Discussione:Forza di Coriolis
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[modifica] Errori nell'articolo
Gameover 17:36, 5 lug 2006 (CEST) :
Nella sezione "Altre manifestazioni del fenomeno" è scritto "Sebbene la forza di Coriolis è debole e non ha influenza rilevabile su piccoli sistemi come i vortici nei lavandini, può però avere conseguenze a lungo termine. È stato osservato un consumo anomalo sulle rotaie ferroviarie riconducibili all'effetto Coriolis, che è anche causa dell'erosione più marcata su un lato dei letti fluviali.".
Nel collegamento Uno studio sugli scarichi dei lavandini viene discreditata questa teoria, con dei calcoli per me corretti.
Inoltre le immagini della formazione dei ciloni sono errate. Le correnti che provengono da est e da ovest in realtà non sono deviate. Esse hanno una velocità lineare, tangenziale rispetto alla rotazione della terra. Moltiplicando quindi vettorialmentele le velocità, la direzione risulta essere ortogonale all'asse di rotazione della terra.
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- La conversazione è attiva alla seguente pagina: Discussioni_Wikipedia:Progetto_Fisica#Errori_nell.27articolo_Forza_di_Coriolis.3F
[modifica] The counterintuitiveness of the coriolis effect
- Guam ha scritto: ( Progetto Fisica )
- Piccolo problema con l'articolo in oggetto. [...] en:User:Cleon Teunissen. Ho mantenuto quella che ho chiamato "Descrizione intuitiva", originariamente chiamata "An image of the Coriolis effect", tolta invece dalla wiki inglese. L'utente Teunissen mi segnala che secondo lui questa sezione è in contraddizione con il resto dell'articolo. --Guam 09:43, Lug 18, 2005 (CEST)
In the current coriolis force article on it.wikipedia.org the animation Image:corioliseffectanimatie.gif is used to construct the opening section. Unfortunately, there is a problem with that. The animation Image:corioliseffectanimatie.gif does not depict the coriolis effect, but something that only bears a resemblance to it. This is why I have removed that animation from the english version of the article.
The following text is exceedingly long, and I know I cannot reasonably expect anybody to read all of it.
Most of the things I discuss are well known standard physics concept. My intention is not to lecture, I know I am not saying anything new, my intention is to show the paths of my thought processes, to show exactly how I have arrived at my conclusions.
I discuss general relativity, in order to show why any reference to relativity in the Coriolis effect article is quite unnecessary.
[modifica] Inertia
First, I need to lay some foundations. I will start by writing about Inertia. I define inertia as the property of matter that in order to accelerate a force must be exerted. Inertia itself is not categorized as a force, because it is not an interaction between objects, there is no exchange of momentum as in for example the dynamics of the gravitational slingshot. Or, phrased in other words: Newtons third law, that for every action there is an opposite reaction does apply in the case of gravitation, but it does not apply in the case of inertia. It would be a contradiction of physics axioms to categorize inertia as a force. (From a logical point of view, the expression 'centrifugal force' contradicts the axioms of physics.)
Inertia is one of the fundamental elements of any theory of motion. It is equal in importance to the four fundamental forces of nature: Gravitation, Electromagnetism, Weak nuclear force, Strong nuclear force. Any theory of motion must acknowledge the part that inertia is playing, and newtonian dynamics and its successor relativistic dynamics both describe the part that inertia is playing.
Often inertia is loosely described as a form of resistance, but inertia is quite unlike that. Friction is a form of resistance. Friction is usually roughly proportional to the velocity. If the driving force is doubled then a higher top speed may be attained. When inertia is the only impediment, then doubling the exerted force doubles the acceleration. Inertia does not relate to velocity, inertia relates to the rate of change of velocity, inertia relates to the second derivative of position with respect to time.
There is an analogy between inertia and inductance. Inductance is the opposition to change of current strength in a coil with self-induction. Take a current circuit with a coil with self-induction. The coil does not resist current strength, but it does oppose change of current strength. Inertia is like that. Space-time does not put up any resistance against velocity. To velocity space-time acts like a superconductor. But space-time does oppose change of velocity. (It is because of this opposition that acceleration is proportional to the exerted force. Without inertia, objects would be accelerated to lightspeed instantly.) It must be recognized that inertia arises from an interaction of matter with the very fabric of space-time.
[modifica] the Flow model of space-time in cosmology
I came across a astrophysics article with an intriguing interpretation of the mathematics of general relativitythe Schwarzschild metric, River model of the Schwarzschild metric by Andrew J S Hamilton and Jason P. Lisle.
- As we show in §II, the Gullstrand-Painlevé metric provides a delightfully simple conceptual picture of the Schwarzschild geometry: it looks like ordinary flat space, with the distinctive feature that space itself is flowing radially inwards at the Newtonian escape velocity.
[...]
The picture of space falling like a river into a black hole may seem discomfortingly concrete, but the aetherial overtones are no more substantial than in the familiar cosmological picture of space expanding [...].
Professor Max Tegmark of MIT has incorporated the river model in his General relativity teaching MIT Course 8.033, Schwarzschild metric & black holes
- A natural interpretation of equation (2) (Hamilton 2004) is that space is flowing radially inward [...] and that particles can travel through this moving space according to the laws of special relativity [...] This is analogous to the FRW coordinates, where the “river” of space was expanding rather than flowing.
The background philosophy, as I understand it, is that the mathematics of General Relativity does not intrinsically enforce one interpretation or the other, so the interpretation of the theory is seen as a heuristic tool. The demands on the interpretation are that the interpretation is consistent with the mathematics, and free of self-contradiction. As I understand Hamilton and Tegmark, the flow model is a heuristic tool, it allows a set of features of curved spacetime to be arranged in a coherent picture.
[modifica] The principle of relativity of inertial motion
The principle of relativity of inertial motion revolves around the criterium of distinguishability. In uniform motion there appear to be two variables: the object being observed, and the observer. You can change the velocity of the object, giving it a new uniform velocity with respect to the observer, or the observer can change his velocity, giving himself a new uniform velocity with respect to the object. As it turns out, the results of those changes are indistinguishable.
Because of this indistinguishability, the inference is that there is in fact only one variable, the relative velocity, in uniform velocity only relative velocity exists.
[modifica] Uniform velocity and angular velocity
In the case of angular velocity it is obvious that unlike in the situation of uniform velocity the two variables are independent. There are two rotational variables, leading to four different outcomes.
A typical example of a rotating point of view is the videocamera in the MIT educational demonstration The videocamera is co-rotating with the rotating turntable, so the video-images show what is seen from a rotating point of view
The animation Image:Coriolis_effect12.gif represents a person sitting on a swivel chair, arms outstretched, holding weights in hands. The four possible situations are:
- 1a The chair is not rotating, the camera above is not rotating.
Since the chair is not rotating, no force is required for the weights (except the the force to withstand gravity) When the weights are being moved, nothing is seen to happen - 1b The chair is not rotating, the camera above is rotating.
Since the chair is not rotating, no force is required for the weights (except the the force to withstand gravity). On the video-images the chair appears to be rotating. When the weights are being moved, then there is no change in what is seen. - 2a The chair is rotating, the camera above is not rotating.
Since the chair is rotating, a force is required to sustain the circular motion of the weights, to pull the weight towards the center of rotation, extra force must be exerted. On the video-images a change of angular velocity is visible, at the moment the position of the weights is shifted. - 2b The chair is rotating, the camera above is rotating too.
Since the chair is rotating, a force is required to sustain the circular motion of the weights. On the video-images a change of angular velocity is visible, at the moment the position of the weights is shifted.
There is a fundamental difference between the space-time physics of uniform velocity and the space-time physics of angular velocity. In the case of uniform velocity only relative velocity exists. In the case of angular velocity it is straightforward to measure an object's angular velocity with respect to the local space-time that it occupies. Rotating objects are rotating in space-time, and you can measure locally, without any outside reference how fast the object is rotating.
In effect: uniform velocity is relative, angular velocity is absolute. (It is absolute in the sense that no outside reference is required to give a measure of it, the fabric of space-time itself provides the reference). Of course, the insights of the general relativity do modify the picture, the phenomenon frame-dragging must be taken into account.
Some physicists have a dislike for the situation that uniform velocity is relative, and angular velocity is absolute. Some physicists feel an urge to show that a scheme is possible to see angular velocity as relative too. Personally I think it is not worth the trouble, because either way the strength of the general theory of relativity is unaffected. Whether the general theory of relativity implements any of the flavours of Mach's principle does not affect the validity of the theory, so why bother to stretch for it? Mach's principle is not a particularly hot issue in cosmology, because it is inconsequential, it does not affect the mathematics of cosmological modeling.
[modifica] Summary
Uniform velocity is relative, angular velocity is absolute.
[modifica] Consequences for the 'Forza di Coriolis' article
In the animation Image:corioliseffectanimatie.gif the disk that is portrayed is frictionless. So that disk is not participating in what happens in that animation. Discarding that disk does not change what happens in that animation. What the animation Image:corioliseffectanimatie.gif shows is a ball cruising in uniform motion in space, no force acting on it. The ball is being videotaped with a rotating video-camera, so on the images of the video-camera the motion appears to be curvilinear motion. I call this alteration of images the 'rotating video-camera effect'. The 'rotating video-camera effect' is not physics. It is just a tumbling of of images.
In my edits of the english version of the article I describe exclusively examples of objects that are subject to one or more forces, enforcing (roughly) circular motion. In Image:Coriolis_effect12.gif there is one force, a centripetal force. Fluctuations in the magnitude of that centripetal force give rise to coriolis effects. It is the coriolis effect when there is a change of the rotational energy due to work being done. If no work is done, either by a torque or a centripetal force, then there is no coriolis effect.
The animation Image:corioliseffectanimatie.gif is about tumbling images, it does not portray the coriolis effect. No force is acting in that animation, no physics is taking place there.
The coriolis effect is about change of rotational energy due to work being done.
Cleon Teunissen
[modifica] Breve riassunto
Breve riassunto (short abstract for italian-speaking readers): Teunissen afferma che:
- La velocità lineare è relativa, in quanto è definibile solamente rispetto ad un altro sistema (e fin qui siamo d'accordo con Einstein). Diversamente la velocità angolare è assoluta, ovvero non richiede alcun sistema riferimento esterno (you can misure [it] locally, without any outside reference). Per questo non si può parlare della forza di Coriolis come di una forza relativistica.
- Nell'esempio illustrato nella sezione descrizione intuitiva, i due sistemi, ovvero la palla in moto lineare e il disco sono da considerare due sistemi distinti. La traiettoria della palla è lineare e uniforme e pertanto rappresenta un sistema inerziale. Non essendoci alcuna forza che compie un lavoro, non si ha alcun effetto Coriolis. La deflessione vista da una telecamera solidale al disco è pertanto solo un effetto visivo.
--Guam->@ 10:11, Lug 21, 2005 (CEST)
[modifica] Replica al punto 2 (reply to #2)
Supponiamo di sparare la palla da una posizione diversa dal centro, con il cannone in rotazione solidalmente al disco. Da quando la palla esce dalla canna si svincola dal disco rotante e per inerzia prosegue soggetta ad un moto trasversale pari alla velocità tangenziale del disco in quel punto. Mano a mano che la palla avanza verso l'esterno la velocità tangenziale del disco sottostante aumenta (raggio per velocità angolare). Per un osservatore in co-rotazione con il disco la palla devia. Questo fenomeno è considerato in balistica esterna e attribuito alla forza di Coriolis. Lo stesso meccanismo è esattamente quello che induce gli alisei a soffiare in diagonale invece che lungo i meridiani, e i meteorologi parlano di forza di Coriolis. Ora, se avviciniamo il cannone al centro, la velocità tangenziale tende a zero, poiché il raggio tende a zero. Se nel caso precedente parliamo di forza di Coriolis, questo è lo stesso caso tendente al limite.
Lets suppose to fire a ball from a gun placed on the disk, in a different point than the center. When the ball exits from the gun it is free respect to the disk, and it has an inertial lateral velocity equals to the tangential velocity of the disk at the radius where it was fired. As the ball go in the direction of the border the tangential velocity of the disk under it increases. This phenomenon is considered in en:external ballistics, where is considered an effect of the Coriolis force. This is exactly the same mechanism that causes trade winds to blow oblique, and meteorologists call this Coriolis effect too. Now, if we approach the gun to the center, the tangential velocity tends to zero (because radius tends to zero). If in the precedent example we talk about Coriolis effect, why we cannot consider this case as a limit (in the mathematical meaning)? --Guam->@ 10:30, Lug 21, 2005 (CEST)
[modifica] The two incarnations of the coriolis effect
There are two phenomena that are commonly referred to as 'coriolis effect'. One of them is what I call the 'rotating video-camera effect' (which is 'una effetto visivo'), the other is what I call 'coriolis effect'. For the time being I shall refer to the latter as 'the physical coriolis effect'
Because of the superficial resemblance, people often assume that the two effects are actually one and the same, and often the 'rotating video-camera effect' is erroneously presented as explanation of the physical coriolis effect.
[modifica] Thought experiment
The defining feature is whether there is physical contact or not.
Thought experiment: a huge space station is under construction: a cilinder a hundred meters in diameter, and hundreds of meters long, and this cilinder is rotating at an appropriate rate to elicit artificial gravity. Imagine this cilinder when it is not yet filled with air. An astronaut is floating around inside the cilinder, weightless, equiped with a space-suit with tiny thrusters. As long as the astronaut avoids physical contact he does not physically experience any coriolis effect.
Suppose a rope is strung from the station's axis of rotaton to the outer wall. Suppose the astronaut is at the axis of rotation, and he grabs the rope, then engages a thruster for a few seconds, giving himself a velocity towards the outer wall, intending to slide along the rope towards the outer wall. Having grabbed the rope there is physical contact with the rotating system!! If the astronaut keeps holding on to the rope, if he attempts to keep sliding along it, then he will physically experience a coriolis effect.
[modifica] Conceptual division
I put the conceptual division at physical contact: as long as the astronaut, floating around inside the rotating space-station is weightless he is not subject to any physical coriolis effect. As long as there is no physical contact the station can have any angular velocity, the weightless astronaut is unaffected.
When he attaches himself in one way or another, when the rotating system can exert a force on the astronaut, then physical coriolis effects may kick in.
For the english coriolis effect article I decided to describe exclusively the physical coriolis effect. Anybody who wants to introduce both effects must acknowledge the distinction between them. Two different names would then be needed, like: 'the illusory coriolis effect', and 'the physical coriolis effect'.
Guam wrote:
- [...]This phenomenon is considered in en:external ballistics,[...]
The discussion in the en:external ballistics article is about a situation that is a form of what I call the 'rotating video-camera effect'.
Cleon Teunissen
I have edited the en:external ballistics article, clarifying why in ballistics calculations a coriolis term is employed.
Cleon Teunissen
