This is an archive of past discussions about Fictitious force. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.
It is possible to characterise a "fictional force" as those which, in principle, do no work. William M. Connolley 13.06 5 Nov 2004 (UTC))
Here's a physicist's point of view:
Imagine the following setup: a beam of charged particles is led trough a succession of 8 chambers. In each chamber there is a uniform magnetic field with exactly the right fieldstrength to make the Lorentz force deflect the path of the particles 45 degrees. (Actually that is quite a normal design for a storage ring. Storage rings ar usually polygons, not the smooth circles of schematic drawings) In each chamber work is done, the momentum of the particle is changed. The 8 chambers together form a closed loop, effectively the charged particles are going around in a circle. In a cyclotron, where the motion is perfectly circular the same is valid. The momentum is changing all the time, so the force is doing work all the time.
That is rather spooky, how can a force that is perpendicular all the time do work?
One way of interpreting that is to consider circular motion as a superposition of two linear independent harmonic oscillations. That is valid in mathematics, and stunningly, it appears it applies to actual physics. If you set up a single harmonic oscillation then there is a constant back and forth conversion between kinetic energy and potential energy in that particular direction, and once a kinetic oscillation is set up its direction is conserved. Conservation of direction is a fundamental property of nature. With a single harmonic oscillation it is quite obvious that there is constant conversion of a conserved quantity of energy, and that there is conservation of direction. In circular motion there is also constant conversion, althought that is less obvious. Cleon Teunissen 22:34, 31 Jan 2005 (UTC)
the Feb 2 version of the article.
What a legacy Einstein left us! In every school physics is first introduced by teaching newtonian physics, and they should. Learning newtonian physics is such a good entry to physics, it teaches thinking physics, without overwhelming, you can't teach basic physics any other way. It's the only way to pave the way for relativistic dynamics. Schools should never skip newtonian dynamics. [...] only particle accelerator physicists and cosmologists use relativistic calculations, that's like point zero zero one percent of the physics community. So there we are: a science with two current paradigms simultaneously.
Err, don't get me wrong; if particle accelerator physicists wouldn't use relativistic calculations the machines wouldn't work, that proves beyond doubt that Einstein was right.Cleon Teunissen 07:50, 30 Jan 2005 (UTC)
I feel the article should start with discussing in newtonian terms and that it should subsequently move on to relativistic dynamics. the reader doesn't have to plough through the relativistic stuff, the newtonian discussion is sufficient. The only purpose of the general relativity section is to show that as far as the fictitiousness is concerned general relativity agrees with newtonian dynamics. Cleon Teunissen 07:37, 30 Jan 2005 (UTC)
The importance of inertia
Cleon Teunissen 16:02, 9 Feb 2005 (UTC)
In terrestial kinematics, inertia always manifests itself, and because of that it tends to be overlooked. Newton proposed the following definitions: if two forces balance each other, then there will be no acceleration. If they do not balance, or if there is only one force, then there will be acceleration. These definitions work well, but they tend to relegate inertia to obscurity. Inertia is a force to be reckoned with. But unlike for example friction, inertia cannot prevent motion, nor sustain it. Inertia is not only powerful, it seems, but also quite powerless.
When two objects collide, their inertia and relative velocity determine how much kinetic energy can be converted into damage and heat. There is one reference frame in which all kinetic energy is converted to damage and heat, that is de reference frame that is co-moving with the common center of mass of the two colliding objects. So single objects do not have an intrinsic kinetic energy, but if two objects are considered as a single system, then this system does have a unique amount of kinetic energy.
Gravity accelerates without manifestation of inertia, and that is very odd, for inertia is the universal "currency" of kinetic energy. When two objects are accelerating towards each other under the influence of each others gravity, and they collide, their kinetic energy has been building up, leading up to the collision. How can that be if there was no manifestation of inertia during the acceleration?
I don't quite understand that, but it seems to me that it must be related to the fact that gravity changes the progression of time.
The name 'fictitious force' is ill chosen, because the force involved is real physics; inertia is far from imaginary. It just that inertia cannot prevent acceleration. --Cleon Teunissen 16:02, 9 Feb 2005 (UTC)
Now the stage is set to get an understanding why gravity fits the description of a fictitious force rather well:
(William M. Connolley 17:10, 9 Feb 2005 (UTC)) Now the stage is set to get an understanding why gravity fits the description of a fictitious force rather well... which is a problem, because very few people would agree with you that gravity is fictitious. In some ways, what you have achieved is an a proof-by-contradiction: you have managed to show that your definition of fictitious force is not a good one. I would rather the article began with something like: "the concept of fictitious force is not clearly defined and is of no obvious physical use" and perhaps continued from there.
I shall rephrase that tongue-in-cheekish part of the article in order to avoid the misunderstanding you mention. Gravity fits the discription of fictious force to an extend, but not completely. I get the impression you have read the article only partly.
It says in the article that in the case of for example centrifugal force, releasing grip is the first step to getting out of trouble. After you have released grip (say a helicopter has lifted you from the rotating disk) you stil have a velocity, but that velocity won't increase; and it won't increase in any perspective, from local to universal. In the case of gravity, releasing grip is the last thing you want to do if you are hanging on to a rope under a helicopter.
But this does not address the deeper issue. Your paradigm is a different one. Your physics paradigm is alien to general relativity.
(William M. Connolley 17:20, 11 Feb 2005 (UTC)) I disagree. But I think yours is...
As is the way with paradigms, all words and expressions have a different meaning in your world. This is making communication very difficult.
Einstein was interested in what formulations are invariant under relativistic transformation,and the phyiscs community has followed his example. In special relativity there is the space-time interval:
(dx)2 + (dy)2 + (dz)2 - c2(dt)2
The space-time interval is invariant under Lorentz transformation, hence it is seen als more fundamental than space and time separetely. --Cleon Teunissen 22:32, 10 Feb 2005 (UTC)
(William M. Connolley 17:10, 9 Feb 2005 (UTC)) I would rather the article began with something like: "the concept of fictitious force is not clearly defined and is of no obvious physical use"
A statement of that sort would be an flagrant contradiction of physics facts.
(William M. Connolley 17:20, 11 Feb 2005 (UTC)) I disagree. 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?
Manifestation of inertia is definable and it is very important to take account of inertia correctly. I am in favor of replacing all mention of 'fictitious force' with 'manifestation of inertia'. --Cleon Teunissen 22:32, 10 Feb 2005 (UTC)
I mean: what I am in favor of is that in all future physics textbooks the expression 'fictitious force' is avoided, and that it is called 'manifestation of inertia'. In order to explain why the expression 'fictitious force' was introduced in the first place, an encyclopedia should devote an article to it. --Cleon Teunissen 22:48, 10 Feb 2005 (UTC)
(William M. Connolley 17:20, 11 Feb 2005 (UTC)) You are coming very close to (may have exceeded) the "no original research" guidelines. Wiki is not here for your personal opinions (or mine) but to report what is out there.
The invariance principle of relativistic physics
Cleon Teunissen 11:33, 11 Feb 2005 (UTC)
In relativistic phyisics, expressions that are invariant under transformation are considered the true 'nuts and bolts' of the underlying physics. An example from relativistic dynamics at non-relativistic velocities: when two objects collide, kinetic energy is converted to damage and heat. The amount of kinetic energy that is converted can be calculated in any inertial reference frame, it will allways come out the same: it is an invariant quantity. At relativistic velocities the full relativistic expressions must be used, and they have the same property.
The aim of relativistic physics is to allow observers in different reference frames to agree. Each observer has the full set of transformations at his disposal, so he can transform his instrument readings to what these readings would be in another reference frame than his own. Thus, two observers, performing transformations, can verify that they agree on the underlying physics.
The invariance principle of relativistic physics is that in all reference frames the same physics is going on.
An example from the history of phyisics. Before 1900 it was widely known in the physics community that the Maxwell equations are not invariant under galilean transformation. That seemed to indicate that going from one velocity to another, the physics of elektromagnetism would change. Lorentz showed that there is a set of transformations under which the Maxwell equations are invariant, these transformations are known as the Lorentz transformations. Einstein concluded that the Lorentz transformations are the fundamental transformations between inertial reference frames. Thus the central principle of relativistic physics is satisfied: in all reference frames, the same physics is going on. Any hypothesis that doesn't satisfy that criterium is alien to relativistic phyisics. --Cleon Teunissen 11:33, 11 Feb 2005 (UTC)
Variance theory is the opposite of the relativistic physics
But this does not address the deeper issue. Your paradigm is a different one. Your physics paradigm is alien to general relativity. --Cleon Teunissen 22:32, 10 Feb 2005 (UTC)
(William M. Connolley 17:20, 11 Feb 2005 (UTC)) I disagree. But I think yours is...
I think the paradigm you believe in is 'variance theory'. I call it variance theory because it is the opposite of the invariance principle of relativistic phyisics. I get the impression that you believe that a coordinate transform is an act with physical consequences. In variance theory, coriolis force and centrifugal force are considered to be the result of a coordinate transform. Conversely, according to variance theory it is possible to make a force disappear with the help of a suitable coordinate transform.
Can you confirm that this is your belief system? 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.
Do you reject the invariance principle?
Second question: is there something that would falsify your theory? What is your explanation of the behavior of gyroscopes? I insist that you show how your theory takes account of the behavior of gyroscopes. All frictionfree mounted gyroscopes on Earth will, after they have been spun up, display the sidereal rotation period of Earth, 23 hours, 56 minutes, 4 seconds. How do they obtain that information? --Cleon Teunissen 08:01, 12 Feb 2005 (UTC)
The no original research guidelines
You are coming very close to (may have exceeded) the "no original research" guidelines. Wiki is not here for your personal opinions (or mine) but to report what is out there. (William M. Connolley 17:20, 11 Feb 2005 (UTC))
That is an important guideline. Let me discuss an example where that guideline was neglected. In weather patterns the coriolis effect is causing air movement that would be absent on a non-rotating planet. In calculating weather, it must be taken into account that on Earth the manifestation of inertia is doing work. Yet in the coriolis effect article somebody stated that 'coriolis force doesn't do work'. This opinion is novel, and it is wrong: the coriolis effect is causing air movement that would be absent on a non-rotating planet; work is being done.
I support the policy that Wiki should report what is out there, and not personal opinions.
In science, I believe that Wiki should report the best that is out there. I mean: in science Wiki should not report folklore, it should report the best of scientific knowledge. For example, I think the Wiki article on 'tidal forces' (that you have contributed to) is excellent. Many if not most books and articles get it wrong, the current wikipedia article gets it right. (Explaining that a gravity gradient alone is sufficient for a tidal effect, and subseqently explaining in what way the geometry of orbiting contributes to tidal effect.) --Cleon Teunissen 08:33, 12 Feb 2005 (UTC)
GPS incorporates the Sagnac effect for optimal accuracy
--Cleon Teunissen 21:57, 15 Feb 2005 (UTC)
The Global Positioning System is incredibly precise, its accuracy is being pushed to below the one meter range. GPS achieves its accuracy by correcting for a range of effects, among them the geometric phenomenon called Sagnac effect. A GPS around a non-rotating planet would not have to deal with a Sagnac effect.
The non-rotating reference frame is the only frame in which there is no Sagnac effect; this is one of the ways rotating reference frames can be distinguished from each other. The angular velocity of a reference frame can be measured by several methods, one of which is to measure the magnitude of the Sagnac effect in that reference frame.
The laws of physics are identical in all inertial reference frames: this does not extend to rotating reference frames. This is of course recognized in general relativity. Special relativity was developed in response to the finding that all available evidence indicates that the same physics is going on in all inertial reference frames. General relativity is called general because it provides a complete set of mathematical tools for all transformations, whereas special relativity only covers the relativity that is expressed in the Lorentz transformations. In other words: general relativity not only describes gravity, but it also enables physicists to calculate for example the relativistic coriolis effect. (And general relativity predicts that the Sagnac effect, like all other physics, will be subject to frame dragging, whenever there is frame dragging.)
Rotating reference frames are distinguishable. Therefore rotating reference frames are not equivalent for describing physics. --Cleon Teunissen 21:57, 15 Feb 2005 (UTC)
Now consider a process in which observers in the rotating frame attempt to use Einstein synchronization (that is, the principle of the constancy of the speed of light) to establish a network of synchronized clocks.
Observers fixed on the earth, who were unaware of earth rotation, would use just use the coordinate distance for synchronizing their clock network. Observers at rest in the underlying inertial frame would say that this leads to significant path-dependent inconsistencies, which are proportional to the projected area encompassed by the path. Consider, for example, a synchronization process that follows earth’s equator in the eastwards direction.
From the underlying inertial frame, this can be regarded as the additional travel time required by light to catch up to the moving reference point. Simple-minded use of Einstein synchronization in the rotating frame uses only the coordinate distance, and thus leads to a significant error. Traversing the equator once eastward, the last clock in the synchronization path would lag the first clock by 207.4 nanoseconds. Traversing the equator once westward, the last clock in the synchronization path would lead the first clock by 207.4 nanoseconds.
In an inertial frame a portable clock can be used to disseminate time. The clock must be moved so slowly that changes in the moving clock’s rate due to time dilation, relative to a reference clock at rest on earth’s surface, are extremely small. On the other hand, observers in a rotating frame who attempt this, find that the proper time elapsed on the portable clock is affected by earth&'s rotation rate.
Path-dependent discrepancies in the rotating frame are thus inescapable whether one uses light or portable clocks to disseminate time, while synchronization in the underlying inertial frame using either process is self-consistent.
GPS can be used to compare times on two earth-fixed clocks when a single satellite is in view from both locations. This is the common-view method of comparison of Primary standards, whose locations on earth's surface are usually known very accurately in advance from ground-based surveys. Signals from a single GPS satellite in common view of receivers at the two locations provide enough information to determine the time difference between the two local clocks. The Sagnac effect is very important in making such comparisons, as it can amount to hundreds of nanoseconds, depending on the geometry. In 1984 GPS satellites 3, 4, 6, and 8 were used in simultaneous common view between three pairs of earth timing centers, to accomplish closure in performing an around-the-world Sagnac experiment. The centers were the National Bureau of Standards (NBS) in Boulder, CO, Physikalisch-Technische Bundes-anstalt (PTB) in Braunschweig, West Germany, and Tokyo Astronomical Observatory (TAO). The size of the Sagnac correction varied from 240 to 350 ns. Enough data were collected to perform 90 independent circumnavigations. The actual mean value of the residual obtained after adding the three pairs of time differences was 5 ns, which was less than 2 percent of the magnitude of the calculated total Sagnac effect.
External link: GPS overview According to this source, the first GPS satellites that were launched could switch to either a newtonian compliant mode, or a relativistic compliant mode. It soon became clear that the system had to comply with general relativity in order to be accurate.
The part ot the article on general relativity has been moved
I have shortened the article by moving the 'general relativity' section.
The method I used is an improvisation, and probably a method that is recommended against. I have used image space for extra article length.
I have modeled this on the tidal force article, where the reader can follow a link to a larger image. The page with that image has some explanatory text.
If there are disadvantages to this method that I am unaware of please inform me. My guess is that image space is not to be used for article; I guess on that page the history won't be recorded.
I hope there is another way of achieving this. I don't think it is enough just to link to the general relativity article, for that article does not focus on discussing the similarity and difference between gravity and fictitious force.~--Cleon Teunissen 13:16, 16 Feb 2005 (UTC)
I moved it back; it was not a good idea. --Cleon Teunissen 16:51, 20 Feb 2005 (UTC)
Perp to V
(William M. Connolley 20:03, 23 Mar 2005 (UTC)) Coriolis is f K cross V. It is always perpendicular to the motion, independent of what the motion is. Indeed, since the coriolis is an instantaneous force (so to speak) it doesn't "know" if the motion is circular or not, not being able to perceive the past or the future.
The coriolis force is certainly perpendicular to the direction of motion in the case of inertial wind. When the motion is inertial, when only the coriolis effect is involved, the coriolis force is perpendicular to the direction of motion.
But when a mechanical force tends to accelerate an object in one direction, while a coriolis force tends to accelerate it in another direction, what is the formula then? What formula describes that dynamic equilibrium? That is a more complicated situation than in the case of centrifugal force.
When motion is circular the direction of acceleration is perpendicular to the direction of velocity. In other non-circular motion, the direction of accelereation is at another angle than perpendicular to the direction of velocity. I'm not sure how that affects the coriolis inertia.
We may not be using exactly the same demarcation of 'centrifugal force' and 'coriolis force' I have encountered two quite different conventions: Coriolis_effect#Defining_coriolis_force --Cleon Teunissen | Talk 14:37, 24 Mar 2005 (UTC)
The analysis of Dragon flight of the Lorentz force example was good. I will remove the remark about not perpendicular. --Cleon Teunissen | Talk 14:43, 24 Mar 2005 (UTC)
Revert
I've long disliked CTs version of this article, as the "discussion" above makes clear. I've really never been sure that he knows what he is talking about, and following the discussion at coriolis effect and Inertial frame of reference has made me even more doubtful. CTs articles are characterised by the "image a spaceship" style of writing, which I find very unhelpful and handwavy. So... here is my preferred version. William M. Connolley 14:56:54, 2005-08-02 (UTC).
I will not contest the reversion. However, much of the ground that Cleon was trying to cover is valid. The examples of what you feel when a car accelerates are good ones, and I call for that material to be restored to this page, but in context with what is now here and in a more conpact form.
OK. Thanks for taking a look. I too won't contest the re-introduction of some of CTs stuff in context and (as you say) more compact. I'm taking your Therefore gravity acts like a fictitious force, and in general relativity, it is one. on faith, since my GR doesn't go nearly that far. Is that (forgive me) a generally recognised view of gravity in GR? I suppose I mean, does anyone call it "fictitious" in papers and suchlike? William M. Connolley 17:23:16, 2005-08-02 (UTC).
Under the equivalence principle as it was originally stated, freefall is inertial motion. Therefore gravity is not a real force in general relativity. I don't know that anyone actually calls gravity "fictitious". After all, that is not going to keep you from getting killed if you jump off a cliff. However, it is a key insight of Einstein's that it is in the same "boat" as centrifugal "force", etc.
If you really don't like it, you can delete it. I won't mind that. What I do mind in GR being brought up incorrectly. --EMS | Talk19:00, 2 August 2005 (UTC)
Sorry: I wasn't complaining. You know more about it than me: I was trying to understand it for myself. William M. Connolley 19:23:15, 2005-08-02 (UTC).
No need to apologize. I did mean to accuse you of complaining. Instead, it is a matter that I debated between removing or updating the GR part of this article. Gravity as a "pseudo-force" is relevant but it is also a subtle point (as well as being amazing). If it seems to be causing people grief, then feel free to remove it. It is nice to have, but it does not need to be here. --EMS | Talk20:42, 2 August 2005 (UTC)
I'm happy with it in. Can you (for my own enlightenment really rather than the article) point me towards any kind of discussion of gravity-as-a-psuedo force (if at all possible, with comparisons to Coriolis) in any of the sci literature (it would even be worth renewing my university library card for...). William M. Connolley 22:22:16, 2005-08-02 (UTC).
I sometimes get the impression that William M Connolley feels that 'coriolis force' ought to be recognized as a fundamental force in itself. William M Connolley has expressed serious doubts whether any of the current meanings of "fictitious" applies at all for 'coriolis force'.
William M Connolley doesn't commit himself, and I think he should: there are the four fundamental interactions of nature: Gravitation, Electromagnetism, w. nuclear, s. nuclear. Does William M Connolley feel that 'coriolis force' ought to be recognized as a fifth fundamental?
Of course, coriolis does not constitute a fifth fundamental, looking at coriolis is looking at the role that inertia plays in motion, which of course is a very important role. --Cleon Teunissen | Talk08:16, 3 August 2005 (UTC)
EMS - thanks. CT: you're talking nonsense. William M. Connolley 17:19:33, 2005-08-03 (UTC).
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 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)
The Physics is weak, to say the least.
You should explain more clearly what you mean by "but the fictitious forces do not necessarily obey Newton's first law". If your claim is really true, then also define clearly what are the physical difference between a "real" and a "fictitious" force. First law defines mass (same as inertia) and the second law defines force. Force is not defined by interaction. Certain kinds of forces give rise to a potential. Instead of using a vector force field, it is more convenient to use a scalar potential field. Interaction is often used to define an arbitrary energy (potential energy).
For a clearer idea of the centrifugal force, consider this "thought experiment". You are sitting at the centre of a infinite disk rotating at a constant angular velocity. You gently (no initial velocity) release a ball of mass m at any point (except the centre point) on the surface of the disk and observe its motion. Then you realize that you are sitting in a force field. If you want, you can map this force field. This is what is commonly called the centrifugal force.
You next tie this ball with a thread and attach the other point of the thread to the centre. You will notice that there is a tension in the thread and the ball is now stationary. The tension in the thread is called the centripetal force. The ball is stationary as the centrifugal and centripetal forces cancel exactly at any given point. What happens if you put a needle suspended at the centre (mid-point of the needle)?
Next: all inertial frames are force-free. All non-inertial frames contain forces. The nature of the force field depends on the nature of the inertial frame. Using the example you have cited, an accelerating (or decelerating) frame of reference (in a straight line; it is really necessary to add?) is equivalent to a constant force. Your example of a passenger in a car merits more discussion. It is not a rigid body (passenger and the seat being considered) and the internal forces do not cancel (they do some work). The car, moving at a constant velocity is an inertial frame and no forces at on the passenger. When accelerating (or decelerating) there is a force on the car which is also felt by the passenger and this in turn causes the compression of the seat. The description written is wrong but the effect shown is correct. This compression, which is a change in the thickness, is same in all frames. What is "fictitious" here? Remember that the force is defined by its effect (second law) and not whether you feel or think about it (you do not feel the forces, you feel the effects).
The explanation given in ref 13 is wrong. Ground reaction is always perpendicular to the surface and if the road is not a straight line, we use the potential energy. Consider a circular track which is banked at a suitable angle. The driver thinks that he is moving in a straight line and if the banking is correct, the vehicle does not see any centrifugal force (it is taken care of by the banking). It is a real force and the passengers will still feel the effects of the centrifugal force (the car is not a rigid body with passengers). In absence of friction, the car will always skid (except at the speed for which the banking is designed).
My major objection to the use of "fictitious" is clear: what about the work done by the "fictitious" forces? Are they also equally "fictitious"?
You claim that the total energy (KE + PE) can be different in different frames. You can certainly make a perpetual motion machine if you stick to these ideas!
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. Salsb11:42, 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? ‣ᓛᖁᑐ06:10, 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? Salsb11:47, 21 September 2005 (UTC)
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. Connolley08:17, 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? ‣ᓛᖁᑐ08:27, 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. Salsb11:57, 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 Iblis12:46, 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. Connolley13:07, 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 Salsb13:30, 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 | Talk14:16, 21 September 2005 (UTC)
Let me state some points very clearly:
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. ISBN0070350485.
"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. -- SCZenz15:16, 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.
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. Connolley21:31, 21 September 2005 (UTC).
William, I have been absolutely clear. I'll try saying everything a different way.
"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.
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?
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.
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.
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.
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. -- SCZenz22:01, 21 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. Connolley08:40, 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. -- SCZenz00:35, 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. -- SCZenz00:35, 22 September 2005 (UTC)
Oh, and one more question... What do you mean there aren't any examples in the article? The examples include:
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. -- SCZenz01:26, 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. Connolley08:40, 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. Connolley15:41, 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. Connolley15:41, 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. -- SCZenz13:15, 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. Connolley15:41, 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. Connolley17:02, 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.
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 Salsb17:37, 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. Salsb18:59, 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. ‣ᓛᖁᑐ17:33, 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. Connolley18:08, 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. Salsb18:59, 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! -- SCZenz17:34, 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. -- SCZenz17:34, 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:
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. Connolley18:34, 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. -- SCZenz17:34, 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. -- SCZenz01:54, 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. ‣ᓛᖁᑐ12:51, 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. Salsb12:33, 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. -- SCZenz13:15, 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. Salsb13:58, 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? -- SCZenz18:05, 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. -- SCZenz21:38, 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 | Talk18:55, 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. -- SCZenz19:04, 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. Salsb19:31, 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 | Talk03:29, 23 September 2005 (UTC)
All forces act in direct proportion to the mass of the object. What is new in this? This is part of the second law.
If you mean model that defines the force (e.g., in electrostatics) then it is not part of the Newtonian Mechanics. In electromagnetic theory, we have a vector potential and some mechanics and we land up with Maxwell's equations. You see, these forces are also "fictitious" in your sense but real in any way you see.
Any force (must be conservative) can be locally "eliminated" by a suitable transformation of the local coordinate frame. Does this mean all forces are "fictitious" in nature?chami 11:38, 3 August 2010 (UTC) —Preceding unsigned comment added by Ck.mitra (talk • contribs)
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! -- SCZenz19:00, 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. Connolley18:34, 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. -- SCZenz19:00, 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.) -- SCZenz19:00, 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! -- SCZenz17:34, 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
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. -- SCZenz19:00, 22 September 2005 (UTC)
These are not part of the Newtonian Mechanics.
Four fundamental forces are strong forces (nuclear), electromagnetic, weak and gravitational.
Force is a concept; only conserved quantities are observables. We use mass, momentum, total energy etc. In relativity, we have one quantity called the energy momentum tensor that is conserved (total energy is not conserved).
We can figure out whether we live in an inertial frame or not.
Mechanics does not permit us to look at the origin of the forces.
Although forces are useful concepts, potentials are more convenient.
What do you mean by contact forces?
chami 11:52, 3 August 2010 (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 Iblis21:37, 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. Connolley15:05, 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. -- SCZenz18:05, 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 Iblis16:17, 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. Connolley16:41, 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 Iblis17:20, 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. -- SCZenz18:05, 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 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. –Joke13721:59, 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? -- SCZenz00:33, 24 September 2005 (UTC)
"It is extremely rare for space-time geometry to rotate significantly with respect to the universe as a whole,"
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 | Talk22:39, 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 | Talk22:46, 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. -- SCZenz04:14, 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 | Talk16:16, 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 | Talk23:01, 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. -- SCZenz23:05, 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 Iblis00:20, 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.. ;) -- SCZenz00:25, 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 | Talk03:42, 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. –Joke13702:09, 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. GangofOne03:10, 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. -- SCZenz04:10, 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. –Joke13702:09, 28 September 2005 (UTC)
Sorry for the slow response. Here is my point. If you have a simple equation of motion and change to curvilinear coordinates y, then the transformed equation of motion is the second total derivative , given by
or
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 in cylindrical coordinates. –Joke13718:00, 2 October 2005 (UTC)
In fact, this is kind of like the geodesic equation in GR, which is
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? -- SCZenz18:56, 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 | Talk22:16, 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!) -- SCZenz23:05, 2 October 2005 (UTC)
Map and car frame perspectives of physical (red) and fictitious (blue) forces for a car driving from one stop sign to the next.
In this illustration the car accelerates after a stop sign until midway up the block, at which point the driver is immediately off the accelerator and onto the brake so as to make the next stop.
For the accelerated frame this shows both the real and fictitious force. Which is incorrect in this case.
Rain and shell frame perspectives of physical (red) and fictitious (blue) forces for an object that rolls off a cliff.Note: The rain frame perspective here, rather than being that of a raindrop, is more like that of a trampoline jumper whose trajectory tops out just as the ball reaches the edge of the cliff. The shell frame perspective[1] may be familiar to planet dwellers who rely minute by minute on upward physical forces from their environment, to protect them from the geometric acceleration due to curved spacetime.
For the rain frame it shows the ball going down as the rain drop falls, whereas it should be coming up because of the fictitious force in the rain frame. Including these figures, although were well intentioned, ends up misguiding the reader. These need to be revised by the uploader. --Fatka (talk) 23:20, 4 November 2008 (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?--24.202.163.19403:27, 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 Makholm10:33, 2 January 2006 (UTC)
By coordinate system do you mean like the rectangular coordinates, the cylindrical polar coordinates and the spherical polar coordinates?--24.202.163.19402:34, 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? --24.202.163.19415:04, 3 January 2006 (UTC)
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.
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ᑐ 05:25, 21 September 2005 (UTC)
ᑐ 08:20, 21 September 2005 (UTC)
