Talk:Centrifugal force

Animation

I've drawn some pictures for de.WP, which might be of interest for english language to. On request I'll change annotaion, but I don't know their conventions in other parts of the world.

Animation

Picture with annotation

Translated annotation

--Stündle (talk) 08:59, 26 September 2011 (UTC)

Nice work. However, without a more detailed explanation of the meanings of the symbols $\stackrel{\rightarrow}{F}_{Zf}$ and $\stackrel{\rightarrow}{F}_{C}$ I believe the lower half of the diagram on the right might be misleading. Presumably $\stackrel{\rightarrow}{F}_{Zf}$ denotes the centrifugal pseudoforce which apparently acts on the man on the revolving beam from the point of view of the rotating coordinate system. But what does $\stackrel{\rightarrow}{F}_{C}$ represent? Is it just the Coriolis pseudoforce apparently acting on the other man, or is it the total pseudoforce apparently acting on him—i.e. the resultant of the Coriolis and centrifugal pseudoforces. I believe it would be preferable to represent both of those pseudoforces on the diagram, with the latter, of course, acting radially outwards and having half the magnitude of the former.

David Wilson (talk · cont) 11:25, 26 September 2011 (UTC)

Annotation:

$\stackrel{\rightarrow}{F}_{C}$: Coriolis force

$\stackrel{\rightarrow}{F}_{Zf}$: Centrifugal force (Fictitious centrifugal force)

$\stackrel{\rightarrow}{F}_{Zf}$: Centripetal force

$\stackrel{\rightarrow}{v}_{B}$: velocity

$\stackrel{\rightarrow}{v}_{B}'$: velocity seen from rotating reference frame

Red person is resting in a inertial frame

Green person is in a uniform rotating reference frame

ω = konst.: rotation speed is constant

Actually I can't figure out what you are talking about. In germany I don't think there is something like total pseudoforce in use. These pictures refer to “Newton's laws of motion”. --Stündle (talk) 12:55, 26 September 2011 (UTC)

By "total pseudoforce" I simply mean the (vector) sum of all the pseudoforces apparently acting on a body. My point is that in the rotating reference frame in the bottom halves of your diagrams, the Coriolis pseudoforce Is not the only one apparently acting on the person in the red shirt. There is also an outwardly directed centrifugal pseudoforce apparently acting on him. The magnitude of the Coriolis pseudoforce is $2 m \omega \mid\displaystyle{\stackrel{\rightarrow}{v}_{B}'}\mid = 2 m \omega^2 r$, where m is the mass of the person in the red shirt, and r is his distance from the centre of the beam. The magnitude of the centrifugal pseudoforce is mω²r. The (vector) sum of these two pseudoforces is a vector of magnitude mω²r directed radially inwards, which is exactly the product of the red-shirted person's mass and his (apparent) acceleration (i.e. the acceleration of his coordinates in the rotating system).

David Wilson (talk · cont) 14:02, 26 September 2011 (UTC)

Well, I tried to translate and improve annotation of my picture. Results see above. --Stündle (talk) 07:30, 27 September 2011 (UTC)

It would also be better if the rotating chair was facing inwards. The poor chap must find it hard to stay in his seat as it is. Also, the distinction between lines representing forces and lines representing velocities should be clearer. Martin Hogbin (talk) 00:11, 14 January 2012 (UTC)

Where's the high school section

i got onto wikipedia to try to find out: if you spin a bucket on a string, given the weight, rate, and the length of the string: what's the force? I dare you to try and find out from Wikipedia without already having a thorough knowledge of the subject.

All the physics articles have this problem: they are all university level explanations of lagrangian mechanics, and any sections containing just the equations you need to solve basic problems actually get erased. Any complaint gets the answer "well, you can derive it from what's on the page", which completely misses the point.

Where's the centrifugal force page for the bottom 99%?

Paul Murray (talk) 23:18, 10 October 2011 (UTC)

It's a fair point, but the reason why anything is on Wikipedia is because someone cared enough about its absence to write it. If this is you, then...

This article doesn't need to be readable in its entirety by 99% of the audience, but some sections of it should be, it should be clear which these sections are, and reading these sections should be sufficient to answer their questions. I don't see this as a reason to remove the Langrangian, or to have a separate article without, but there's certainly scope for improving the readability of some other sections. Andy Dingley (talk) 00:14, 11 October 2011 (UTC)

Paul has made a valid point. Wikipedia has some valuable guidelines at WP:Make technical articles understandable.

At the beginning of every technical article there should be information that is understandable to young people, and people who are new to the field. After that, there should be a progressive increase in the level of complexity. Dolphin (t) 02:36, 11 October 2011 (UTC)

This is a summary-style article that's supposed to help you find the article you want, but people have a tendency to keep making it more complicated. Probably what you're looking for is the first equation in Centripetal force. Dicklyon (talk) 02:56, 11 October 2011 (UTC)

Dick, good to see you back. I responded to your comments on this subject on my talk page. Martin Hogbin (talk) 08:11, 11 October 2011 (UTC)

@Paul, when you say 'what is the force?', what force exactly do you mean? Martin Hogbin (talk) 08:29, 11 October 2011 (UTC)

Paul's question can be made more meaningful if re-worded to say What is the tension (or tensile force) in the string?Dolphin (t) 06:41, 10 January 2012 (UTC)

Yes, if we assume that that is what Paul meant. What is particularly unhelpful to beginners (and pointless for experts) is to have one name for several different things. Martin Hogbin (talk) 09:26, 10 January 2012 (UTC)

I can't see anything else that Paul could have meant. What one name for several different things do you have in mind? Dolphin (t) 12:40, 10 January 2012 (UTC)

Paul just says, "what's the force?". Does he mean, the 'Fictitious centrifugal force, acting in the water and bucket, the 'Reactive centrifugal force' exerted by the bucket on the string, or is he contemplating using Lagrangian mechanics? I fully understand why he finds the subject confusing. Martin Hogbin (talk) 16:39, 10 January 2012 (UTC)

Images

Forces on a ball in circular motion as seen in the inertial frame where the ball is seen to rotate (center) and as seen in a co-rotating frame where the ball appears stationary (right).

A ball in circular motion has a centripetal force on it; in the case of that force being applied by a massless string, the reaction force is an equal force, centrifugally directed, on the string, and via the string tension on the post holding the string.

Forces on a ball and a string keeping it in circular motion. Left: inertial frame where the ball is seen to rotate. Right: co-rotating frame where the ball appears stationary. Forces on the ball at center, forces placing the string under tension shown separately below.

Perhaps we need a diagram and a discussion of this example? Here's a diagram that might be used to show the centrifugal force and the reactive centrifugal force. The explanation accompanying the figure could be:

In the inertial frame the ball is subject to the centripetal force provided by the string tension, and the string (below) is subject to the reactive centrifugal force and the force due to the center-post which is the same as the centripetal force on the ball. In the co-rotating frame the ball appears stationary with zero net force because the string tension is balanced by the centrifugal force, and the string is subject to the same tensional forces seen by the inertial observers.

Formulas could also be provided? Brews ohare (talk) 18:23, 10 January 2012 (UTC)

I have added this material. Brews ohare (talk) 15:56, 11 January 2012 (UTC)

The figure is about as clear as mud; can anyone but Brews get info out of them? The annotations rotating and stationary are opposite to what would be applied to the frames of reference, so you need a lot of text to interpret that. The upper-left bit is not needed. And why do we want to talk about a reactive CF in a rotating frame? Dicklyon (talk) 18:51, 11 January 2012 (UTC)

I have to agree: the image tries to cram far too much into a small space, losing some important details as it does. It also is far to abstract for an 'example', while the use of colours is just confusing and will render the diagram useless for someone with poor colour vision or using a non-colour display. I doubt this is fixable without making the diagram too big. Perhaps because of this the text makes no sense: what co-rotating frame, what observers? So generally as an "Example" the whole section seems very abstract and unclear.--JohnBlackburne^words_deeds 19:54, 11 January 2012 (UTC)

Dick and Blackburne: You all haven't provided much guidance on how to improve the figure; just rained on the parade. What details are lacking? What isn't clear? For example, is it unclear that the centripetal force is inward? Is it unclear that the string is under tension? Is it unclear that the centrifugal force is outward? Would the arrows be indistinguishable on a gray scale for the colorblind? Would it be an improvement to have a blank space above the labeling of the arrows instead of a picture?

In the frame of the moving ball, the force on the string is the reaction against the string's pull on the ball to provide centripetal force, and is called "reactive centrifugal force". In the frame where the ball is stationary, the force on the string is again the reaction to the pull supplied by the string, this time to counter the centrifugal force on the ball. Both have the same size and direction, and the same origin, being due to the "action and reaction" Newton's law. The string is under the same forces in either frame. Are we to call the force on the end of the string by a different name in the two identical situations despite their being identical in every respect? . Maybe if it acts like a duck and quacks like a duck...?? Brews ohare (talk) 00:01, 12 January 2012 (UTC)

Have you both read the added section? I believe any confusion about the diagram is resolved if you read the text on the Main page that accompanies the figure. Brews ohare (talk) 05:47, 12 January 2012 (UTC)

I replaced it with a less complicated figure. It's perhaps not ideal, but it's better. Dicklyon (talk) 06:19, 12 January 2012 (UTC)

┌────────────────────────────────────────────────────────────────────────────────────────────────────┘ Hi Dick: I am glad to see you engage in improving the figure. Here are some shortcomings of the new figure:

1. The font is so small that it cannot be read. A larger font could be used if the wording were shortened. For example, it is not necessary to say on the figure that the post is immovable, or that the string is massless.
2. The linewidths are too lightweight for good visibility.
3. The normal understanding is that the centrifugal force acts outwardly upon the ball, but the figure labels instead the force at the post.
4. A reactive force generally is considered to arise at the point of application of the force to which it is a reaction. Thus, having the reactive force at the post, while the force causing the reaction is at the ball is contrary to normal practice.
5. Because only one circle is used to describe both the inertial and the co-rotating frames, it is made to appear that all three forces: centrifugal, centripetal, and the reactive centrifugal forces, are present in all frames. Only the reactive centrifugal force is present in all frames.

I hope that some improvements along these lines can be made. Brews ohare (talk) 15:36, 12 January 2012 (UTC)

I've submitted another alternative here. Brews ohare (talk) 17:03, 12 January 2012 (UTC)

It's not exactly a new figure, but was created almost a year ago. I agree the text is a bit small, but only a bit – it seems to be optimised for viewing at the default size of 300px. It's still easily readable at 200px, except for the red text being both coloured and inclined. But as it's an SVG that's easily fixed.

I don't see any other problems with it. The lines and arrows are clear and visible, the string goes from the ball to the post as it should. The centrifugal force acts both on the ball and the post, so is correctly placed.

As for adding more circles it seems likely that will just make it more confusing. The circles will have to be smaller making existing captions less clear and requiring even more detail and more text so it's clear what they signify. Someone could try adapting this but I would not encourage anyone trying as I can't see how it can be done.--JohnBlackburne^words_deeds 17:05, 12 January 2012 (UTC)

I've retouched the SVG file to make the text larger and clearer, using a darker colour for the coloured text, and editing a couple of things for length.--JohnBlackburne^words_deeds 17:20, 12 January 2012 (UTC)

I've broken out this thread about the images. At 200px the single-circle figure is legible. The alternative showing the inertial and co-rotating frames separately also is very clear at this size, and even at a smaller size. In my opinion treating the two frames on a single circle is not a clear approach because different forces are present in the two frames, and the single circle makes it look like all forces are present in both frames of reference. Brews ohare (talk) 17:32, 12 January 2012 (UTC)

In addition, it is not possible to show the reactive centrifugal force at its correct point of application on the ball-end of the string without making an exploded view. Brews ohare (talk) 17:35, 12 January 2012 (UTC)

I'm afraid the new image has most of the problems of the previous 5-circle image. It is really unclear what's going on: even if you know what it's trying to show it takes some time looking at it before you work out what it's meant to be showing, and we can't assume readers will have that knowledge. What does it mean 'stationary ball' ? Only one circle is labelled as "Rotating Ball", but only one, a different one, has an arrow that indicates it's turning. There is no indication what the colours mean, though presumably they are significant. No indication these are force at all ("Post"?) I am still unconvinced this can be done in a reasonable space with multiple rings. I certainly can't think how I'd do it, and both images here with five rings show how difficult it is.--JohnBlackburne^words_deeds 22:37, 12 January 2012 (UTC)

John: Thanks for trying to make clear what the problems are here. The diagram is organized in two columns and three rows. The left column is "rotating ball" and the right column is "stationary ball". So the bottom labels refer to columns, not solely the diagram immediately above. The colors simply connect the label of a particular color with the arrow of the same color. I am a bit bewildered that "stationary ball" is a confusing label. Do you have some further suggestions? Brews ohare (talk) 23:30, 12 January 2012 (UTC)

I've changed the figure slightly to emphasize the column organization and changed the labels of the columns to refer to frames. The label "post" has been changed to "post reaction". Any better? Brews ohare (talk) 23:59, 12 January 2012 (UTC)

The main problem you are trying to do too much in a small space: it still looks like a cluttered mess with the individual elements too similar, the colours and text labels confusing and it being unclear what's being represented. Even something simple like is it five things, five views of one thing, or something inbetween? As I noted above I don't think its any particular failing of your art, it's that you're trying to do the impossible.--JohnBlackburne^words_deeds 00:14, 13 January 2012 (UTC)

It doesn't sound like breaking it up into more figures would make you happier, eh? Brews ohare (talk) 01:38, 13 January 2012 (UTC)

I revised the labeling to mitigate the clutter a bit. Brews ohare (talk) 12:50, 13 January 2012 (UTC)

Brews, I know from experience that it would be more than a full-time job to try to moderate your aggressive rapid-fire nonstop editing. If you could work on making the article more concise, well sourced, and correct, that would be a huge benefit, in my opinion, compared to your usual adding of more fluffy-looking hard-to-interpret diagrams, examples, essays, and general bloat. I don't have time for this. Good luck to you guys. Dicklyon (talk) 16:41, 13 January 2012 (UTC)

Dick: Probably the balance between being concise and being clear will strike different contributors differently. For one thing, the audience conceived by one author is different than that of another. The addition of this example was in response to a legitimate reader complaint, and it was my hope that this example would prove helpful. Constructing an intuitively obvious diagram is difficult, and I understand that you are repulsed by the free body diagram common in engineering mechanics. The present image requires too much attention from the reader, but it's OK, it's accurate, it fits the explanation in the text. Your characterization of my contributions in general as pretty much worthless is not constructive, Dick, and I don't think it is objective. Brews ohare (talk) 18:49, 13 January 2012 (UTC)

Meaning of centrifugal force

Dick, I hope you will eventually come round to my way of thinking that this article should concentrate on meaning of CF that is almost universally taught and used by mathematicians, physicists, and engineers today. That would make it much simpler to understand. Martin Hogbin (talk) 18:21, 13 January 2012 (UTC)

Martin: I don't think your concept of restricting the article to the single version of centrifugal force called here the "fictitious centrifugal force" is responsive to the history of these WP articles. If you look back over the morass of Talk page episodes and Main page edits you will find that many readers use the term "centrifugal force" to mean "reactive centrifugal force". You will find that usage even in textbooks. You also will find in the history of these WP articles that many readers believe that centrifugal force means the Lagrangian "generalized centrifugal force" and that opinion also is found in textbooks. So, in my view, a general article on Centrifugal force should cover all these readers' expectations, and hopefully will widen their view to see that there are several usages that are related, but not identical, and that none can claim to be the centrifugal force. By including all the usages, maybe WP can avoid repeating the long exchanges of the past that result primarily because each reader tends to take their own view as the view and the other views as those of the great unwashed that need to wake up to the one true meaning. Brews ohare (talk) 18:39, 13 January 2012 (UTC)

I am happy to mention historical meanings of the term but today it is almost universally used with one meaning only. That is not a decision I have made, it is how things are. WP should reflect that. Nobody (well in infinitesimal specialist minority) uses the term CF to primarily refer to a Lagrangian "generalized centrifugal force" . Martin Hogbin (talk) 20:10, 13 January 2012 (UTC)

Martin: The above remarks about the meanings of "centrifugal force" are not historical curiosities, but are found in use today. You might take the time to look at the Lagrangian sources mentioned here. Some discussions are just a bit murky, like this, where the term centrifugal reaction is coined, and this one and this one where the reactive centrifugal force is called the centrifugal force. Brews ohare (talk) 23:22, 13 January 2012 (UTC)

Yes, I know that you can find the odd example where the reactive centrifugal force is used but this confusing and unnecessary usage of the term is strongly discouraged by most current educational sources. Martin Hogbin (talk) 00:02, 14 January 2012 (UTC)

As shown by the extended debates in the Talk pages here on WP, ignoring some usages doesn't make things easier on WP. Editors come to WP with their ideas as a first prejudice, and they won't go away. If a visitor finds their view of "centrifugal force" is not present, they'll add it to the main page. (They may even replace the meaning already on the main page because they think it's wrong.) Then a "simplifier" or a believer that only the "commonly accepted" view should be presented, and other views are "bloat", will revert the addition. And away we go.

As you know, reading about various views at the outset is an easier form of persuasion than pushing only one view on the article Main page and ending up debating what is the "real" meaning on Talk pages all over again. And again. And with sources to support all the opinions, which all have adherents in the published record. And is your source better than my source? And on, and on.

It's stretching things to say presenting different usages is confusing and unnecessary - it's just becoming aware there is a variety of things denoted by the technical term "centrifugal force" before confrontation erupts on a Talk page over a revert of one's edit adding your take. Brews ohare (talk) 04:52, 14 January 2012 (UTC)

Where is the high-school section? (Part II)

To return to the title of the above thread: Where is the high-school section? This is an important general question and runs into the balance between the accessibility of WP to the nonspecialist and the guidelines like WP is not a textbook. Simple examples with the associated math are an anathema to some editors like Dicklyon who invokes his concepts of "bloat" and "symbol soup" and has drastically cut examples from this article, and not just those with formulas, but those with qualitative points and diagrams to boot.

Possibly it is time to lay out a formal set of classifications for articles into, for example, "Broad introductions", "High-school treatments", "Undergraduate treatments" and "Specialist discussions". In the last category are a great many treatments of mathematical topics on WP that I find incomprehensible myself despite a PhD in solid-state physics and having published articles of my own in Physical Review on the application of space groups to crystalline phase transitions. Brews ohare (talk) 15:51, 10 January 2012 (UTC)

We already have beginnings of such a system without formal introduction in having articles like Quantum mechanics & Introduction to quantum mechanics & Particle in a box & Quantum harmonic oscillator, and also Theory of relativity & Special relativity & Introduction to special relativity. The recasting of the criteria applicable to such categories is nowhere stated, to my knowledge. Brews ohare (talk) 16:00, 10 January 2012 (UTC)

Lagrangian version of generalized centrifugal forces

FyzixFighter, your revision here was not correct. For more detail about the Lagrangian version, please look at the discussion later in the article and also in this subsection of another article. I believe these sections are accurate. In particular, please look at the boxed quotation in the latter section by Ge et al..

The situation is somewhat like this: the Lagrangian approach is based upon generalized coordinates and the generalized forces are related to the time derivatives of these coordinates. As such, these forces need have no connection with forces as defined in Newton's laws, although sometimes there is a correspondence. In particular, the so-called "centrifugal forces" referred to for example in the field of robotics is normally not a force from Newton's standpoint at all. An exception is the case of polar coordinates as discussed explicitly, where it happens that the the Lagrangian approach results in the same equations as Newton's laws in a co-rotating frame. This example is unusual however, as is explained at the above links. Brews ohare (talk) 05:36, 10 January 2012 (UTC)

It is not the case that the Lagrangian approach is more general than the Newtonian approach, and the generalized forces do not include the Newtonian vector forces as a special case. The interpretations of the two methods do sometimes overlap, but mostly they are disjoint, and not all Newtonian forces are generalized forces, nor vice versa. Brews ohare (talk) 05:40, 10 January 2012 (UTC)

I'm sorry but I disagree. The description of the three contexts are almost taken directly from the Bini et al 1997 reference. Bini's third context is not tied to the Newtonian approach, although as you note there is correspondence. In the original text that I reverted to, the wording does not limit it to a Newtonian approach so most of your argument above is against a point that neither I or the original text is making. Again, I'm basing this on my reading of the Bini reference which I believe includes but is not limited to the Lagrangian usage, so I don't think it is OR and it is well supported by a reliable source. This text has stood a while as it is, so let's discuss and get consensus since your edit was challenged (per WP:BRD) rather than set off an edit war. --FyzixFighter (talk) 06:00, 10 January 2012 (UTC)

I don't see why this context is mentioned at all. There's a whole separate section for the generalized fictitious forces in Lagrangian mechanics. Few sources mix up "fictitious centrifugal force" with these. Here is the diff from April 2010 where it went from being a brief mention and ref to the section below to the beginning of bloat. How about we put it back to that brief mention? Dicklyon (talk) 06:02, 10 January 2012 (UTC)

We agree on this one Dick. We have already had one complaint that the article is incomprehensible. Martin Hogbin (talk) 22:53, 10 January 2012 (UTC)

Hi Fyzix: The links to the Bini reference are not working for me, so I cannot read what these authors actually said.

The references cited in the two WP sections I have linked above are quite clear. In particular, the "centrifugal forces" of the robotics engineers are simply the squared q-dot terms, and are not Newtonian forces except in the special case of polar coordinates.

In my view the statement "The third context is the most general, and subsumes the first two, as well as stationary curved coordinates (e.g., polar coordinates)..." is quite incorrect. First, Lagrangian and Newtonian mechanics are entirely equivalent within classical mechanics, so this statement cannot mean that Lagrangian mechanics is more general. Second, the article is about centrifugal force, so it is natural to assume that is the subject under discussion. In this arena of centrifugal force it is more correct to say that Lagrangian generalized centrifugal force is unrelated to Newtonian centrifugal force, with perhaps one exceptional case where they overlap. One confusion in comparing Lagrangian centrifugal forces to those of Newton is that these forces do not disappear in an inertial frame of reference, as inertial forces are supposed to do under the Newtonian concepts.

If I have misinterpreted the paragraph, it should be rephrased to say what is meant. Brews ohare (talk) 06:32, 10 January 2012 (UTC)

I'd go so far as to say that if the formulation you have proposed, Fyzix, is adopted, in my opinion it contradicts the following sub-section in this article and the more extended discussion of the other article as linked above. Brews ohare (talk) 06:45, 10 January 2012 (UTC)

I have managed to locate the Bini references and have updated these links. These authors' discussion on p. 5 does not support the language that I've removed. Brews ohare (talk) 15:16, 10 January 2012 (UTC)

To DickLyon: The reason for bringing up the Lagrangian ideas is the following: First, the two come up all the time with both being called simply "centrifugal forces" and this confusion of terminology has led in the past to very prolonged confusion because the inertial forces of Newton disappear in an inertial frame, while the generalized centrifugal forces do not. This article is an opportunity to set the the distinction forward. Second, the Lagrangian generalized centrifugal force is very commonly used in several branches of physics and engineering, particularity in robotics, so a reader might well google this article looking for some illumination on this usage. Brews ohare (talk) 06:32, 10 January 2012 (UTC)

Dick, perhaps your support for including reactive centrifugal force has bearing here? Brews ohare (talk) 14:59, 10 January 2012 (UTC)

I'd support a return to this formulation, which as you say precedes the introduction of "bloat". Brews ohare (talk) 16:28, 10 January 2012 (UTC)

The Bini reference is about general relativity. This article is about, or should be about, Newtonian physics. Martin Hogbin (talk) 22:59, 10 January 2012 (UTC)

Martin: I think you have to actually read the article and not just go by its title. Brews ohare (talk) 23:26, 10 January 2012 (UTC)

Putting the terminology aside and focusing on the physics: as the section now reads it's bit misleading, because if you want, you can also switch to a co-rotating frame and derive the expression for centrifugal force, coriolis force etc. Count Iblis (talk) 22:58, 10 January 2012 (UTC)

Count: You can switch to a co-rotating frame in simple systems, but the term "centrifugal force" is applied to the q-dot squared terms in complex systems where there is no frame that co-rotates with all the spinning elements of the system, which rotate about many individual centers. Brews ohare (talk) 23:26, 10 January 2012 (UTC)