Difference between centrifugal force vs reactive centrifugal force?

[etotheipi]

Well, that depends on what you take as the "center". If one uses the barycenter then gravity is purely centripetal at all times. If one uses the instantaneous center of curvature of the trajectory then gravity is only purely centripetal at two points, as you point out.

I was under the impression that it is by definition always the latter, i.e. the normal component in intrinsic coordinates, $\frac{mv^2}{\rho}$.

The component toward the barycentre would I guess be towards a centre of some sort, but it wouldn't be what we call "centripetal force"?

 

[jbriggs444]

But then in that case, can one use the formula for centripetal force $\frac{mv^2}{r}$? I guess not, since this formula adapts the second convention.

Yes, $a=\frac{v^2}{r}$ fits the instantaneous center of rotation while $a=\frac{GM_r}{r^2}$ fits the barycenter [here, $M_r$ is the "reduced mass" of the other body].

I was under the impression that it is by definition always the latter, i.e. the normal component in intrinsic coordinates, $\frac{mv^2}{\rho}$.

The component toward the barycentre would I guess be towards a centre of some sort, but it wouldn't be what we call "centripetal force"?

In my book, "centripetal" and "centrifugal" are directions. Literally toward the center and away from the center. But I am not a definition Nazi. Use words as you wish. Just be aware that others may use them differently.

 

[vanhees71]

There is the gravitational force of the sun acting on the planet. The third law pair to that force is the gravitational force of the planet acting on the sun. Both forces are centripetal.

Sure, but nowhere (using the usual terminology) is there any centrifugal force.

 

[Ibix]

Sure, but nowhere (using the usual terminology) is there any centrifugal force.

Indeed. There's at least one seventeen page thread on the topic here. However in the ball on a string example, there is a third law pair to the inward pointing centripetal force on the ball and this force is outward pointing. This would be the "reactive centrifugal force". It isn't present for planets because the third law pairs are both centripetal, since there is no mechanical connection between the planets.

I think the problem is that some sources do use the words "centrifugal force" to mean the thing I've called a "reactive centrifugal force", which is most definitely a proper force in the cases where it's present. This is sloppy terminology - even the Wikipedia article on centrifugal force notes this usage as "deprecated" - and is what's causing all the confusion here.

I also agree that "reactive centrifugal force" is not a term I'd come across before the first time I saw this argument on here. I mostly remember it just to answer people like the OP who've confused it with the inertial force.

 

[Dale]

Sure, but nowhere (using the usual terminology) is there any centrifugal force.

Yes. This is why I dislike the term “reactive centrifugal force”. Sometimes the reaction force to a centripetal force is centripetal, not centrifugal.

 

[jbriggs444]

Yes. This is why I dislike the term “reactive centrifugal force”. Sometimes the reaction force to a centripetal force is centripetal, not centrifugal.

Meanwhile, I dislike the "reactive" part. It reinforces the incorrect notion that there is some sort of asymmetry in a third law force pair. Cause and effect, action and reaction rather than simply two facets of the same interaction.

 

[Dale]

Meanwhile, I dislike the "reactive" part. It reinforces the incorrect notion that there is some sort of asymmetry in a third law force pair. Cause and effect, action and reaction rather than simply two facets of the same interaction.

Excellent! Yet another reason to not like the term

 

[vanhees71]

Indeed. There's at least one seventeen page thread on the topic here. However in the ball on a string example, there is a third law pair to the inward pointing centripetal force on the ball and this force is outward pointing. This would be the "reactive centrifugal force". It isn't present for planets because the third law pairs are both centripetal, since there is no mechanical connection between the planets.

I think the problem is that some sources do use the words "centrifugal force" to mean the thing I've called a "reactive centrifugal force", which is most definitely a proper force in the cases where it's present. This is sloppy terminology - even the Wikipedia article on centrifugal force notes this usage as "deprecated" - and is what's causing all the confusion here.

I also agree that "reactive centrifugal force" is not a term I'd come across before the first time I saw this argument on here. I mostly remember it just to answer people like the OP who've confused it with the inertial force.

What's the normal terminology what you rename "reactive centrifugal force"? There's a string tension here, and it's as centripetal a force as the gravitational force in the planet-Sun (Kepler) example.

Also third law refers always to interactions, i.e., to the forces acting on two different bodies or, for the most general case of general $N$-body forces, it boils down to the center-of-mass motion, which for a closed system is always uniform and thus finally to global momentum conservation for closed systems as a consequence of homogeneity of space according to Noether's theorem.

 

[vanhees71]

Meanwhile, I dislike the "reactive" part. It reinforces the incorrect notion that there is some sort of asymmetry in a third law force pair. Cause and effect, action and reaction rather than simply two facets of the same interaction.

action and reaction is not a cause-and-effect relation. It's an artifact of Newtonian physics, where you have action-at-a-distance ineractions, which of course is an approximation to real interactions mediated by fields.

 

[jbriggs444]

What's the normal terminology what you rename "reactive centrifugal force"? There's a string tension here, and it's as centripetal a force as the gravitational force in the planet-Sun (Kepler) example.

The outward force of rock on string is clearly centrifugal.

Of course, strictly speaking, string tension is not a force at all. It is a condition in the string. A component of a stress tensor. It is more akin to a force pair rather than to a single member of a force pair.

 

[vanhees71]

Which outward force? In the inertial frame there's a centripetal force pointing towards the center caused by the tension of the string. Of course tension is a force binding the atoms/molecules together the string is made of. What else should it be? Maybe you are referring to the rigid-body approximation, where you substitute the (elastic) forces by the rigidity constraint?

 

[jbriggs444]

Which outward force?

The outward force of rock on string, as I said. That force exists at the interface between rock and string. It has a direction and a location. The direction is unambiguously away from the center of rotation.

 

[vanhees71]

Sure, but it's not a centrifugal force but just a force in an inertial frame. As you say, it's a "contact formce". So don't call something a centrifugal (inertial) force in an inertial frame. By definition there are no inertial forces in inertial frames of reference. The wrong wording leads to utmost confusion of the students, as is demonstrated by this thread which consists of 63 contributions just to eliminate the wrong thinking from this wrong wording again!

 

[jbriggs444]

Sure, but it's not a centrifugal force but just a force in an inertial frame. As you say, it's a "contact formce". So don't call something a centrifugal (inertial) force in an inertial frame. By definition there are no inertial forces in inertial frames of reference. The wrong wording leads to utmost confusion of the students, as is demonstrated by this thread which consists of 63 contributions just to eliminate the wrong thinking from this wrong wording again!

We do not seem to be communicating well.

You referred to string tension as centripetal. I took issue with this. String tension is neither centrifugal nor centripetal. It is a force pair, not a force.

There is a force pair that exists between rock and string. As you agree, it is a contact force. One member of that force pair is directed toward the center. One member of that force pair is directed away. I do not know of a pithy name other than "reactive centrifugal force" to refer to the outward force of rock on string.

Your words seem to suggest that you want to label the outward force of rock on string as "centripetal". But I am sure that is not what you intend.

 

[vanhees71]

It's in any case NOT a centrifugal force, because there is no centrifugal force in an inertial frame of reference by definition. In you example the force on the rock is due to the tension in the string. At the end of the string the rock's contact force is opposite and of equal magnitude of course (Newton's 3rd law).

 

[jbriggs444]

It's in any case NOT a centrifugal force, because there is no centrifugal force in an inertial frame of reference by definition. In you example the force on the rock is due to the tension in the string. At the end of the string the rock's contact force is opposite and of equal magnitude of course (Newton's 3rd law).

It is a force that acts in the centrifugal direction. Taking "centrifugal" here to mean literally "away from the center".

 

[etotheipi]

It is a force that acts in the centrifugal direction. Taking "centrifugal" here to mean literally "away from the center".

I think perhaps the intention was that the terminology "centrifugal force" is so overwhelmingly associated with the $-m\vec{\omega} \times (\vec{\omega} \times \vec{r})$ that arises in a rotating frame of reference, that the term "reactive centrifugal force" (even though it's perhaps correct in the sense "fugal $\equiv$ away") is unhelpful for students. Not least because it presents the risk of conflating two very different ideas, just as these last few threads have demonstrated!

 

[vanhees71]

I fight against the use of the word "centrifugal" in the context of forces at all as long as we discuss physics from the point of view of an inertial observer, because it's much more economic to avoid misleading language and stick to clear definitions. A centrifugal force is an inertial force and as such can by definition only present in non-inertial (in this case rotating) frames of reference.

 

[Dale]

I fight against the use of the word "centrifugal" in the context of forces at all as long as we discuss physics from the point of view of an inertial observer, because it's much more economic to avoid misleading language and stick to clear definitions. A centrifugal force is an inertial force and as such can by definition only present in non-inertial (in this case rotating) frames of reference.

I don't think that anyone here disagrees with you. The term "reactive centrifugal force" is rather like "relativistic mass". It has a standard known definition that is not from Physics Forums and is not liked by most participants here, but sometimes you see someone who is confused by it and so we have to explain what it means and also why we dislike it.

 

[A.T.]

I fight against the use of the word "centrifugal" in the context of forces at all as long as we discuss physics from the point of view of an inertial observer, ...

I am even more annoyed by the "reactive" part because I dislike the "action/reaction" terminology with respect to Newtons 3rd Law.

 

[Dale]

So on PF out of the term "reactive centrifugal force" the only non-annoying part is "force"

 

[jbriggs444]

So on PF out of the term "reactive centrifugal force" the only non-annoying part is "force"

It's a momentum flow, darn it :-)

Edit: And may the Schwartz be with you always.

 

[Dale]

It's a momentum transfer, darn it :-)

Haha! But Star Wars just wouldn't be the same if the Jedi had to say "Use the momentum transfer, Luke!"

I just watched Rogue One again, so I guess it would be "I am one with the momentum transfer, the momentum transfer is with me ..."

 

[Aeronautic Freek]

I was allways excelent at math but allways strugle with physics and find very harder to understand than math..
I don't know if I am stupid or I have poor bascis of physics so it is hard to follow you ,but I find most of people consider physics harder to learn...

A.T. ,Dale,jbbriss44,Ibix etc,,are you proffesors of phyiscs or normal people,how you know physics so well?

 

[jbriggs444]

A.T. ,Dale,jbbriss44,Ibix etc,,are you proffesors of phyiscs or normal people,how you know physics so well?

Purely amateur here. I do IP networking for a living. Though the team members have nick-named me "The Professor".

 

[Dale]

A.T. ,Dale,jbbriss44,Ibix etc,,are you proffesors of phyiscs or normal people,how you know physics so well?

I teach physics and programming professionally, but as an instructor in industry rather than at a university in academia

 

[Ibix]

A.T. ,Dale,jbbriss44,Ibix etc,,are you proffesors of phyiscs or normal people,how you know physics so well?

It's a spare time thing for me as well, although I have done postdoctoral work in physics in the past.

Frames, switching between them, and keeping track of how representations change as you do so is non-trivial, I think. This confusion of terminology about centrifugal forces doesn't help.

 

[vanhees71]

It's a momentum flow, darn it :-)

Edit: And may the Schwartz be with you always.

Are you a proponent of the Karlsruhe Physics Course? This is however the worst product of physics didactics ever. It's maximizing the confusion of students and is in some parts conceptually wrong (applying the Gauß integral theorem to non-closed surfaces, selling entropy as the same as heat or as a modern form of phlogiston etc. etc.).

 

[vanhees71]

I was allways excelent at math but allways strugle with physics and find very harder to understand than math..
I don't know if I am stupid or I have poor bascis of physics so it is hard to follow you ,but I find most of people consider physics harder to learn...

A.T. ,Dale,jbbriss44,Ibix etc,,are you proffesors of phyiscs or normal people,how you know physics so well?

Well, if you are excellent at math, try theoretical physics!