Don't Ever Mention "Centrifugal Force" to Physicists

[PhDeezNutz]

Why not rename resistance to voltage? Why not rename force to energy?

Why not call everything "Bruce", just to prevent confusion?

 

[sophiecentaur]

Who says we must be slaves of old confusing terminology? Langauge changes. New students won't be confused.

I think you may well have something there. It does rely on teachers having a higher level of Science knowledge, though.

Many of the comments I read on PF are clearly from members who had a better than average grasp of the subject when young (bright kids or good teaching or both). Sometimes they forget the problems suffered by their peers. Personally, I was lazy rather than dim.

 

[vanhees71]

Who says we must be slaves of old confusing terminology? Langauge changes. New students won't be confused.

In this case, I don't see any confusion. Centripetal and centrifugal forces are well-defined for centuries.

 

[Vanadium 50]

Are people arguing that centrifugal and centripetal force are the same thing and the two names are what's confusing? That is not the case.

 

[bob012345]

Are people arguing that centrifugal and centripetal force are the same thing and the two names are what's confusing? That is not the case.

No, I was suggesting changing the name of the latter to the former because that's the more common word in people's minds. It was a sort of tongue in cheek but also half serious comment.

 

[Vanadium 50]

but also half serious comment.

I doubt changing the names would do anything besides increasing confusion. Of which there is already plenty.

 

[hutchphd]

No, I was suggesting changing the name of the latter to the former because that's the more common word in people's minds. It was a sort of tongue in cheek but also half serious comment.

There is also the matter of the 3000yr etymology of the word from Latin centrum (see center) + -fugus ‘fleeing’ (from fugere ‘flee’). This is just ridiculous.

 

[bob012345]

There is also the matter of the 3000yr etymology of the word from Latin centrum (see center) + -fugus ‘fleeing’ (from fugere ‘flee’). This is just ridiculous.

Does that really matter in the grand scheme of things?

 

[jbriggs444]

It leaves on a tangent and is not 'thrown outwards', as school kids had been told by generations of non-physicists.

Depends on your frame of reference.

Viewed from the rotating frame, it begins accelerating radially outward. But then begins curving due to the addition of Coriolis to the centrifugal force.

Viewed from an inertial frame, it moves tangentially in a straight line as the platform accelerates away in its circular path.

 

[bob012345]

Depends on your frame of reference.

Viewed from the rotating frame, it begins accelerating radially outward. But then begins curving due to the addition of Coriolis to the centrifugal force.

Viewed from an inertial frame, it moves tangentially in a straight line as the platform accelerates away in its circular path.

Why do we call an inertial frame one that does not have inertial forces but a non-inertial frame is one that does?

 

[Vanadium 50]

Why do we call an inertial frame one that does not have inertial forces but a non-inertial frame is one that does?

Why do we drive on a parkway and park on a driveway?

 

[sophiecentaur]

Depends on your frame of reference.

Of course it does. But a school student is standing, looking at the event and frames of reference are not dealt with early on in school. Just ask a kid to draw a spinning disc and what will happen to an object when released from the edge. 100:1 they will draw a radial line (or a curve). This is the level that we're (I'm) dealing with and, to my mind, forbidding centrifugal force is forgivable at that stage. Using the concept of a fictitious force could turn the whole business into magic and we don't want that.

I already made a comment about the superior knowledge of many / most PF members. Just read the large number of crazy versions of QM that are posted; it's just the same, even with basic Newtonian Mechanics. You can lead a horticulture . . . . .

 

[vanhees71]

Are people arguing that centrifugal and centripetal force are the same thing and the two names are what's confusing? That is not the case.

In my experience the main confusion comes from not clearly distinguishing centrifugal force (an "inertial force", only present in the description of the motion in a non-inertial frame) and centripetal force (a "true force" occuring in the description of the motion in a non-inertial frame).

Of course, with clear definitions, there's no confusion in the first place, but it's nonsense to say that "inertial force" wouldn't exist. They clearly occur in the description of motions using non-inertial reference frames. It's also clear that the corresponding equations of motion are the same Newtonian equations of motion as in inertial frames and derived from them, but expressed in coordinates defined in a non-inertial frame.

 

[hutchphd]

“When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean—neither more nor less.” “The question is,” said Alice, “whether you can make words mean so many different things.” “The question is,” said Humpty Dumpty, “which is to be master—that’s all.”

 

[vanhees71]

Obviously Humpty Dumpty is a philosopher not a natural scientist ;-)). SCNR.

 

[A.T.]

Why do we call an inertial frame one that does not have inertial forces but a non-inertial frame is one that does?

Why is "velocity" relative, but "velocity relative to X" absolute?

 

[Demystifier]

Some of you have seen this already when a similar discussion flared up and I apologize for the repetition. I resisted at first, but it is worth reviving because it encapsulates the controversy well.

View attachment 317957

The wheel acts by centripetal force on Bond. But then, by the 3rd Newton law, Bond acts on wheel by the opposite force. Isn't this opposite force the centrifugal force?

 

[Demystifier]

Why is "velocity" relative, but "velocity relative to X" absolute?

The latter is absolute precisely because it's relative.

 

[jbriggs444]

The wheel acts by centripetal force on Bond. But then, by the 3rd Newton law, Bond acts on wheel by the opposite force. Isn't this opposite force the centrifugal force?

That opposite force is sometimes called the "reactive centrifugal force". It is not, of course, the same as the inertial "centrifugal force" that is most often meant by the phrase.

One can argue that what is important to Mr. Bond is the stresses and associated deformation that he experiences as a result of the applied centripetal force. The essential problem is that this force is unevenly applied. Unlike inertial forces which are normally distributed evenly throughout a body (though see Niven's "Neutron Star"), the centripetal force on Mr. Bond is applied only to his back side.

Edit: In "Neutron Star", our hero, Beowulf Shaeffer pilots a craft build from a General Products hull to investigate a neutron star to which several missions have previously failed. Upon close approach to the star, he experiences a strange force that is able to penetrate the impenetrable hull. The "strange force" turns out to be tidal gravity. Beowulf then extorts compensation from the paranoid Puppeteers who produce the hull because he now deduces one of their secrets -- they evolved on a planet without a moon.

An amusing afterthought by the author:

"Niven writes: "I keep meeting people who have done mathematical treatments of the problem raised in the short story 'Neutron Star' ... Alas and dammit, Shaeffer can't survive. It turns out that his ship leaves the star spinning, and keeps the spin."

Thus we come full circle to a spinning man.

 

[sophiecentaur]

That opposite force is sometimes called the "reactive centrifugal force". It is not, of course, the same as the inertial "centrifugal force" that is most often meant by the phrase.

But if it quacks like a duck . . . . . Those two forces have the same values and would have the same visible effect (wherever you're looking from).
But isn't it all to do with what is implied by the word "outwards"? It is straining to go outwards but, once released it actually goes tangentially (to the outside observer). To Mr Bond, what he releases seems to go away and 'backwards'.
As a kid in school, how could you describe it?

 

[jbriggs444]

But if it quacks like a duck . . . . . Those two forces have the same values and would have the same visible effect (wherever you're looking from).

They do in some cases. Not all. The inertial centrifugal force continues to exist (in the rotating frame) even after the string breaks or the centrifuge explodes or the car hits a patch of black ice. The reactive centrifugal force ceases to exist when the string breaks, etc.

The centrifugal force also exists (in the rotating frame) even though it may be cancelled by a more-than-equal and opposite Coriolis force. In that circumstance, the reactive centrifugal force does not exist. [e.g. an object at rest in the inertial frame as viewed from the rotating frame. It has outward centrifugal force, inward Coriolis force and inward centripetal acceleration. But zero net external interaction forces.

Pick a frame. Then pick the associated inertial force laws. Objects will move as the forces demand.

Kids in school... are taught that there is one right answer to any question. The truth is otherwise.

 

[sophiecentaur]

The inertial centrifugal force continues to exist (in the rotating frame) even after the string breaks

That's because of the apparently curved / zig-zag path seen by Mr. Bond? Sounds very non-Newtonian, though.

 

[jbriggs444]

That's because of the apparently curved / zig-zag path seen by Mr. Bond? Sounds very non-Newtonian, though.

Archimedian spiral, approximately. It is Newtonian -- provided that you ignore the third law. Inertial forces such as centrifugal and Coriolis are not interaction forces. They have no third law partners.

As the cartoon suggests, perform a coordinate system transformation and force-like effects pop out.

 

[sophiecentaur]

Archimedian spiral, approximately. It is Newtonian -- provided that you ignore the third law. Inertial forces such as centrifugal and Coriolis are not interaction forces. They have no third law partners.

As the cartoon suggests, perform a coordinate system transformation and force-like effects pop out.

Well put. And something else to confuse young minds with - just after they learn to distinguish between third law pairs and forces in equilibrium.

 

[vanhees71]

The 3rd law is an approximation anyway ;-)). It's fully substituted by the momentum-conservation law (synmmetry under spatial translations) in special relativity and the necessity of dynamical fields for local interaction laws (there's no interacting-point-particle dynamics in SR anyway).

 

[Omega0]

I've just come across the following line while studying (Young & Freedman) and found it amusing.

It sounds like a dirty family secret we discuss once and then should never mention again

I very well remember that a prof told me the same. "This is not a force!"
Well, to be honest: If it feels phyisically like a force and I could measure it as a force it is force - for me.

 

[Ibix]

If it feels phyisically like a force and I could measure it as a force

You can't - that's kind of the point. You can measure things that (in a rotating frame) you might call a reaction to a centrifugal force, but you cannot measure the centrifugal force itself.

 

[A.T.]

it feels phyisically like a force

It doesn't. When analyzing from a rotating frame, the inertial centrifugal force applies to everything that is not on the frame rotation axis. Even to things that are perfectly inertial and definitely don't "feel" any force. It's just there to make the accelerations in the rotating frame consistent with Newtons 2nd Law, not to explain any deformation or stresses.

 

[Dale]

If it feels phyisically like a force and I could measure it as a force it is force - for me.

As others have said, you cannot feel or measure an inertial force like the centrifugal force. All that you can do is to infer it through the motion of the object wrt some frame given all of the directly measurable real forces.

For example, in a rotating reference frame you do not measure the centrifugal force on a co-rotating object. You measure the real centripetal force, and then because the object is not accelerating you infer that there must exist a centrifugal “inertial” force which balances the real centripetal force.

 

[Omega0]

You can't - that's kind of the point. You can measure things that (in a rotating frame) you might call a reaction to a centrifugal force, but you cannot measure the centrifugal force itself.

I am speaking about local measurements in spacetime. Is it just wording?

Say, you have a cylinder with a quite huge radius, rotating. You are inside and grown up inside and you can't see the other side, no curvature, nothing. You jump and you fall back. You call it, whyever, centrifugal force. In other topologies you may have called it gravitation, which I can measure, but in this world I call it "centrifugal force".

What is so wrong about this picture?

 

[Ibix]

You jump and you fall back. You call it, whyever, centrifugal force.

But that is a mistake. A careful analysis will show that you are moving inertially while not in contact with the cylinder - you move in a straight line.

In other topologies you may have called it gravitation, which I can measure,

Well, gravity is another thing you can't measure. You can measure the force from the floor on your feet (that's what a weighing scale does), but when you are in free fall there is no detectable force. That's a key insight (the equivalence principle) on the road to general relativity.

(Actually in a true gravitational field you will see tidal effects that are measurable, which you won't in free fall inside a spinning cylinder.)

 

[Dale]

Is it just wording?

It is not just wording. It is a matter of experiment. There is no physical experiment you can make to actually measure an inertial force. You can only infer it from the acceleration combined with measurements of all of the real forces.

You jump and you fall back.

I.e. you infer the force from your motion.

You cannot measure it in any other way. It does not produce a reading on an accelerometer or a strain gauge or anything else you can think of. While you are only under the influence of that force all local experiments will be the same as in free fall far from gravity.

 

[Ibix]

I.e. you infer the force from your motion.

Just to expand on that, you can infer from the changing relative motion between you and the floor that there's a force on at least one of you and the floor. And accelerometers attached to the floor will read non-zero while those attached to you read zero. Conclusion: there's a force on the floor and it's accelerating, you are inertial.

The centrifugal force (and other inertial forces) are essentially the terms you have to take from one side of $F=ma$ to the other so that you can say that the floor is not accelerating. That's certainly a useful perspective, but you choosing to juggle terms in the maths doesn't make your sensors behave differently - so centrifugal force is not a thing you can measure.

 

[Omega0]

Oh, okay, shame on me. I really thought that something which is actually called a force could be called a force. I know the laws from mechanics, we have
$$ \vec{F}_z = -m \vec{\omega} \times (\vec{\omega} \times \vec{r})$$
and that this has for example the name "centrifugal force". I mean, it is obvious that you can cancel the "thing" as an observer outside via a simple transformation. A "real force" is one you will get never get rid off, for example in a free falling system in a gravitational field. If you can't find a transformation from your IS to get rid of the "forces" like unusual displacements, they are forces, correct?

 

[Omega0]

You cannot measure it in any other way. It does not produce a reading on an accelerometer or a strain gauge or anything else you can think of. While you are only under the influence of that force all local experiments will be the same as in free fall far from gravity.

Ahhhh I got it, sorry. Absolutely right. Thanks. Maybe I watched too many movies where I thought afterwards that rotation is a nice alternative to gravitation but it is not. So clear. My fault.