Talk:Reactive centrifugal force

This entire article is false.

The centrifugal force arises as a consequence of both newtons 1st law and newtons 3rd law, however his 2nd law only applies to the centripetal force, a real force. There is no real centrifugal force, even if you call it a reactive centrifugal force. Simply calling it a reactive force already implies that it is a fictitious force. When a real force is present there is always a reactive force, but in this case which force is reactive is completely arbitrary. Meaning neither force is inherently the reactive force. For the centrifugal force, that is all it is. As soon as the centripetal force disappears the centrifugal force would as well and motion would continue in a direction tangentially to the rotational curve. It cannot arise by itself. There is so much false information throughout this article, but the since its main topic is already false, I suppose that doesn't matter much. I could go on, but I've got a feeling I'm wasting my time. But this article needs to be removed.

EnemyTortoise15 (talk) 06:08, 3 May 2018 (UTC)

Wikipedia also has an article on the luminiferous aether even though it doesn't exist. Similarly, an article on fictitious forces even though they are fictitious. They qualify for an article of their own because they are notable; not because they represent the truth. They are notable because published sources exist to allow independent verification of all that is written in these articles. Dolphin (t) 13:17, 3 May 2018 (UTC)

There is a difference between an article on centrifugal force that identifies it as a non-Newtonian "force" arising in a rotating reference frame (See: https://en.wikipedia.org/wiki/Centrifugal_force), and this article. Unfortunately, that article also references this article.

The fact is that in any rotation about an inertial point or axis, all accelerations are centripetal. There is no outward acceleration. So to speak about an outward real Newtonian force is potentially very confusing. What you have in any rotating body are accelerations and tensions. If the centre of rotation is an inertial point or axis, accelerations are all centripetal. The tensions can be in any direction as they depend on the mass distribution within the rotating body or system. The tensions all net out to zero, of course because the centre of mass is not accelerating. I don't know why anyone would want to give a name to the components of these tensions that are directed outward. These tensions can NEVER give rise to outward acceleration. Any movement of matter away from the centre that might occur will occur because of inertia not a Newtonian force or tension. I don't know what purpose this article serves except to confuse people.

The one "authority" that is used in this article, Delo E. Mook & Thomas Vargish (1987). Inside relativity. Princeton NJ: Princeton University Press. p. 47. ISBN 0-691-02520-7 is incorrect. The authors describe the reaction force to the gravitational centripetal force that the Sun exerts on a planet, such as Jupiter, as a centrifugal force of Jupiter on the Sun. In fact, although it may be difficult to measure, we know that the Sun and planets and other matter in the solar system orbit all about their common centre of mass. So the force of Jupiter on the Sun is in fact a force that causes centripetal acceleration of the centre of mass of the Sun toward the centre of mass of the entire solar system (the inertial point). So it is clearly centripetal, not centrifugual.

Nothing can save this article. Even the supposed example of a "reactive centrifugal force" clutch is at best misleading. As noted, there are balanced tensions within any rotating body. But the "force" that causes the outward motion of the moving clutch is inertia! The "force" that causes the clutch surface to move outward to reduce the separations between molecules on the surface of the clutch and those of the outer casing surface and "engage" the clutch is... "inertia"!!

I have tried over the years to reason with the authors of this article, but, unfortunately, they seem to be very determined to keep this article alive. AMSask (talk) 15:56, 17 April 2019 (UTC)

I don't understand the complaint here. The fictitious centrifugal force and the reactive centrifugal force are entirely different things, the latter being a real outward force on the wall, rails, string, or whatever is supplying the centripetal force to accelerate a mass to move in a circle. Treating the one as the other doesn't do any good. Dicklyon (talk) 14:14, 21 April 2019 (UTC)

I hope that we both agree that there is no "reactive centrifugal acceleration". If so, we should agree that any outward movement of matter in a rotating system is inertial and not due to a force.

To analyse the forces in a rotating system one has to start with the inertial point or points (ie. inertial axis). All accelerations are toward the inertial point or axis. So all forces are toward that inertial point or axis. The reaction force to a force that causes centripetal acceleration is a force that gives rise to an acceleration of matter in the opposite direction. Since all accelerations are centripetal, the reaction force to a centripetal force must be centripetal. That is the issue. To suggest that there are centrifugal forces at play suggests, wrongly, that there are centrifugal accelerations. There aren't any.

This whole concept of a reactive centrifugal force seems to be based on a flawed analysis of a the rotation of a body tethered to a fixed post by a string. The centripetal acceleration of the rotating body comes from the centripetal force exerted on the rotating body by the string tethered to the post. So, it is said, the reaction force (ie. the force of the rotating body and string on the post) is directed away from the post and is, therefore, centrifugal. But it only appears to be centrifugual because of the assumption that the post axis is the centre of rotation. If the post was the top of the mast of a small boat and the rotating body was a heavy ball tethered to the mast top, you would see that the post and the ball actually rotate about a central inertial point that is between the mast central axis and the ball. So the "reaction" to the force causing the centripetal acceleration of the ball toward the centre of rotation is a force that causes the acceleration of the mast toward the same central point: ie. it is centripetal. The principle is the same when you fix a post to the earth. It is just that the centripetal acceleration of the earth is negligible due to the relative mass of the earth. But the physics is exactly the same: since all accelerations are centripetal, all forces are centripetal. There is no centrifugal acceleration.

The fact is that in any rotating system rotating about an inertial point or axis, there is only centripetal acceleration and, therefore, only centripetal real forces. So we complain that this article does not make this clear and needs to be rewritten.AMSask (talk) 20:45, 21 April 2019 (UTC)

Ah, yes, I see what you're doing there, and how that could be formally correct even though it doesn't represent the directions that a normal person would call "away from the center". When there are forces in opposite directions, and you call both of them centripetal, even when one is on a wall or rail in a direction outward relative to any plausible center of rotation, you're just denying what's there and what people sometimes call it. Dicklyon (talk) 04:24, 22 April 2019 (UTC)

I am not sure if you actually do see the point of my complaint because I don't think a normal person would call a centripetal force one that is directed away from the centre. If it is directed toward the inertial axis of rotation it is centripetal. If you disagree, which apparently you do, why don't you explain where my analysis of the tethered rotating ball is wrong. How do you distinguish between the ball/mast rotation and a rotation about a post fixed on the earth?AMSask (talk) 05:29, 22 April 2019 (UTC)

Just a further comment: In my view, this article makes a fundamental mistake in the explanation of Newton's Third Law. When rigid body A collides with rigid body B, A exerts a force on B and B exerts an equal and opposite force on B - as measured in any inertial frame of reference (e.g. the centre of mass of the colliding bodies). This does not mean that A exerts a force only on the atoms of B that it is in contact with. A's mass exerts a force on the entire mass of B as does B on A. It does this through tensions between the atoms in each body during the collision. The tensions are not material for the purpose of the third law analysis. One has to analyse the resulting motion of the centres of mass of A and B relative to the inertial frame (for convenience, the centre of mass of the system).

Similarly, with a rotating system, one has to analyse the forces from an inertial frame of reference by examining the motions of the centres of mass of the bodies in question. Example: A circular space station in interstellar space creates "artificial gravity" by rotating so that a person on the outer rim would feel a force directing them toward the centre of rotation. Consider an astronaut walking on the perimeter rim who "feels" a force like gravity that enables him to walk along the space station outer rim. Is the reaction force of the astronaut against the space station centripetal or centrifugal? Well, there are two bodies whose motion one has to analyse: the astronaut in question, and the rest of the station and contents (including other astronauts). One first has to choose the inertial frame of reference for analysis. The centre of mass of the whole system is such a point. The whole system rotates about that centre of mass. The astronaut rotates about the centre of mass of the whole system, as does the centre of mass of rest of the space ship. So, the reaction force to the centripetal force exerted on the astronaut is a force that the astronaut exerts on the rest of the space station system that causes it to rotate about that same inertial point - their common centre of mass. As the astronauts move around in the physical space station structure, that structure will change position relative to the inertial centre of mass of the entire system.

The problem with calling the force of the astronaut on the outer rim of the space station "centrifugal" is that it is impossible to distinguish that force from the fictitious centrifugal force that any rotating body experiences. While it may appear to be outward from the centre of rotation at the point of application, it causes the centre of mass of the rest of the system to accelerate toward the centre of rotation. It is only by analysing the forces relative to an inertial point that one can see that the force of the astronaut on the space station and that of the space station on the astronaut cause each to experience centripetal acceleration, which is always directed toward the inertial centre of mass of the system. All accelerations are centripetal. There is no centrifugal real force. AMSask (talk) 22:51, 23 April 2019 (UTC) updated last para. AMSask (talk) 14:38, 27 April 2019 (UTC)

The real force that you're calling centripetal on the space station is the same real force that's sometimes called a reactive centrifugal force. Your different point of view on it does not make it not exist. I don't see why you say " one has to analyse the forces from an inertial frame of reference by examining the motions of the centres of mass of the bodies in question", but even if you do you can describe the resulting acceleration relative to that astronaut's path if you want to; it's not a different answer, just different words. Dicklyon (talk) 15:39, 27 April 2019 (UTC)

If you think both are the same force and the force causes acceleration of a body toward the centre of rotation, why would you call it centrifugal? All that does is lead to confusion! If it is centrifugal, where is the centrifugal acceleration?

The reason you have to analyse the forces in an inertial frame of reference is because one gets inertial effects that look like forces if the analysis is done in an accelerating frame.

The reason you have to examine the motions of the centres of mass of the bodies in question is so you can analyse the forces. Newton's third law is about "actions" which is an impressed force for a duration of time or a "change of momentum". In a freely rotating body, there is no change in total momentum so the forces must be equal and opposite: The force of the station on the astronaut that causes the astronaut to change momentum (toward the centre of rotation) must be accompanied by a force of the astronaut on the space station that causes a change in momentum that is equal in magnitude and in the opposite direction. But since we are describing the forces in relation to the inertial centre of rotation one has to relate the change in momentum relative to that point or axis. The forces are equal and opposite but because the centre of mass of the other body (the rest of the space station) is on the opposite side of the centre of rotation to the astronaut, both forces are directed toward the inertial centre of rotation. AMSask (talk) 15:14, 28 April 2019 (UTC)

That's not the only way to describe the reactive forces we're talking about here. And it really doesn't work for the centrifugal clutch, where there's no acceleration due to the two balanced reactive centrifugal forces, but the forces are real, outward with respect to the obvious center of rotation, and important to understanding the friction. Is there a problem with that description, other than that you prefer the reserve the word centrifugal for the fictitious force? Dicklyon (talk) 16:24, 28 April 2019 (UTC)

The centrifugal clutch is an example of the inertial centrifugal "force". The expanding clutch moves outward due to inertia. It is a real phenomenon. It just isn't a real force. There is no outward acceleration. It is true that there is a force of friction between the clutch and the housing and you can analyse it as if it is the centrifugal force acting on the rotating clutch that pushes against the housing. Nothing wrong with that at all. But in order to analyse the physics, which is what we are doing here, we should do it correctly.

Here are some uses that appear to make good sense: [1], [2], [3]. In the first one, for example, the forces of orbiting planets on the sun are described as centrifugal with respect to each planet's direction. There is no consistent center of rotation of the sun that could be used to describe these forces, and describing each with respect to its own contribution to the sun's tiny wobble wouldn't clarify anything. Similarly, if you want to look at the forces on your support brackets from a fluid going around a curve in a pipe, it makes most sense to look at it with respect to the center of curvature of that pipe, not the center of wobble of the world. Dicklyon (talk) 16:37, 28 April 2019 (UTC)

There is a consistent centre of rotation of the sun: the axis through the centre of mass of the solar system. It defines an inertial frame. All the bodies in the solar system, including the sun, rotate about it. The earth and moon rotate about the earth-moon barycentre. This is not controversial at all. The earth and moon both accelerate toward an axis through the barycentre. The 'reaction' force of the centripetal gravitational force of the earth on the moon is the centripetal gravitational force of the moon on the earth. The same analysis applies to any gravitational centripetal force. It doesn't change just because one body is much more massive than the other.

For the fluid going around a curve in a pipe, I agree it makes sense to analyse it that way if you are designing the pipe and brackets. If the brackets break and the pipe moves outward, it is because the pipe can't provide the required centripetal force to the fluid. So you cannot say that the pipe accelerates due the centrifugal reaction force (i.e the reaction force to the centripetal force of the pipe on the fluid).AMSask (talk) 06:53, 1 May 2019 (UTC)

d'Alembert force

The d'Alembert force "act's" in the inertial frame (force of inertia ==> -m*a). To describe the feelings of an passenger in an car is the d'Alembert force the right explanation. What you call the "reactive centrifugal force" is the reaction in the example from the centripetal force on the ball to the string. Nobody need's such an article.--Wruedt (talk) 07:25, 2 May 2019 (UTC)

The "force of inertia" and the general centrifugal force of rotating frames are not the same thing. Note that eq 45.15 in Lanczos makes a distinction between the centrifugal force and the force of inertia. Take for example an object that is stationary in the inertial frame, described from a rotating frame (with the object not at the origin). There is no "real" centripetal force in the inertial frame and the "force of inertia" is zero, but in the rotating frame there is both the centrifugal and Coriolis forces. --FyzixFighter (talk) 23:14, 2 May 2019 (UTC)

That is exactly what i say. When there is no centripetal force, then there is also no d'Alembert force. So the d'Alembert force is in an example with only 1 real force=centripetal force the opposit of the centripetal force and is also called centrifugal force. The d'Alembert force "act's in the inertial system and not in an rotating frame.--Wruedt (talk) 07:27, 3 May 2019 (UTC)

No, that is not what I am saying. The presence of the centrifugal d'Alembert force is present when there is a rotating frame of reference. The object that is stationary in the inertial frame has not real centripetal force, yet in a rotating frame there will be a centrifugal force due to the frame's rotation. Whenever the motion of the reference system generates a force which has to be added to the relative force of inertia, measured in that system we call that force an apparent/fictitious/d'Alembert/pseudo-force. The name is well chosen, as that force does not exist in the absolute or inertial system and is created solely by the fact that our reference system moves relative to the inertial system. It is only "real", ie indistinguishable in nature from other impressed forces, in the moving frame. --FyzixFighter (talk) 13:08, 3 May 2019 (UTC)

Look at Lanczos. The d'Alembert force is defined in the inertial frame and is the resistance of a body against the external force. See also all sources in technical mechanics books under prinziple of d'Alembert.--Wruedt (talk) 13:57, 3 May 2019 (UTC)

@Wruedt: Honestly, it appears that you are confusing the term "force of inertia" of d'Alembert's principle with the term "d'Alembert force". Yes, let's look at Lanczos, specifically page 100 (emphasis mine): "If a reference system is in accelerated motion, this motion produces the same effect as if external forces were present which have to be added to the impressed forces. These forces are called the "d'Alembert forces" or "apparent forces" because they are generated by the motion of the reference system but are not present in an absolute frame." According to Lanczos, d'Alembert forces (aka apparent forces) are not present in the absolute (inertial) frame. --FyzixFighter (talk) 20:56, 3 May 2019 (UTC)

d'Alembert force is not a common name for apparent forces. D'Alembert is referenced with the principle of d'Alembert. The force here introduced is the opposit of the external force. The principle is based on Newton 2, which is formulated in the IS. I don't know what this article want's to say. A simple reaction to the external force named "centripetal force" is the reaction to that force and could be named "reaction of centripetal force" (Newton 3).--Wruedt (talk) 07:04, 4 May 2019 (UTC)

I think the name "d'Alembert forces" as an alternate for apparent/fictitious forces is more common than you may think. A search at books.google.com returns multiple hits. It is connected to d'Alembert's principle, which states that F_net-m*a=0. This equation is true in inertial frames, but in accelerated frames (when a is expressed in terms of the coordinates of the accelerated frame) it fails. Additional forces must be added to F_net to make it true again - hence the name d'Alembert forces, or forces that restore the validity of d'Alembert's principle in accelerated frames. I admit I did not learn the name in university - only after dealing with the madness of fringe science pushers at centrifugal force did I learn of the term.

While almost all physics sources use the term "centrifugal force" to refer to the fictitious force of rotating frames, there are a few other uses such as how it is used for terms in Lagrangian mechanics (strangely this is principally in robotics sources). In engineering sources that look at the internal stress and strain of rotating objects, they talk about centrifugal forces, which are not the fictitious but one half of the action-reaction pair of centripetal-centrifugal forces between internal elements of the spinning object. As to the purpose of this page, I am torn. I was in favor of its creation several years ago, but that may have been a knee-jerk reaction to certain fringe science pushers (see User:FDT). If there was a merge discussion today, I think I would probably support merging it into the "Other uses of the term" section (it's mostly there already) of the centrifugal force article. --FyzixFighter (talk) 15:15, 4 May 2019 (UTC)

The engineering sources that look at the internal stress and strain of rotating objects are talking about real tensions within the rotating body. These are not outward forces, however. All net tensions are inward. If a piece breaks off and flies away, there is no longer any centripetal force, so the outward movement of matter is not due to a 3rd law pair force to the centripetal force. One can analyse the tensions as outward forces and that's fine for design purposes. But in terms of Newtonian physics, what the engineer is doing is ensuring that the rotating system can provide sufficient centripetal force to all of its parts during the rotation.AMSask (talk) 18:16, 9 May 2019 (UTC)

I would disagree that there are no outward forces - real tensions are made up of real forces. Take for example a chain being swung around in a circle. A free body diagram of any individual link (B) (which is not one of the end links) will have a centripetal force exerted on it by the next inward link (A) and a centrifugal force from the next outward link (C). The centrifugal force of C on B is equal and opposite to the centripetal force of B on C because they are 3rd law pairs. But the force of C on B is less in magnitude than the force of A on B so that the net force on B, and therefore the acceleration of B, is centripetal. --FyzixFighter (talk) 00:27, 10 May 2019 (UTC)

The centripetal force acting on a single link is supplied by the rest of the rotating system, not just the links that are immediately connected to it. The third law paired force to the centripetal force acting on one link is the force that the link exerts on the rest of the system, which includes the rest of the chain and the person swinging it around. All those things are experiencing centripetal accelerations. Saying there are centrifugal forces suggests that there are forces that are tending to cause matter to flee the central point of rotation. That is not what is happening.

Would you say that the 3rd law pair to the centripetal gravitational force that causes the moon to rotate about the earth-moon barycentre is a centrifugal force on the earth? If so, you would be wrong, even though the force that the moon exerts on me and other parts of the earth is away from the earth-moon barycentre at times. AMSask (talk) 15:01, 11 May 2019 (UTC)

Caution: The statements in this Article do not represent a consensus among physicists

The idea that there is a "real" ie. Newtonian centrifugal force is controversial. On the one hand, we have physicists who contend that when analysed in an inertial frame of reference, the reaction force to a centripetal force is always another centripetal force. Others contend that in a rigid or tethered body rotation, the reaction force to the rotating outer mass is a centrifugal (outward) force on the immediately adjacent inner mass. [Note: There is also a text (Delo E. Mook & Thomas Vargish (1987). Inside relativity. Princeton NJ: Princeton University Press. p. 47.) which states that the third law reaction force to the Sun's gravitational centripetal force on Jupiter is a force of Jupiter on the Sun that is centrifugual. This is simply wrong. While the force on the Sun by Jupiter is in the direction of Jupiter from the centre of mass of the Sun, it is toward the centre of mass of the two-body Sun/Jupiter system and is, therefore, centripetal.]

Certainly, in any rotating system all accelerations are centripetal and on this everyone seems to agree. Where there appears to be disagreement is with the analysis of the direction of forces in a rotating rigid body. In a rotating rigid body there are tensions between each part of the body and all other parts of the body. Some suggest that it is appropriate to refer to the outer mass pulling centrifugally on the adjacent inner mass. Others state that this not only confusing but also incorrect. AMSask (talk) 16:37, 11 October 2024 (UTC)