

I agree. I think the examples should probably just be blanked and started over again. Salsb 12:33, 22 September 2005 (UTC)

The examples are valuable; they do make the concept easier to understand, even if they could be better. If one could be cleaned up and made concise (preferably regarding centrifugal force, as that seems most obviously fictitious), and placed before the formulas begin, it would be very helpful to laypeople. ‣ᓛᖁ♀ᑐ 12:51, 22 September 2005 (UTC)

I disagree with blanking them. I think they can be fixed, but seriously rewritten of course. -- SCZenz 13:15, 22 September 2005 (UTC)

I also just added a section on fictitious forces as they arise from rotating references frames. I followed Kleppner, since the section above did, but the derivation is essentially identical in Fetter and Landau, along with the admonishments, about the forces not being real, and presumeably in any other mechanics text, although I only cited the ones I have on had to consult. I had to hurry at the end, since I'm supposed to be in a meeting one minute ago, but if someone could make the equations look nicer, I would apprecitate it. Salsb 12:33, 22 September 2005 (UTC)

I think we should be careful about making this article too much more technical than it now is. We need to have examples of applications and real-world effects, and move them to the top. -- SCZenz 13:15, 22 September 2005 (UTC)

I agree, I think its pretty much at the limits of technicality. I put it in mostly to make it obvious -- to the mathematically able -- where the fictitious forces arise, and since there was a request for more citations, as this is a concept found in essentially every intro mechanics book. It might be fruitful to move the technical parts to the end, so readers get the examples then the math. Salsb 13:58, 22 September 2005 (UTC)

I went ahead and reorganized the sections. It still needs ample work, but thats all I can do for now Salsb 14:18, 22 September 2005 (UTC)

I have rewritten "acceleration in a straight line" to be a useful general illustration of the concept. Does it serve this purpose? Are there other comments? -- SCZenz 18:05, 22 September 2005 (UTC)

Since we have rewritten all of the article except the relativity section, I've moved the cleanup tags to just that section. Does someone want to rewrite it? If not, I will likely replace it with something much shorter in a few days' time. -- SCZenz 21:38, 23 September 2005 (UTC)

relationship to mass

I have given the article more of a reading, and it is missing a very essential point: Fictitious forces act in direct proportion to the mass of the object. This of course is due to their presense being due to the acceleration of the observer.

It is a very essential point since that is what Einstein noticed in defining his original equivalence principle: that the action of gravity is identical to the action of being accelerated in a rocket ship, with the strength of the force being directly proportional to the mass of the object being operated on.

Note that for the "real" forces, the strength of the action is independent of the mass of the object being operated on. --EMS | Talk 18:55, 22 September 2005 (UTC)

I object to this point, at least if I understand it right. The fact that gravity, like fictitious forces, is proportional to mass is suggestive of the ideas that led Einstein to general relativity. However, this does not change the fact that, in Newtonian Mechanics, gravity is a real force. Nobody teaching introductory physics ever draws their free-body diagrams without gravity. -- SCZenz 19:04, 22 September 2005 (UTC)

As written this is not true. Whether or not the force depends on the mass of the object is irrelevant to it being real or fictitious. It so happens that fictitious forces from rotating coordinates, for example, depend on mass, but that isn't due to their being fictitious as such. Salsb 19:31, 22 September 2005 (UTC)

The point is that the reason for the "force" is that the observer is being accelerated. Think about it a bit: If you are being accelerated, the objects in inertial motion will in your frmae of reference will be seen to accelerate. If you say that this acceleration is due a force on those objects, then that force must be proportional to the mass of the object. So any force which acts in proportion to the mass of the object is automatically suspected of being "fictitious".

As for Newtonian physics and gravity in particular: Do note that there is no way to explain gravitation without a force being involved given the implicit Newtonian assumptions of a flat spacetime and inertial motion being motion at a constant rate with repect to a Cartesian coordinate system. Of course Einstein chose to toss those assumptions out of the window, but the result is a very, very different and non-intuitive view of spacetime. However, the point remains that in Newtonian physics the force of gravity is necessary to explain things as they are object on a massive body.

Finally, I should make it clear that this article is under no obligation to go into any depth about general relativity and in fact should not do more than mention the connection through gravity and let that be that. In fact, it is a legitimate question as to whether general relativity and gravity as a fictitious force should be mentioned here at all. I like mentioning it, but it does seem to put a strain on this article. --EMS | Talk 03:29, 23 September 2005 (UTC)

Remaining objections

I am starting a new section, as the other one was waaay too long and I've lost track of it all. I will try to copy in William's most recent comments and work on addressing them. If I've left anything out, William, can you put it in again? Thanks! -- SCZenz 19:00, 22 September 2005 (UTC)

No problem within Newtonian mechanics

OK, you're right there - I read through too quickly. I agree: within newtonian mechanics this provides a consistent definition. I maintain my other objections. William M. Connolley 18:34, 22 September 2005 (UTC).

Ok, I claim there is also a consistent definition in generally relativity, that is also mathematical. Unfortunately, writing it down would be too complex, especially since forces and reference frames are handled differently in GR. (Plus it would take me many hours of reading to understand it, I suspect.) I can certainly find a book that goes over this, and cite it, however. -- SCZenz 19:00, 22 September 2005 (UTC)

I'd like to note, though, that there is not consistent mathematical definition between newtonian mechanics and GR, again because reference frames, forces, and equations of motion are handled differently in the two theories. The definitions would be conceptually equivalent, however. The question of whether gravity is a fictitious force in GR arises from the fact that gravity is not a true force in GR. (iIn non-inertial reference frames, there is no force of gravity, although obviously objects still fall into each other.) -- SCZenz 19:00, 22 September 2005 (UTC)

ScienceWorld blank page

The Scienceworld link is "under construction," so I don't think that we can draw any conclusions from the fact that they don't define the term. It has no text at all! -- SCZenz 17:34, 22 September 2005 (UTC)

Yes. Precisely. Scienceworld came to write that article and realised (this is my interpreation of course) that they could find no source or definition of fictitious force, and so wisely they didn't define it. -- William M. Connolley

I don't think you can ascribe that motivation. Most likely they just haven't gotten to it yet. -- SCZenz 19:00, 22 September 2005 (UTC)

What's wrong if you call them real forces?

OK, but I still think you're missing my point. Take the coriolis force. We can be in a rotating frame - say on the sfc of the earth - and yes the equations balance when you add in the coriolis force. Thats all agreed. But what isn't obvious at all is that calling it fictitious helps at all. What would go wrong if you called it a real force? -- William M. Connolley

In Newtonian mechanics, there are a few fundamental forces: things like electromagnetism, contact forces, and gravity. These are all you need to explain all motion (within the capabilities of the theory) as long as you use an inertial reference frame. It is helpful to differentiate between these forces, and the additional effects (which you can treat as forces) that arise in non-inertial frames. The term used for this is "fictitious force." Even if we agreed that this definition wasn't necessary, Wikipedia can only report what scientific terminology is used, not change it. -- SCZenz 19:00, 22 September 2005 (UTC)

Things that would go wrong are quantities that depend on accelerations in inertial frames, such as the radiation emitted by an accelerated electrical charge. One can put this right by imposing the right kind of boundary conditions at infinity for the electromagnetic fields, though. Another effect worth mentioning in this article is the unruh effect. Count Iblis 21:37, 22 September 2005 (UTC)

I don't think either of those are very satisfactory. Also, SCZ objected violently to my Within physics, there is no obvious use for the term "fictional", or even any precise definition. It is not clear that this characterisation is particularly useful, and many deny that forces are "fictitious" or "imaginary" in any real sense.. Thinking about this, I have conceeded too readily the point about "precise definition". In newtonian mech; yes. In GR, possibly (though, err, its too complex to actually explain, it seems). But in *physics*? No. There is no definition that allows you to say: "hmmm... I'm measuring a force. I wonder if its fictitious or not? Oh, the defn is...". Or is there? If there is, please present it. William M. Connolley 15:05, 23 September 2005 (UTC).

Fictitious force as a concept is the same in either GR or Newtonian Mechanics: it's an apparent force that would not be present in an inertial reference frame, but is present in a particular non-inertial frame. But it doesn't, and needn't, have any meaning to say a particular force is fictitious independant of which model of the universe you're using. Gravity is the key example of this: if gravity is a force in GR, it's fictitious, but gravity is not fictitious in Newtonian Mechanics. Why not? Because the notion of an inertial reference frame is different in the two models. We're classifying apparent forces in relation to the theory we're using, not in relation to some absolute quality they have. -- SCZenz 18:05, 23 September 2005 (UTC)

Well, one can define what an inertial frame is, although there are some issues here having to do with Mach's principle. So, you know how to relate observed accelerations to the accelerations that would be observed in an inertial frame. Also note that forces that are proportional to mass cannot be detected except for the acceleration they cause. E.g. Earth's gravity (ignoring tidal effects) can be measured either by measuring the gravitational acceleration of falling objects, or by the effects of other forces that counteract it. So, you'll never have to worry about whether or not a weighing scale is indicating a real force or not. It's always real, because what you see is the result of normal forces which have an electromagnetic origin. The difference between fictional forces and gravity is that gravity only acts locally and thus gives rise to tidal forces.Count Iblis 16:17, 23 September 2005 (UTC)

I would be inclined to agree that gravity is always real, from intuition. But EMS clearly says that it isn't, in GR; the article is a bit vague, and says it sometimes (err, no, it says: gravity may appear as a fictitious force - what is this supposed to mean?). So are you asserting that gravity is always real, then? William M. Connolley 16:41, 23 September 2005 (UTC).

In GR gravity generated by massive bodies is real but it isn't a force. It is related to the curvature of space-time. Fictitious forces that act like gravity can be transformed away by a coordinate transformation. These transformations cannot transform a nonzero curvature away to zero. I guess that what matters is what you can objectively detect and how much relevant information that contains. If you see an object accelerating toward you in an inertial frame then that means that at the location of the object some physical effect is causing that acceleration (interaction with the gravitational filed or whatever). But if you are accelerating yourself while making this observation, then you must first subtract your own acceleration. If you don't then the force you attribute as acting on the body is the real force plus the fictional force. This fictional force is actually due to the physical effects that are accelerating you, they have nothing to do with the body you are observing.Count Iblis 17:20, 23 September 2005 (UTC)

What I meant by "may appear as a fictitious force" is that if it appears to be a force at all in GR, it is fictitious. I'll clarify that. -- SCZenz 18:05, 23 September 2005 (UTC)

I don't think the idea of "fictitious force" is a particularly useful one in general relativity. In GR, there are inertial and non-inertial frames. The inertial ones are freely falling, which is different from classical mechanics, where inertial frames are determined relative to a universal system of Cartesian coordinates.

The concept is principally useful in Newtonian mechanics, where there exists a simple, universally defined inertial Cartesian coordinate system in which the equations of motion are ${\ddot {x}}_{i}=0$ in the absence of external forces. In non-inertial frames, other terms appear. You can call these fictitious forces, because try as you might with strain gauges, you'll never measure them, as opposed to, say the Coulomb force. One property that the fictitious force shares with gravity (and indeed, a property that is crucial in GR) is that both terms are proportional to a particle's mass.

Now I don't understand why this page is so incredibly long. It seems to me that this topic would be better served by a punchy article with a couple of simple examples (especially rotating frames of reference), rather than a long, rambling and likely error-ridden treatise. –Joke137 21:59, 23 September 2005 (UTC)

The GR stuff needs to be fixed, ideally by someone who knows more than me. Would you like to do it? The rest I and a couple of others have rewritten in the past couple of days, in the face of rather intense pressure to justify even the existence of the concept. Are there specific changes you are suggesting, or is it ok? -- SCZenz 00:33, 24 September 2005 (UTC)

"It is extremely rare for space-time geometry to rotate significantly with respect to the universe as a whole,"

Does it at all? Or are you refering to frame-dragging here, or is there some other case? GangofOne 00:34, 23 September 2005 (UTC)

Good move (removing relativity stuff)

Even though I would like a mention of relativity to be in here, I approve of the deletion of the section on it. I had not had much time to look it over, but once I did, I realized the SCZenz was right to want it out. All that is needed here is a short blurb on gravity being "fictitious" under the equivalence principle of general relativity. A few extra sentences to somewhat explain that is advisable, but discussion of the technical details of GR (such as the description of geodesics) does not belong here.

For now, my feeling is that it is best left out until this article has had a chance to stabilize and mature some more. Then it can be brought in as an aside. However, the editors of this page can and should be willing to veto any such addition (even if it is mine) if in their opinion it does not "fit". --EMS | Talk 22:39, 27 September 2005 (UTC)

Just saw the current introduction

Now that I have looked at that, all that is needed is already in this article. Indeed, this is a very well-written paragraph on it. I can quibble with the parenthetical remark at the end noting the gravity is "real" in Newtonian physics, but I will edit that when I can. As I see it, the use of the "force" of gravity is required in Newtonian physics, independent of whether that force is fictitious or not. --EMS | Talk 22:46, 27 September 2005 (UTC)

Heh. I'm glad you like it. You can quibble about the perentheticla remark, but it's important for clairty to people who don't know a lot of physics. Also I think your quibbling would be wrong. ;) But if you want to argue it, let's take it to user_talk pages, rather than risk fanning any more debate on this article. -- SCZenz 04:14, 28 September 2005 (UTC)

I am going to edit that stuff at some point, to tighten it up and make it technically correct (or at least more correct). A major issue is how to do this in a way that enhances your excellent work instead of muddying it up again. The existing text communicates well without being buzzwordy, and this is essential to the success of this article. So you have left me with a interesting puzzle to work through. --EMS | Talk 16:16, 28 September 2005 (UTC)

Modifications done

I have put my changes in. I think that I have succeeded in explaining how GR turns gravity into a fictitious force in the main narrative, while having put some needed elaboration into the footnotes. So hopefully readers will be able to get the gist of what is going on in the view of general relativity. --EMS | Talk 23:01, 29 September 2005 (UTC)

My only concern now is that we have two sets of footnotes (yours, and the references), with identical labelling. ;) Not sure what to do with that. -- SCZenz 23:05, 29 September 2005 (UTC)

The comments can be combined with the paper/book reference in the references. That's not my style, but sometimes you see this in scientific articles.Count Iblis 00:20, 30 September 2005 (UTC)

Hmm... Not my style either. And, generally, not Wikipedia's. Let's think about it, or failing that leave it and hope nobody notices.. ;) -- SCZenz 00:25, 30 September 2005 (UTC)

I did notice and correct that. I am not sure that I like the corrections, as that is not my style either. (I would like the letters to be in square brackets.) However, this at least this resolves the conflict. Unfortunately, footnoting is not robustly supported in Wikipedia at this time. I will keep an eye out for a better way to do this, but for now I think that this the best I can do. --EMS | Talk 03:42, 30 September 2005 (UTC)

Comment

I actually quite like the introductory paragraph now. One important point not made in the article is that fictitious forces not only arise from using non-inertial frames of reference, they also arise from using curvilinear coordinates (e.g. the centripedal force in polar coordinates). This is an important distinction: curvilinear coordinates are not a non-inertial frame of reference, since they are not time dependent. The effect of using a curvilinear coordinates is to obtain fictitious forces proportional to velocity squared, whereas accelerated rectilinear coordinates give forces independent of velocity. –Joke137 02:09, 28 September 2005 (UTC)

Could you give a simple example? I don't get it. The derivations given are all vector equations that nowhere mention Cartesian coordinates. GangofOne 03:10, 28 September 2005 (UTC)

Good point. The textbook I used didn't talk about fictitious forces arising from curvilinear coordinates, and their definition doesn't admit them. In fact, this idea doesn't make sense to me--centripedal force arises from a rotating reference frame, not from the polar coordinates!--but I could be wrong. -- SCZenz 04:10, 28 September 2005 (UTC)

Also, a smaller comment: it would be nice to also mention that these are sometimes called "pseudo-forces". Or does anyone have an introductory textbook that they can check? I think that is what mine called them. –Joke137 02:09, 28 September 2005 (UTC)

And rotating curvilinear coord. lead to pseudo-fictitious-forces, I guess. GangofOne 03:10, 28 September 2005 (UTC)

Nonono, I think they're saying that pseudo-force is a synonym for fictitious force. -- SCZenz 04:10, 28 September 2005 (UTC)

Sorry for the slow response. Here is my point. If you have a simple equation of motion ${\ddot {x}}_{i}=0$ and change to curvilinear coordinates y, then the transformed equation of motion is the second total derivative $d^{2}x(y,t)/dt^{2}=0$, given by

${\ddot {x}}_{i}=\sum _{j}{\frac {\partial x_{i}}{\partial y_{j}}}{\ddot {y}}_{j}+\sum _{jk}{\frac {\partial x_{i}}{\partial y_{j}\partial y_{k}}}{\dot {y}}_{j}{\dot {y}}_{k}+2\sum _{j}{\frac {\partial x_{i}}{\partial y_{j}\partial t}}{\dot {y_{j}}}+{\frac {\partial ^{2}x_{i}}{\partial t^{2}}}=0$

or

${\ddot {y}}_{\ell }+\sum _{ijk}{\frac {\partial y_{\ell }}{\partial x_{i}}}{\frac {\partial x_{i}}{\partial y_{j}\partial y_{k}}}{\dot {y}}_{j}{\dot {y}}_{k}+2\sum _{ij}{\frac {\partial y_{\ell }}{\partial x_{i}}}{\frac {\partial x_{i}}{\partial y_{j}\partial t}}{\dot {y_{j}}}+\sum _{i}{\frac {\partial y_{\ell }}{\partial x_{i}}}{\frac {\partial ^{2}x_{i}}{\partial t^{2}}}=0$

The first term corresponds to the acceleration in the new coordinates. If x(y) doesn't depend on t, then the last two terms vanish. The second term is the v²/r term that produces the centrifugal force: this comes from curvilinear coordinates, not from a rotating reference frame. The last term is a fictional force from being in an accelerated frame of reference. My point is that it is a bit misleading to say that the centrifugal force comes from being in a rotating reference frame: you can actually think about it as a calculational trick useful for writing $F=ma$ in cylindrical coordinates. –Joke137 18:00, 2 October 2005 (UTC)

In fact, this is kind of like the geodesic equation in GR, which is

${\ddot {x}}^{i}+\Gamma ^{i}{}_{jk}{\dot {x}}^{j}{\dot {x}}^{k}=0.$

Joke137 18:02, 2 October 2005 (UTC)

I can't think of any way of clairfying this point to the lay reader. In fact, I'm still not 100% sure I understand it myself. You claim that, if you specify some coordinate system so that the path of a particle following a straight line is no longer "straight" (e.g. polar coordinates), then terms that look like fictitious forces arise to keep the particle on its original path... is that right? -- SCZenz 18:56, 2 October 2005 (UTC)

We could take a roller coaster as an example.... Count Iblis 20:56, 2 October 2005 (UTC)

I may like GR, but the last thing that I would want to do is to bring the geodesic equations of GR into this. If the observer is spinning, his view of events is such that the use of centrifugal force is needed. If he is not spinning but instead is using curvilinear coordinates: Well yes there is a problem if he considers his "directions" to be "radial" and "tangential". However, those are not the kinds of directions that Newton was refering to. In the end the deviations from "in the same direction" seen are not due to a force of any kind. Instead it is due to the fact that his coordinate system is not rectilinear to begin with. That is a whole different ball of wax, and while the geodesic equations do explain that quite elegantly, it is not in my opinion germane to this article. --EMS | Talk 22:16, 2 October 2005 (UTC)

I think I'm gonna back EMS on this one. Objects need not persist in straight-line motion if you define "straight line" in a screwy way. Whereas centrifugal force arises in a rotating reference frame independant of the coordinates chosen for that frame. We have now cited a number of mechanics textbooks that define fictitious forces in terms of non-inertial coordinates, whereas I have yet to see one that defines them in terms of changes to non-rectilinear coordinates. Yes, I would believe there is one somewhere. And yes, I know the two notions are easy to interchange (and impossible to separate???) in GR. But we can keep them separate in Newtonian Mechanics, and limit this article to that. (Bringing in GR has caused us a lot of trouble before!) -- SCZenz 23:05, 2 October 2005 (UTC)

Well done

I have not looked at this article for ages, but I am very impressed with the way it has turned out. My compliments to the authors. -- ALoan (Talk) 13:58, 28 November 2005 (UTC)

Acceleration in a straight line

In case 2, the observer being attached to the box, he does'nt move in respect to the box i.e. he is at rest in the box as everything else which is fixed to the box, except the passenger who is lousy attached. This passenger moves in respect to the observer, first by being accelerated backward, secondly by being desaccelerated till he stop i.e. becomes at rest again. Thus for the observer, two forces have acted on the passenger, but not at the same moment, for in this case, the passenger would not have moved at all (case of seat in plain steel). Thus for the observer, these two forces are real, not imaginary. If the first was not real, there would not be a real acceleration, and thus no desacceleration, but these two are real. One force cannot be real and the other not! Consequently, for this observer, no force is fictitious, perhaps mysterious, but real. So where is the fictitious force?--24.202.163.194 03:27, 2 January 2006 (UTC)

Beware: fictitious does not mean imaginary. A fictitious force describes an effect that really does happen when you view the system from an accelerated frame. In this context "fictitious" is simply a technical term meaning that the force is caused by the choice of coordinate system rather than by interaction between objects. It does not imply that there is anything wrong with the force. Henning Makholm 10:33, 2 January 2006 (UTC)

By coordinate system do you mean like the rectangular coordinates, the cylindrical polar coordinates and the spherical polar coordinates?--24.202.163.194 02:34, 3 January 2006 (UTC)

Not as such. "Coordinate system" in this context was used synonymously with "frame of reference". Henning Makholm 13:32, 3 January 2006 (UTC)

Thus, if I understand what you are saying, if we use an accelerated reference frame, we have to intoduce what is called a fictitious force, and if we use another reference frame ( I suppose you mean an inertial one), we don't have to use a fictitious force at all. Is it that? --24.202.163.194 15:04, 3 January 2006 (UTC)

Yes, that is it. (An "accelerating" and "non-intertial" frame is pretty much the same thing). Henning Makholm 15:14, 3 January 2006 (UTC)

Another way to see what is a fictitious force and what is not:[2]

--Aïki 06:14, 17 January 2006 (UTC)

Earth, planet and accelerated frame

In the introduction, it is said that the surface of the Earth is a rotating frame of reference. and at the end, that an observer on the surface of a planet (thus in general, which means the Earth too, and include the rotating as well than the non rotating planets) is in a accelerate frame.

What I understand from that, is that the rotating frame of the Earth is the accelerate frame of 'a planet'. If it is so, we are talking here of a rotating planet, not a planet having no rotation at all.

Is everybody here understand the same thing or understand it in another way? --24.202.163.194 15:33, 3 January 2006 (UTC)

Centrifugal force: 2 citations

1- 'A great deal of confusion has arisen regarding the term 'centrifugal force'. This force is not a real force, at least in classical mechanics, and is not present if we refer to a fixed coordinate system in space. We can, however, treat a rotating coordinate system as if it were fixed by introducing the centrifugal and coriolis forces. Thus a particle moving in a circle has no centrifugal force acting on it, but only a force toward the center which produces its centripetal acceleration. However, if we consider a coordinate system rotating with the particle, in this system the particle is at rest, and the force toward the center is balanced by the centrifugal force.'

Source: Mechanics, by Keith R. Symon, University of Wisconsin, Addison-Wesley publishing company, inc.

2- 'Centrifugal force is the inertial (or fictitious) force radially outward on objects when they are view from a rotating frame of reference. The force arises solely from choosing an accelerated frame, and 'disappears' when the problem is viewed from a stationnary frame. ... Remember: 'In a non-rotating frame of reference, there is no such thing as centrifugal force.' '

Source: Physics - A new introductory course, Particles and Newtonian Mechanics, by A. P. French and A. M. Hudson, by the Science Teaching Center at the Massachusetts Institute of Technology. --24.202.163.194 01:38, 4 January 2006 (UTC)

Euler force

Does anyone have a citation for the term Euler force? Salsb 19:20, 27 January 2006 (UTC)

added to article GangofOne 19:42, 27 January 2006 (UTC)

Physlets by Brian Fiedler

I have added the following two links to the external links section: Motion over a flat surface Java physlet by Brian Fiedler Motion over a parabolic surface Java physlet by Brian Fiedler

The following GIF-animations illustrate the same thing as the 'motion over a flat surface' physlet:
Inertial motion as seen from non-rotating perspective
Inertial motion as seen from rotating perspective
6 images with vectors depicting centrifugal term and coriolis term.
--Cleonis | Talk 11:53, 23 February 2006 (UTC)

Object dropped a equator example: isn't Coriolis zero at the equator?

This article has an example: "Neglecting air resistance, an object dropped from a 50 m high tower at the equator will fall 7.7 mm eastward of the spot below where it was dropped because of the Coriolis force.[1]" Whereas Coriolis effect states: "...in the equatorial region the coriolis parameter is small, and exactly zero on the equator." subasd 07:25, 22 April 2006 (UTC)

I now removed that example, because the fall eastward doesn't have to do with Coriolis, which is indeed zero at the equator.[3]subasd 07:43, 22 April 2006 (UTC)

Um, did you notice that that fact was cited from a well-known physics textbook? I'll double-check on monday (the book is in my office) if it makes you feel better. -- SCZenz 08:28, 22 April 2006 (UTC)

Your reference doesn't say the coriolis force is zero at the equator, as far as I can tell. -- SCZenz 08:31, 22 April 2006 (UTC)

In the CE article, that bit is talking about the CE in the plane tangent to the surface as a result of motion in that plane. So, that excludes things falling vertically. At the equator, since CE is perp to the motion and the rot axis, any CE will be perp to the plane and therefore the Coriolis parameter for Met equations (but not the CE) is zero William M. Connolley 08:42, 22 April 2006 (UTC)

Very well, maybe that should be clarified in the CE article then. And SCZenz, my reference (pretty much the first google result) does say "The Coriolis force is zero right at the equator." subasd 08:51, 22 April 2006 (UTC)

You're absolutely right, it does say so and I missed it (likely because I was tired). However, I do think you should be a bit careful removing explicitly cited facts because the first google result contradicts them. -- SCZenz 17:38, 22 April 2006 (UTC)

William is correct. If the distance to the rotational axis isn't stationary, then you have a corriolis force. This is all very elementary stuff, no need to look up anything, just use your brains! If the motion is along the surface of the Earth, then the corriolis force is zero at the equator. Count Iblis 12:31, 22 April 2006 (UTC)

No, not quite! If the motion is along the sfc, but not || to the rot axis, the CF isn't zero, but it is perp to the sfc (and hence disappears from the 2D equs) William M. Connolley 14:13, 22 April 2006 (UTC)

Yes, of course! I guess my brains weren't working properly :). An extreme case of this is mentioned in the article: A star viewed from the rotating spacecraft which I put in some time ago, so I should have known! Count Iblis 16:09, 22 April 2006 (UTC)

this article is misleading

Problem point: it claims that " There are two ways of analyzing the problem", and next it forces the choice between the use of real forces in an inertial frame, and pseudo forces in a more handy frame - thus denying the way it's analyzed in classical mechanics. Probably that author doesn't know better; but as it stands, this article is making propaganda for the use of pseudo forces, and is thus not conform to Wikipedia standards. Harald88 10:37, 22 April 2006 (UTC)

I disagree. First of all this article is about fictitious force. So, most of the article is about how to make use of this technique. Second, the fictitious force method is part of the curriculum for first year students when they learn classical mechanics. In high school, teachers will usually avoid the concept of fictitious force. However, high school physics education is so low in standard, especially in the US, that we shouldn't pay attention to that here. Count Iblis 12:22, 22 April 2006 (UTC)

Your point one is a faulty argument: an article about a technique should be focussed on it, but not make faulty claims about competing techniques. That's definitely a NPOV violation (your point two didn't address the subject matter). Harald88 10:33, 23 April 2006 (UTC)

In the introduction it is clearly mentioned that sometimes it is useful to use this technique. And it is also clear from the very defefinition of fictitious force that it is just a mathematical technique. So, you don't need to use it, but then you need to work in an inertial reference frame. This is all such elementary stuff that it is hard to see how anyone could be misled.

It's elementary stuff that your claim is erroneous. That approach may sometimes come handy, but working in an inertial reference frame if one doesn't use fictitious forces isn't the only alternative. Instead, the standard alternative is to simply map to a rotating frame, based on an inertial frame - without exchanging potential and kinetic energies. Harald88 13:59, 23 April 2006 (UTC)

B.t.w., perhaps you should take a look at maths articles here. The article on differentiation suggests that to find the N-th derivative you must differentiate a function N-times. It fails to explain that you only need Log[N]/Log[2] steps. That makes a huge difference if you want to compute the billion-th derivative of some function. Is this misleading, POV? Count Iblis 12:42, 23 April 2006 (UTC)

Nice article! But I don't see the claim that you must use that method to find the answer. And exactly that is the issue here; if that article would similarly deny the existance of other methods, it would indeed be misleading (false claims are worse than just "POV"). Harald88 13:59, 23 April 2006 (UTC)

This article doesn't deny alternative methods either. In fact, it is completely trivial to see that you can do without fictitious forces. Count Iblis 17:27, 23 April 2006 (UTC)

I now changed the sentence in order to make your above claim true. Remains that next the statement that Neither viewpoint is more "correct" in any sense recognized by physicists isn't encyclopedic: has an opinion poll about this been published? Idf so, where? Almost certainly the majority of physicists has a contrary opinion. Harald88 17:52, 23 April 2006 (UTC)

Physicists generally recognize the difference between calculational tools and truth claims. One could cite a wealth of physics textbooks that use both methods, and exhort the student (as long as he is careful) to use the one most convenient to the problem at hand. This is not controversial. -- SCZenz 03:45, 24 April 2006 (UTC)

In order to keep such a claim in wikipedia, it must be possible to corroborate it with citations. Harald88 18:04, 24 April 2006 (UTC)

Uncontroversial statements that appear in an overwhelming number of sources usually don't need to be cited. — Laura Scudder ☎ 18:25, 24 April 2006 (UTC)

This one certainly is controversial, as the whole subject is controversial: looking at on the web published comments by academics, apparently many physicists are opposed to the use of fictitious forces. With Google you can readily find sites such as http://regentsprep.org/Regents/physics/phys06/bcentrif/centrif.htm. Some go to extremes in their aversion against that concept wanting to ban it (http://www.physicsclassroom.com/Class/circles/U6L1d.html and http://www.antonine-education.co.uk/Physics_GCSE/Unit_3/Topic_2/topic_2.htm), while others go to extremes in their defense of it, claiming that it's a "real" force. Thus the claim as appears in this article needs at least corroboration, and most probably it needs correction. Harald88 20:17, 24 April 2006 (UTC)

These are all web sites, and they are all designed for teaching; they certainly oversimplify. I see no reason to conclude, for example, that someone who tells a high school student that "there is no such thing as centrifugal force" actually means that centrifugal forces aren't useful for solving problems if you know how to handle them. You are only presenting evidence that people like to teach about fictitious forces in different ways. That's reasonable, since fictitious forces can be philisophically troublesome in certain ways; that's why physicsts don't go in for philosophy much. ;) -- SCZenz 20:48, 24 April 2006 (UTC)

No problem. Now please provide a reference that backs up the claim about physicists in general, or we must change it to make it encyclopedic (no OR). Harald88 20:54, 24 April 2006 (UTC)

Kleppner and Kollenkow, page 340: "By introducing non-inertial systems we can simplify many problems; from this point of view, the use of noninertial systems represents one more computational tool." Now your turn: provide evidence that this statement is controversial among physicists (i.e. not among people who make websites). -- SCZenz 20:58, 24 April 2006 (UTC)

Thanks; note that no-one challenges that it can be used as a tool. Please back up that "Neither viewpoint is more "correct" in any sense recognized by physicists", or modify it such that it corresponds to your source. And do I correctly hear you claim that those web courses are not given by physicists? Harald88 21:23, 24 April 2006 (UTC)

I can find many sources of physicists suggesting different frames be used interchangably; you have yet to present any of physicists claiming that one frame is more correct than another. None of the webpages you cited were created by physicists as far as I can tell; they are high-school-level or corporate created sites. -- SCZenz 21:30, 24 April 2006 (UTC)

See Twin paradox for the claim that inertial frames are special. And so far you have provided nothing that backs up your above counter claim. If it can't be backed up by decent sources, it is unsuited for Wikipdia. Harald88 11:40, 25 April 2006 (UTC)

I'll thank you to stop lecturing me on WP:NOR; we've been disputing whether a statement was original research, not what OR is or whether it's bad. Your understanding of the claim in twin paradox is incorrect: special relativity, like Newtonian mechanics, must have modifications made to make calculations in a non-inertial frame; the paradox arises because calculations are made in a non-inertial frame as if it were inertial. (Of course inertial frames are unique in that they have no fictitious forces, but that doesn't support your claim that people don't use them or don't think of them as equally correct.) -- SCZenz 23:00, 25 April 2006 (UTC)

As far as I know there was no dispute nor any "lecturing"; of course, a dispute will arise if someone here insists on including his/her unsupported POV as fact in this article. And you will have a hard time to find even one ~single physicist who agrees that a frame in which all stars have superluminal velocities is "equally correct" as frames in which they all move at less than c. In general we don't agree with that. Harald88 20:28, 26 April 2006 (UTC)

I don't think I really follow exactly what you dislike about that statement. I mean, the discussion above that is merely comparing the experience of two observers. How can anyone say that one observer is more correct than another? It seems to me like the very experimental nature of science itself forces physicists to accept both observers' experiences on equal footing. — Laura Scudder ☎ 22:53, 24 April 2006 (UTC)

See above. Harald88 11:40, 25 April 2006 (UTC)

I think that Harald's point has more to do with the fact that fictitious forces are not due to any "real" interactions of the physical system and merely an artifact of the choice of coordinates. We discussed this here some time ago. But in this article this isn't very relevant, because we focus on the use of the fictitious force technique. You could say the same thing about virtual particles being an artifact of perturbation theory.... Count Iblis 01:17, 25 April 2006 (UTC)

Yes indeed; and that makes unfounded statements about physicist's opinions of what point of view is or is not equally correct off-topic and thus soapbox, apart of being unencyclopedic. I'll now simply "be bold" and correct it, as too much time is wasted for a minor correction. Harald88 11:40, 25 April 2006 (UTC)

Your minor correction, aside from reflecting your POV, is rather misleading. Physicists commonly use the inertial frame for certain problems, and commonly use other frames for other problems. I'll see if I can't change it to something that's both accurate and won't annoy you, but it's frankly beyond me why people have so many philisophical issues with this article. -- SCZenz 22:50, 25 April 2006 (UTC).

That's fine of course. Note that your remark that "so many people" have issues with this article" is a real surprise of me for I understood from earlier in this conversation that all this was "non-controversial". In any case, we all agree that when presented as a calculation tool, it's indeed non-controversial. Regards, Harald88 20:28, 26 April 2006 (UTC)

If you like, I can refer you to some introductory university-level texts that discuss this technique in some detail. You did remind me to restore the emphasis that fictitious forces are a tool or technique—as such, there's really no such thing as propaganda for them since people can either use them or not and they'll get the same answer. Fictitious forces are used a lot in climatology, because a reference frame fixed to the surface of the earth is rotating once every 24 hours, as we discuss above. -- SCZenz 17:50, 22 April 2006 (UTC)

Thanks but I don't need to be instructed about that technique, it's pretty straightforward. And there is no need to introduce fictituous forces when mapping to a rotating frame; I can refer you to an introductory university mechanics book by Alonso&Finn on that. Harald88 10:33, 23 April 2006 (UTC)

Perhaps you can state clearly for us which sentences in the article are wrong or misleading, or write down what information we should add? -- SCZenz 17:23, 23 April 2006 (UTC)

I did at the start, and corrected it; but I haven't checked the whole article yet. Harald88 17:52, 23 April 2006 (UTC)

What's in a Name?

We observe an aircraft looping the loop and infer that 'real' forces are present, and can quantify them from the trajectory curvature, etc.. On board the aircraft, we can measure the acceleration directly. Contrary to the fundamentals of empirical science, we view the inference as 'real' and the measurement as 'fictitious'. Maybe the barf bag is fictitious also, and we are left with a not so fictitious mess to clean up.

If we combine the forces acting on bodies within the aircraft with those acting on the aircraft itself, and resolve them into inertial axes, divide by the mass, integrate twice, taking account of the change in direction of the forces over the integration period, and then resolve back, we get the correct motion within the moving frame of reference. But this is an extremely tedious process. It is far simpler to work in the non-inertial frame and introduce the additional terms. In so doing, force no longer equals rate of change of momentum, so we must introduce additional terms, derived from the known motion of the frame of reference, basically to recover Newton's Second Law in its simplest form.

In fact, all we are saying in introducing 'fictitous forces' is that the body moving in an accelerating frame of reference, itself has the acceleration of the frame of reference. The two accelerations must be added vectorially, and resolved to the moving frame for the motion to be predicted correctly. Resolving the acceleration of the moving frame from inertial axes into itself gives rise to the so-called 'fictitious' forces.

In short 'fictitious forces' are 'real' forces acting on a moving frame of reference which are resolved into the moving frame. Somewhere in the process of resolution they apparently left 'reality' behind.

Gordon Vigurs 09:56, 9 July 2006 (UTC)

You wrote "In fact, all we are saying in introducing 'fictitous forces' is that the body moving in an accelerating frame of reference, itself has the acceleration of the frame of reference."

That however is (indeed!) not fictitious at all! There is no necessity when doing that to introduce fictitious forces, and consequently most(?) textbooks don't do that. For the distinction, see the article centrifugal force. Harald88 20:59, 10 November 2006 (UTC)

still misleading statements

Checking the situtaion now, I immediately hit on an erroneous statement: "fictious forces could be felt easily by humans, as they are on a spinning carousel."

No fictitious forces can be felt; only true forces can be felt. Fictitious forces are calculation aids for rotating frames. One can't feel what doesn't exist!

What one feels in a spinning carousel is the contact force from the carousel which pushes against the body as it deviates the person from an inertial trajectory. Harald88 20:55, 10 November 2006 (UTC)

But a person's perception is that they are being pushed outward for some reason, and they have to hold on to avoid flying off. -- SCZenz 21:45, 10 November 2006 (UTC)