I Don't Ever Mention "Centrifugal Force" to Physicists

[hmmm27]

air compressor vs rare compressor.

Now, if somebody could come up with a way to remember which frame is "inertial" and which "non-inertial".

 

[vanhees71]

Why not just rename the centripetal force as the centrifugal force, the force a centrifuge accelerates an object towards the center of motion?

No, there is established terminology for centuries, and if you use it right, there's no problem. Inertial forces by definition occur only in non-inertial frames. That's all.

 

[sophiecentaur]

Can I just go back to School and the basics. When an object is constrained to move in a circle and then 'let go'. It does not move directly away from the centre of rotation. It leaves on a tangent and is not 'thrown outwards', as school kids had been told by generations of non-physicists.

What can a zealous Physics Teacher tell them? When released, the object (of course) goes further and further away from the centre and it travels in a straight line - blah blah - Pythagoras etc.. Pupils' eyes glaze over at an argument involving more than just one step.

So the teacher just tells them there is no such thing as centrifugal force. They take it on board and all go away and tell their Dad that he got it wrong because Mr. Scales said so. The term 'Imaginary Force' really doesn't help them, either.

It's just another of those "Mass is not Weight" and "Volts are not Pressure" things that Broadcasters love to ridicule.

 

[vanhees71]

It's not that difficult, and indeed our teacher in highschool managed to explain it to us very well:

In an inertial frame a body moves along a straight line with constant velocity (a velocity is a vector is vector...!). In conclusion if you want the body to move on a circle you need to apply a force of some kind, and that's called a centripetal force. You need this force also when the speed (magnitude of the velocity) is not changing. In this case you have a force of constant magnitude always pointing radially inwards. There are no centrifugal (or any other inertial) forces in an inertial frame of reference.

The restframe of a body in rotational motion is in accelerated motion relative to any inertial reference frame, and that's why in this frame there are inertial forces acting on the body in addition to the "true forces" (i.e., due to interactions, acting also in the inertial frame of reference). That's why you have in this rotating frame also a centrifugal force (and other inertial forces like the Coriolis force).

 

[bob012345]

No, there is established terminology for centuries, and if you use it right, there's no problem. Inertial forces by definition occur only in non-inertial frames. That's all.

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

 

[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).