Talk:Centrifugal force/Archive 5

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PeR added a comment to an existing post, way at the top of this talk page. I copy PeR's comment here:

The scope of this article extends beyond the first month of high school physics. What high school physics teachers should say is "The centrifugal force does not exist in an inertial frame of reference". But then they would have to explain what a non-inertial frame of reference is, and that is usually saved for the university courses. --PeR 09:02, 11 January 2007 (UTC)

The condition "does not exist in an inertial frame" is blurry, and it does not qualify as a physics statement. Physics is about the entities that are frame-independent. That is: independent from the way we assign numbers to certain states. For example, for temperature there is the scale of Fahrenheit and there is the scale of Celsius. Each scale assigns different numbers to the melting point of ice and the boiling point of water, and this difference is irrelevant for physics considerations. Calculations can be interconverted between units of Fahrenheid and units of Celsius. Likewise, calculations can be interconverted from mapping motion in an inertial coordinate system or a rotating coordinate system. Either way, inertia exists, independent of how a particular motion is mapped. Inertia exists, and when an object is forced into non-inertial motion, the object's inertia manifests itself.

This is a matter of principle: the building blocks of physics theories are entities that are independent of the way that motion happens to be mapped.

Summary:
Independent of whether motion is mapped in a inertial coordinate system or in a rotating coordinate system, one can recognize the role that inertia of objects is playing in the physics taking place. What teachers should teach is that inertia exists, and how it plays a part in physics taking place. --Cleonis | Talk 13:50, 11 January 2007 (UTC)

Cleonis, you and I have iterated this discussion ad nauseam already on my talk page and in other places. I agree with everything that you say above. Fictitious forces do not exist in the sense of fundamental interactions, and often physics teachers don't explain inertia in the best way.

However, the question was whether the page should be replaced with a short text referring readers to the centripetal force page, because of what the anon's physics teacher had said. This I disagree with. The concept of fictitious forces does exist, as a useful mathematical tool, when dealing with non-inertial reference frames. As an analogy, negative numbers do not exist from a certain philosophical perspective, but the concept of negative numbers is very useful in mathematics, and hence it is a good idea to keep that page as it is rather than replacing it with a text stating that they don't exist, and then pointing readers to the natural numbers page. The same is true for this page. --PeR 12:21, 12 January 2007 (UTC)

I suppose this relates to ambiguity in the verb 'to exist'. Often in physics a discussion may arise as to whether something actually exists. For example, from a human perspective, we may well get the impression that there is some cold-suction action, where a particular cold actively sucks warmth away. From a physics point of view, there does not exist an active suction from cold being exerted on warm regions. So I prefer to be very cautious with the verb 'to exist'. If a calculation would be set up in which a fictitious cold-suction-force is applied, I'd still emphasize that no cold-suction force exists. In the case of applying a mathematical tool, there is no need to assert that it "exists" (inviting the ambiguity of that verb), it suffices to assert that the tool is regularly applied, and that it is very useful.

Ambiguity cannot be eliminated entirely, but every opportunity where it is easy to avoid ambiguity should be used. --Cleonis | Talk 14:21, 12 January 2007 (UTC)

Heat transfer works just the same with signs reversed. In fact, on the temperature scale which Anders Celsius originally used, increasing numbers represented colder temperatures. Absolute temperature is a rather recent invention.

One could say that the centrifugal force does not exist in the same sense that "cold" does not exist. But I think you're right in that one probably does best in avoiding the word "exist" altogether. --PeR 21:38, 13 January 2007 (UTC)

So do we have a consensus to rewrite or remove all assertions that such-and-such concept does or does not "exist"? Henning Makholm 00:49, 14 January 2007 (UTC)

It does seem that there is such a consensus; asserting the usefulness, without using the verb 'to exist'. For the example of heat transfer it means that it would be asserted that what exists (according to our theories) is 'entropy'; heat will flow from the warmest regions to any less warm regions. --Cleonis | Talk 11:17, 14 January 2007 (UTC)

I originally created this section about a year ago as part of an attempt to make what the article now describes as "fictitious centrifugal force" its main topic and then explain how in limited cases the "reactive centrifugal force" also makes some sense. It appears that my point was rather poorly made, and that the section has survived to this day mainly because no subsequent editors have felt they understand it well enough to remove it. In the context of how the article looks now, I think the section is confusing and misplaced; I'd like to delete it all. Has anybody secretly grown so fond of it that they would complain if I did? Henning Makholm 00:46, 14 January 2007 (UTC)

I think the opening sections already cover what is addressed in the section 'centrifugal force in statics', and that the opening sections do a better job.

Anyway, the scope of statics is objects that are in equilibrium. To test the load-bearing capacity of a suspension bridge, a large amount of heavy vehicles is parked halfway the length of the span of the bridge. Another part of testing a bridge is to find out whether winds can trigger an oscillation of some parts of the suspension bridge. That is dynamics. Calculations/modeling in statics do not involve inertia, calculations in dynamics do. --Cleonis | Talk 11:39, 14 January 2007 (UTC)

So removed. For what it's worth, the intended relevance of "statics" was a situation where one wants to design a complex system that rotates (i.e. where all the constituent parts are supposed to follow the same constant rigid rotation) such as a centrifuge or a flywheel, and we must find out whether the various contact forces and internal stresses will suffice to supply the necessary centripetal forces. This can be done most easily in a rotating frame where nothing moves and the various shortcuts of statics are thus available, if only we add in artificial loads from the centrifugal force. Henning Makholm 01:11, 17 January 2007 (UTC)

Of course, in the case of rotation of solid bodies the analysis reduces to statics.

Interestingly, it's quite rare that the analysis reduces to the statics case. For example, in the case of helicopter blades, a major concern is that when the rotating helicopter blade bends upwards, it's center of gravity moves closer to the rotation axis, resulting in an increase of the blade's velocity. More blade velocity gives more lift, which tends to increase the upward bending. When the frequency of the blades flapping up and down coincides with the rotation rate, this mechanism of self-reinforcing vibration can cause the helicopter blades to shatter. Rotor designs must incorporate dampening to dissipate the energy of vibrations. Any device with rotating parts is prone to developing self-reinforcing vibrations.

Generally, what needs to be modeled is rotation with a variable rate of rotation. When a calculation is set up for helicopter blades with an oscillating rotation rate due to vibration, the coordinate system that will be used is a coordinate system with a constant rotation rate, hence the centrifugal term will be constant. The amount of reactive centrifugal force will oscillate, since the rotation rate of the blade oscillates. Whenever rotation is accompanied by vibration, the centrifugal term and the reactive centrifugal force are distinct.

Summerizing:
Generally in using rotating coordinate systems, the centrifugal term and the reactive centrifugal force do not coincide, they coincide only when the rotating body can be considered vibration-free. --Cleonis | Talk 02:14, 17 January 2007 (UTC)

I'm really not seeing the relevance of this reference, even applied inline. What does it have to do with centrifugal force? Particularly odd with the comment that reactive centrifugal force can 'even smash DVDs'. Vandalism/SPAM? Or am I missing something? Skittle 23:20, 10 February 2007 (UTC)

There is a single line in there that if a DVD should spin fast enough, the disk will shatter. So the reference is relevant, but the relevant line is hard to find, and that makes it a lousy reference. In addition, I don't see that the article quite counts as a reliable source. My advice is to remove the DVD shattering phrase and the reference. --EMS | Talk 05:16, 11 February 2007 (UTC)

I agree. The DVD shattering phrase seems to try to convince a reluctant reader that the "reactive" centrifugal force is a real force. This is problematic for two reasons. First, nobody disputes the reality of the reaction force. (Some of us do hold that it is not conceptually interesting, but that is a different question). Second, it is not NPOV, by the rule of thumb is that if you find yourself trying to convince an imagined reader who you think would not accept a plain statement of fact, you're pushing a POV. An encyclopedia should not aim to stuff truth down the reader's throat; it should simply state plainly that such-and-such is the accepted truth, and then leave the reader to wallow in his delusions if he so chooses. Henning Makholm 16:55, 11 February 2007 (UTC)

So removed. Henning Makholm 21:21, 13 February 2007 (UTC)

Since Cintrifugal force doesn't exist i think we should just move the article to be called centripital or w.e

No. Read the section entitled "Confusion and misconceptions". --PeR 12:14, 17 March 2007 (UTC)

To change the name of the article is an absolutely ludicrous suggestion. The argument has been over whether or not the centrifugal force is real or fictitious. Even both sides in the argument are agreed that it exists one way or the other. And both sides in the argument are in total agreement about the fact that whatever centrifugal force is, it is most certainly not the same thing as centripetal force. David Tombe 15th April 2007 (61.7.159.103 08:38, 15 April 2007 (UTC))

I removed a line which read as "Note that this real centrifugal force does not appear until the person touches the body of the car." This is erroneous, as the car exerts this force on the person whether or not they are touching the body of the car.

You're wrong. First, the force this sentence speaks about is not one that the car exerts on the passenger, it is the force with with the passenger pushes the car door outwards. Second, this force does only appear once the passenger makes contact with the car door. (We ain't having no action at a distance here). You may not want to call this force a "centrifugal force", and you'll be very welcome not to; I don't either. But it seems to be fact that some people do call it a centrifugal force; consensus ended up being that this meaning of the word should be explained in the article too. I will revert. –Henning Makholm 19:45, 30 April 2007 (UTC)

I'm not sure the line: "In this case the centrifugal force is canceled by the centripetal force, and the net force is zero, thus the person does not accelerate with respect to the car." is particularly well chosen. In what sense are these forces "canceled"? You state correctly that the "centripetal" force is the force of the door on the passenger, and that there is a force that some refer to as "centrifugal", and that this is actually the (reaction) force of the passenger on the door of the car. These are simply the Newton's 3rd Law action and reaction pair of forces. To say they "cancel" is very misleading! They act on different objects. They do not cancel. The "net" force you refer to would need to be the result of two or more forces on the same object. ChrisBSc 12:22, 30 May 2007 (UTC)

You're right: that sentence is wrong when speaking about the reaction force. I have removed it. –Henning Makholm 15:05, 31 May 2007 (UTC)

The tone and some of the content of this section should be kept for the talk page. Not everyone teaching or learning in a high/secondary school lacks an understanding of what centrifugal force is. This is an encyclopaedia not a gossip column, you don't want to insult future contributors or readers. If this section really needs to stay then where are the referenced school graduate statistics to back it up? MattOates (Ulti) 08:29, 27 August 2007 (UTC)

I can see that the text could be misunderstood as meaning that the teachers lacked understanding. What it was meant to say was that teachers (who presumably do understand centrifugal force and when it is valid to use it) often do not have time to pass on that understanding to their students, simply because rotating frames are not part of what they are supposed to teach. The students in the mid-bracket who are bright enough to understand what they are taught, but not so good that they'll invent on their own what they aren't, will be left with a half-truth, not because the teacher is ignorant of the full truth, but because the half-truth is all the curriculum requires them to learn.

I have tried to edit the sentence to prevent the misunderstanding. Does the current form address your complaint? –Henning Makholm 21:19, 13 September 2007 (UTC)

I deleted the entire Derivation section because it didn't add any information beyond what was in the previous section, I know for a fact that the large number of equations are imposing and dissuade some readers, and it was very long. Wikipedia is WP:NOT not a how-to manual. If there was actually some useful information imparted beyond an algebra lesson only tangentially related to the subject of the article, then I will apologize. But there wasn't as far as I could see. ←Ben^B4 17:31, 12 September 2007 (UTC)

Well done! --PeR 17:23, 13 September 2007 (UTC)

I must admit, I would love to negotiate a license to get this in the article: [1]; but I'm sure somebody would take it out on tone reasons or something. Lots of physics text books have little cartoons like that in though, so there's quite a bit of precedent.WolfKeeper 22:13, 1 October 2007 (UTC)

I'd support the addition. The motivation for keeping it in the article (except for the fact that it is funny) is that it illustrates the debate on the "existence" of the centrifugal force in a good way. (Because it comes from an external source, it is in a way "evidence" that the debate exists. - The artist knew that people would recognize the discussion. If somebody had drawn the same cartoon after reading the wikipedia article, for the explicit purpouse of inclusion in the article, I'd be opposed.) See also Talk:Fictitious_force#Recommended reading --PeR 08:01, 2 October 2007 (UTC)

I love it - it's nicely pointing at the debate, without taking position. Great! Perhaps they will agree it to be included with mention of the source? That's kind of publicity for that site (allowed, right?) Harald88 21:01, 2 October 2007 (UTC)

You can ask at User talk:Xkcd - I bet he will relicense it without the -NC in the CC license so we can use it. 209.77.205.2 04:13, 3 October 2007 (UTC)

I removed the following subsection (recently added in good faith by User:Agge1000) from the article:

If a problem exhibits polar, spherical or cylindrical symmetry, a polar, spherical or cylindrical coordinate system can be used. These are "rotating reference frames" in themselves, even if nothing is physically rotating. For instance applying spherical coordinates to a system of an object moving around on a non-rotating planet or polar coordinates when addressing planetary orbits will often result in centrifugal forces appearing. The cause is the same as when "attaching" a (in itself inertial) cartesian coordinate system to the surface of a rotating planet or a turning car.

It is true that one needs correction terms that modify coordinate accelerations if one uses non-uniform coordinates such as polar coordinates. However, I do not think these corrections can meaningfully be described as "centrifugal forces". For example, if you place a free particle at rest anywhere in a non-rotating polar coordinate system, it will stay at rest, i.e. its coordinate acceleration (as well as its true acceleration) will be 0. If there was a centrifugal force, the particle should have an acceleration away from the axis, but it hasn't.

It may be meaningful to describe the corrections more generally as fictitious forces, but I am not sure that this is actually a common way of handling the problem. Some sources would be necessary for this. –Henning Makholm 15:11, 16 November 2007 (UTC)

If we formulate it like this. In cartesian coordinates we may have

${\displaystyle {\frac {d^{2}\mathbf {r} }{dt^{2}}}={\frac {GMm}{r^{2}}}{\hat {r}}}$

imagining that "m" is just a test-particle so "M" can be fixed in the Origo. Now if we instead want to use "r" as the radial component in polar coordinates instead we get

${\displaystyle {\frac {d^{2}r}{dt^{2}}}={\frac {GMm}{r^{2}}}-r{\dot {\theta }}^{2}}$

It is the extra fictitious force in the last equation I would like to refer to as "centrifugal". Perhaps it has another defined name? Do anyone know what the proper name is?-- Agge1000 (talk) 17:40, 16 November 2007 (UTC)

Perhaps it could be reinserted formulated alomg these lines?-- Agge1000 (talk) 17:40, 16 November 2007 (UTC)

A lot of this article does not make sense. The concept of centrifugal force only has relevance in a rotating frame of reference, whether used in calculations or empirical observations. An often quoted example used to prove that centrifugal force "is fictitious" is that of spinning with an object held at the end of a string; the hand exerts a force on the string which holds the object in, so the object must be exerting an equal and opposite force on the hand via the string. But somehow that opposing force isn't real, or is qualified by stating it is a reaction force or something like that.

Trying to make this fit it into an inertial frame of reference is subverting the physics behind the phenomenon. When describing the force it is fixed and motionless; in a fixed frame of reference the force vector would be spinning, which is at odds with the description of the physical experience. The object opposing the force (the hand) is fixed (in a rotating frame); if it were fixed in a non-rotating frame, then the string would wrap around the hand and the experiment would end. The observer describes a static system, all under rotation. The observer is less likely to think in terms of pulling the object off its inertial straight line and onto a circular path thereby creating a dynamic force (constant in magnitude but varying in direction). This distinction is more than fleeting; if the observer is inside a rotating cylinder then they would have no idea that a fixed frame of reference even existed.

So if anything, centripetal force is the more obscure 'advanced physics' concept, and trying to wash away the core of the physical experience (the static forces and the obviously rotating frame) as "too advanced" is bound to cause confusion and dissatisfaction. --Adx (talk) 14:43, 18 November 2007 (UTC)

I wonder how you can think that a concept that Newton used in relation to inertial reference frames, "has no relevance". It boggles me how anyone can claim that its original and straightforward meaning "is subverting the physics". Also, this article does not claim that all "centrifugal force is fictitious".

Thus I can't make sense of what you are trying to say; and I saw no reference to any specific part of the article. It's not clear what anyone would be deeming "too advanced". If you want to help improving this article, please study the sections that discuss the usage with which you are familiar and propose specific improvements to those sections.

Harald88 (talk) 16:19, 18 November 2007 (UTC)

I agree that it is hard to tell what Adx's complaint is. He appears to be confused by the fact that the article describes two different concepts, though I cannot see a way to make it clearer in the article that this is going on. We will probably be able to make progress only if Adx points to specific sentences or paragraphs that he thinks are wrong/misleading. –Henning Makholm 17:04, 18 November 2007 (UTC)

Apologies for pointing the finger of blame at the article, that was not my intention, I don't have a problem with its technical accuracy. My point is that many people will come here wanting to know "why centrifugal force does not exist". Rather than provide a physics lesson, the article could start off by breaking down their experience of the real force they feel while rotating, into the equivalent inertial frame explanation (that Newton invented as a tool to deal with exactly this type of conceptual problem). The inertial frame explanation is less physically relevant because it deals with dynamic force vectors, but it does nicely explain what is happening to the system as a whole. (In other words, an article about centrifugal force not textbook physics, if you see my point.) Nowhere did I say the article claims "all centrifugal force is fictitious", just that it gives no straight answer on this point. By "too advanced" I refer to the section dealing with how the confusion is often ignored in high school physics, where if pressed the final answer will be "just take my word for it, centrifugal force isn't real". That is "subverting the physics", this article does a much better job but it isn't perfect. Perhaps it isn't Wikipedia's job to depart from tradition, so feel free to dismiss all this as the idle ramblings of someone who is on a different planet.--Adx (talk) 14:27, 22 November 2007 (UTC)

And they advised me never to use that term in their classes. As they explained it, the proper term would be "intertia"... i.e. the desire to the rotating mass to continue moving straight ahead. It is only the centripetal force (think string) that keeps it from following its inate inertia.

That said, I think this article needs a major rewrite to clarify the centrifugal force is not a force. A more proper term is "inertia". - Theaveng (talk) 12:55, 10 December 2007 (UTC)

Not to be blunt, but your college professors are either (1) wrong, (2) didn't tell you the whole story, or (3) you misunderstood them. Reread the car example discussions in the article (and discuss them with your professors, since it may be helpful for both them and you to figure out how the miscommunication happened). Basically, think about if you turn your car sharply. Your body has a tendency to move outward, which is (as you say) INERTIA, not a "force." People incorrectly call that tendency of your body to move outward a "force," when it is actually just a basic property of matter, namely inertia. That's probably what your professors want you to realize.

However, if your body moves outward, hits the door (or window or whatever), and then PUSHES against the door, you are indeed exerting a FORCE on the door. The direction of that force is OUTWARD and is therefore properly termed *centrifugal*. The door exerts a force back INWARD to hold you from flying out of the car, which is the *centripetal* force. Or, another way to think of it -- Newton's third law states that forces occur in pairs. So if there is indeed some force pushing inward (centripetal), there must somewhere be a force pushing in the opposite direction (centrifugal). The existence of one implies the existence of the other. HOWEVER, that outward-pushing ("centrifugal") force isn't what causes you to move outward in the car... that's simply inertia. Physics professors and teachers are trying combat the misuse of the term. However, if they say a centrifugal force CAN'T exist, then a centripetal one can't exist either... it would violate Newton's third law. 24.147.122.22 (talk) 05:02, 12 December 2007 (UTC)

You appear to intend to be very blunt yet you say "not to be blunt". That doesn't seem to follow the spirit of wikipedia. My college professors also told us not to use the phrase Centrifugal Force. I trust them more than alot of unreferenced stuff on wikipedia. Not that I don't love wikipedia. But it is what it is. And a physics book it is not. Maybe as a result of this article the newer college physics books will insert a footnote about it. But that is all it would be since it is not needed to do the free body diagrams and calculations. The person sitting in the car never experiences a centrifugal force (unless his big brother the physics student shoves him out the door for arguing pro-centrifugal force rhetoric).-Crunchy Numbers (talk) 15:22, 12 December 2007 (UTC)

Thanks Crunchy.

I would love to ask my profs the question, but having graduated that's no longer possible. ----- You are correct that the door is what keeps my body moving in a circle. That is indeed Centripetal force (actually it's the whole car that exerts the force, since the car is all connected as a single piece). However the force that I exert on the door is NOT centrifugal force. False. It is the Normal force... an opposite reaction to what the car is doing to me. Please don't mis-label things. - Theaveng (talk) 19:33, 12 December 2007 (UTC)

This was first discussed several years ago, and references were presented (they are still present in the article) to the effect that some people who do know what they are talking about do sometimes refer to the reaction to the centripetal force as "centrifugal force". I would agree with you that this is a confusing usage, but the mission of Wikipedia is not to try to spread a better way of thinking than that actually used in the sources.

However, the more common meaning of "centrifugal force" is a correction term that one must use if (and only if!) one analyses a situation with a rotating coordinate system. Your professors may well have told you not to use the term in class, because you were not supposed to use rotating coordinate systems in that class, and therefore it would be a sign of misunderstanding if you felt the need to include a centrifugal force in your calculations. However, the fact that there are contexts where it is wrong to use a centrifugal force does not imply that it is always wrong to use a centrifugal force. –Henning Makholm 23:02, 22 December 2007 (UTC)

Hi, Theaveng. I made the original reply, and I do apologize for being blunt. That said, I still would argue that your response is based on a misconception -- i.e., calling the other force a "normal force" (and saying that is the ONLY term for it) seems to be a way of putting it in "second place" to the centripetal force, which everyone seems to think is the "real" force. My point is simply that you can't have a force by itself. They always come in pairs. And if you have one force pushing or pulling inward (a "centripetal" force), then there MUST be a force pushing or pulling outward. (Yes, they are acting on different objects, but they are both there.) Neither is prior to the other; they can only exist together. Therefore I think it's almost as bad a misunderstanding to emphasize centripetal force alone as it is to get confused about the role of inertia in these situations. I think the common terminology you describe (i.e., using "centripetal" but saying you can't even use the word "centrifugal") is thus misleading. There are often forces that are "center-fleeing," and saying we can't use the word "centrifugal" to describe them because many people don't understand inertia is a bit ridiculous.

Let me switch examples to make this clear. Say that your average person was taught that there were such things as "normal forces" but they completely misunderstood the term and thought it referred to "normal" as opposed to "special" or "extraordinary." (Thus, "normal" forces were generally always there in a problem, as opposed to the added forces that generally play a role in non-static situations.) Say this misunderstanding was fairly common. (It has occurred in my high-school physics classes when the term is first introduced, despite many clarifications.) Should we abandon the term "normal" and say it can't be used in physics class to refer to a force just because most people use the term incorrectly?? Or should we instead teach that the word "normal" is actually a general mathematical term that has to do with perpendicular relationships?

So, I really didn't mean to be insulting (apologies Crunchy Numbers). But for the same reason that many people have a pet peeve about using centrifugal force incorrectly to refer to inertia, my pet peeve is people who seem to be intent on stamping out the idea that "centrifugal" forces can exist when in fact they MUST exist whenever centripetal ones do. Etymologically, the word "centrifugal" just means that it's a force going outward away from the center. Why not teach people that they can use such a term to describe actual forces that fit that description rather than banning the word?

Lastly, Crunchy, I'm sorry, but you're wrong in most cases to say that the person in the car "never experiences a centrifugal force." For example, if I'm holding a cup in my hand as the car goes around the turn, the cup will exert a centrifugal force (outwardly-directed) on my hand. Even if I'm not holding anything, generally I'll at least be wearing clothes which will exert centrifugal forces against me. Even the air molecules in the car will have inertia and will push against me, exerting an outward force that is just as real as the force that the door exerts back on me. Why give only one of these things a special name ("centripetal") and say that all the forces from the cup, my clothes, and the air don't get a special name? Only if I were being rotated naked in a vacuum could you really make the claim that I'm not experiencing centrifugal forces external to myself... and even then, the blood vessels walls, etc. inside my body will be.

Simply put, from an etymology standpoint, centripetal and centrifugal just refer to different directions. Using nomenclature that privileges forces in one of those directions (while banning the use of the term for the other direction) is inconsistent and leads to further misunderstandings about the operation of Newton's Third Law... at least from my experience teaching in physics classrooms. 65.96.183.164 (talk) 06:47, 12 January 2008 (UTC)

By the way, I also do agree with Henning Makholm's bit about rotating coordinate systems and the usefulness of "centrifugal force" to describe things in such a case. That's a really useful shortcut in certain situations that are more advanced than introductory physics. But, in my view, it isn't really objectionable to say you can't use a term until you understand the situation where it's appropriate, which seems to be your physics professor's take. What I do object to is the system of terminology that results, which strikes me as inconsistent (and somewhat misleading) for inertial frames. So, I just want to be clear that I have no argument with Henning's take on this, though I still object to explanations that ignore the force-pair involved and just focus on inertia and the effects of centripetal acceleration. But, as Henning points out, this has all been discussed at length in the Talk Page archives.... 65.96.183.164 (talk) 07:45, 12 January 2008 (UTC)

The misconception here is that you can't have a force by itself, and that forces always come in pairs. This is only the case in stasis. Unbalanced forces cause an accelleration. Thus from the perspective of outside the car (the inertial reference frame) the unbalanced force of the car door is causing you to accellerate (towards the other door). We say F=MA, i.e. an unbalanced force causes a mass to accelerate. From the perspective of inside the car, an almighty hand of God forces you towards the door and the door pushes back, which is hardly physics, more theology. 217.169.50.138 (talk) 15:58, 5 March 2008 (UTC)

F=m*a describes the acceleration that corresponds to dynamic force equilibrium. For example, if you accelerate a car by pushing it, the car equally pushs back at you as you can easily see and feel: the flesh of your your hands is being deformed by the force with which the car pushes back at you - it is the inertial force (related to "inertial mass") that regulates the acceleration. Should a clarification like that perhaps be included in the text of this article or (with a link from here) in an article about F=m*a? Harald88 (talk) 21:33, 5 March 2008 (UTC)

I read that there are two types of centrifugal forces : ".../...1) A real or "reactive" centrifugal force occurs in reaction to a centripetal acceleration acting on a mass. This centrifugal force is equal in magnitude to the centripetal force,.../... 2) A pseudo or "fictitious" centrifugal force appears when a rotating reference frame is used for analysis./..."

For the first type, Wikipedia gives the following example: "Both of the above can be easily observed in action for a passenger riding in a car. If a car swerves around a corner, a passenger's body seems to move towards the outer edge of the car and then pushes against the door..../...However, the force with which the passenger pushes against the door is real. That force is called a reaction force because it results from passive interaction with the car which actively pushes against the body. As it is directed outward, it is a centrifugal force."

These last sentences are of course wrong. The passenger doesn't push against the car, but the car is turning and the passenger is just going straight forward, due to his inertial mass, with the consequence that the car pushes the passenger. And that force is of course centripetal, not centrifugal! Thus: centrifugal forces still don't exist. Only the reaction force on a change of direction of a inertial mass is real. The force of the car on the passenger is real.

No, from Newton's third law, if the car pushes on the passenger, then the passenger also pushes on the car with an equal and opposite force. So there is a real force there.- (User) WolfKeeper (Talk) 20:01, 31 January 2008 (UTC)

What is the origin of the misunderstanding? Probably because at school, the teachers have slammed with a hammer the Laws of Newton in our head. And the famous law Action = Reaction is one of them. If there is a centripetal force, where is then the reaction force? Well, there isn't any. Inertial masses are allergic to changes of direction and velocity. And everything that try obstructing the inertia will exert a force to that inertial mass, inwards, thus centripetally.Sipora (talk) 19:52, 31 January 2008 (UTC)

Any 'reactionary force' that may appear to occur in this situation is just a component of the velocity of the car around the corner. There is no centrifugal force in this case; I do not understand the concept fully enough to say that it never exists, but in this case it most certainly does not. If I could draw a diagram it would show that there is a centripetal force, towards the middle of the corner (if we treat it as a circle), and the velocity of the car at any given time, forward. The 'centrifugal' force is actually felt from a combination of these two forces acting upon the car and, transitivley, on the people inside. 17:48, 7 February 2008 —Preceding unsigned comment added by 82.32.75.92 (talk)

The following was added to the intro in a way that made the intro less coherent (it even introduced a third bullet point where only two bullets belong):

Centripetal acceleration is the term for the continuous change in linear velocity as a point curves about the axis of rotation rather than flying off the body tangentially.

• When an external force angularly accelerates a body that remains intact, an equal and opposite tangential force is exerted by the rotating object in accordance with Newton's Third Law of Motion.

It may be useful to put some of it under the first heading ("Reactive centrifugal force"). Harald88 (talk) 13:37, 5 March 2008 (UTC)

In addition, I now see that also the definition was changed without discussion, and it looks less good to me than the one that we had settled on. Thus I revert, moving the alternative version here:

* A real or "reactive" centrifugal force occurs in reaction to the angular velocity of a mass. If a freely rotating body remains intact (i.e., does not distort or break apart), the centrifugal force measured at any point in or on that body exactly equals the centripetal force directed toward the axis of rotation at that same point.

IMHO, that definition corresponds less to the way Newton presented it and obscures the fact that force is due to acceleration; it lacks the clarification that it is the force that originates from the passive body. Harald88 (talk) 21:47, 5 March 2008 (UTC)