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

[Argonaut]

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

 

[Dale]

It is hard to avoid if you use a rotating reference frame.

 

[hutchphd]

And besides we all take delight in our secret perversions....
It actually can be very useful and I confess to uttering those particular profane words on multiple occasions.

 

[hmmm27]

A research scientist at Cambridge suggested the term "centripugal", that being a little less open to misinterpretation than "centrifetal".

 

[Baluncore]

I find "radial" forces less provocative of a hostile response.

 

[kuruman]

Centrifugal force is like chainsaw. In the hands of an expert it can be quite useful; in the hands of novice it can do a lot of damage. Young & Freedman are trying to protect the innocent from themselves, hence the statement.

 

[gmax137]

I like the term "fictitious forces."

I did not like it the first time I heard it, but once the idea sunk in it made sense to me.

 

[DaveE]

"In an inertial frame..."
Context is everything. Don't jump to the exciting bits without pondering the qualifiers first.

 

[topsquark]

I have no objection at all to the term centrifugal force. I simply object to its being incorrectly used, which it almost always is when I see it outside of a textbook.

-Dan

 

[vanhees71]

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

What a nonsense. Of course, in an inertial frame of reference there are no inertial forces, but they are very useful to analyze motions in non-inertial frames like, e.g., the reference frames we use everyday in the lab, i.e., a rest frame wrt. a point (our position) on the surface of the Earth. It's almost always enough to neglect the acceleration, but one of the most famous demonstrations of the Earth's rotation around its axis is Foucault's pendulum. There usually the centrifugal part of the inertial forces is, however, neglected (rightfully), but the Coriolis force must be taken into account.

Inertial forces are of course not "real forces" in some sense, i.e., they are not due to the fundamental interactions (gravity, electroweak and strong interactions) but belong to the left-hand side, $m \vec{a}$, of the equation of motion. Nevertheless it's easier to think of them intuitively as "inertial forces", bringing them on the right-hand side $\vec{F} \rightarrow \vec{F}+\vec{F}_{\text{inertia}}$.

 

[kuruman]

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.

 

[Lnewqban]

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

How should we call centrifugal pumps, fans and compressors?
Centripetal fluid accelerator machines?

 

[Herman Trivilino]

How should we call centrifugal pumps, fans and compressors?
Centripetal fluid accelerator machines?

You either use the terms as they appear in the manufacturer's literature or spend a lot of time explaining to people why they are misnamed. In process technology, pumps move liquids only, and compressors move gasses only. So by that convention a vacuum pump should be called a vacuum compressor. But the manufacturers disagree!

 

[Herman Trivilino]

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.

Note that in an inertial frame the rim of the wheel exerts a centripetal force on Mr. Bond. But Mr. Bond exerts a centrifugal force on the rim of the wheel.

 

[kuruman]

You either use the terms as they appear in the manufacturer's literature or spend a lot of time explaining to people why they are misnamed.

But they are not misnamed. The name depends on an equivalent frame of reference in which the working principles are formulated.

 

[Vanadium 50]

@Argonaut played a trick on you all, and he got you real good. Ha ha ha! The joke's on you!

Here is the whole thing in context:

CAUTION Avoid using “centrifugal force” Figure 5.30 shows both a correct free-body diagram for uniform circular motion (Fig. 5.30a) and a common incorrect diagram (Fig. 5.30b). Figure 5.30b is incorrect because it includes an extra outward force of magnitude v^2/R to “keep the body out there” or to “keep it in equilibrium.” There are three reasons not to include such an outward force, usually called centrifugal force (“centrifugal” means “fleeing from the center”). First, the body does not “stay out there”: It is in constant motion around its circular path. Because its velocity is constantly changing in direction, the body accelerates and is not in equilibrium. Second, if there were an additional outward force that balanced the inward force, the net force would be zero and the body would move in a straight line, not a circle (Fig. 5.29). And third, the quantity m(v^2/R) is not a force; it corresponds to the m\vec{a} side of \sum\vec{F} = m\vec{a} and does not appear in \sum\vec{F} (Fig. 5.30a). It’s true that when you ride in a car that goes around a circular path, you tend to slide to the outside of the turn as though there was a “centrifugal force.” But we saw in Section 4.2 that what really happens is that you tend to keep moving in a straight line, and the outer side of the car “runs into” you as the car turns (Fig. 4.11c). In an inertial frame of reference there is no such thing as “centrifugal force.” We won’t mention this term again, and we strongly advise you to avoid using it as well

So, they are not saying there is never utility in inertial forces. They are saying not to misuse the concept, and 10% of the way through a first course of physics is not the place.

 

[bob012345]

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

 

[Ibix]

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

While we're at it, let's swap the sign on the electron charge so electron flow matches conventional current!

 

[bob012345]

While we're at it, let's swap the sign on the electron charge so electron flow matches conventional current!

I think that's a very positive suggestion.

 

[Ibix]

I think that's a very positive suggestion.

Indeed, and I can't see any negatives either.

 

[bob012345]

Indeed, and I can't see any negatives either.

Ok, we should put you in charge of the process at the current time.

 

[Vanadium 50]

Why not just rename the centripetal force as the centrifugal force,

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

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

 

[Ibix]

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

Excellent idea @Bruce.

 

[Argonaut]

@Argonaut played a trick on you all, and he got you real good. Ha ha ha! The joke's on you!

Here is the whole thing in context:
So, they are not saying there is never utility in inertial forces. They are saying not to misuse the concept, and 10% of the way through a first course of physics is not the place.

I didn't mean to mislead anyone, and if I did, I apologise. I'm probably just too ignorant of physics to even realise. I just found the authors' insistence on avoiding the term funny - that's all!

 

[kuruman]

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

Excellent idea! That would be the first step towards a one-size-fits-all Theory of Everything. For example,

$\mathbf{Bruce}=Bruce~\mathbf{Bruce}$ (Newton's Second Law)
$Bruce=Bruce~Bruce$ (Ohm's Law)
$Bruce=Bruce~c^2$ (the speed of light is the same in all formulations)
etc. etc.

I stop here lest the thread be closed by the mentors for silliness.

 

[kuruman]

I didn't mean to mislead anyone, and if I did, I apologise. I'm probably just too ignorant of physics to even realise. I just found the authors' insistence on avoiding the term funny - that's all!

You did fine and, as you can see, people had fun here.

 

[Ibix]

I didn't mean to mislead anyone, and if I did, I apologise. I'm probably just too ignorant of physics to even realise. I just found the authors' insistence on avoiding the term funny - that's all!

There's more than one concept that students will have heard of but it's better to avoid (at least initially) because they always lead to confusion in novice hands. So you will occasionally come across "I know you know this word and I know you're expecting me to use it but I'm not going to". I suspect it's not an easy thing to use a concept enough to explain why you're not going to use it without using it too much.

 

[Nugatory]

But Mr. Bond exerts a centrifugal force on the rim of the wheel.

It is a radial outward force, but it is not the fictitious force that appears in the rotating non-inertial frame and is generally called “centrifugal”. That fictitious force acts on Bond, not the rim of the wheel.

 

[Baluncore]

While we're at it, let's swap the sign on the electron charge so electron flow matches conventional current!

Just keep in mind that in Italian, "voltae" means to "turn".

 

[nasu]

You either use the terms as they appear in the manufacturer's literature or spend a lot of time explaining to people why they are misnamed. In process technology, pumps move liquids only, and compressors move gasses only. So by that convention a vacuum pump should be called a vacuum compressor. But the manufacturers disagree!

A gas compressor compresses a gas. A liquid pump moves a liquid. A vacuum "device" neither compresses nor moves a vacuum. However, it may do both moving oil and compressing gases. So maybe we should just call it a "vacuum maker". Or maybe just Mister Vacuum.

 

[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.