This page is an archive of past discussions. 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. |
PeR added a comment to an existing post, way at the top of this talk page. I copy
PeR's comment here:
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)
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'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)
Since Cintrifugal force doesn't exist i think we should just move the article to be called centripital or w.e
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.
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)
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 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. ← BenB4 17:31, 12 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)
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:
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)
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)
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)
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.
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)
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.
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)
This page is an archive of past discussions. 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. |
PeR added a comment to an existing post, way at the top of this talk page. I copy
PeR's comment here:
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)
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'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)
Since Cintrifugal force doesn't exist i think we should just move the article to be called centripital or w.e
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.
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)
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 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. ← BenB4 17:31, 12 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)
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:
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)
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)
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)
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.
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)
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.
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)