I Why Does a Glass Move Outward When Opening the Fridge Door?

[A.T.]

Centrifugal force is absolutely a very real inertial force in an inertial reference frame.

In this context, the term "real" usually refers to interaction forces that obey Newton's 3rd Law, which inertial forces do not. Terms like "real" & "fictitious" just lead to pointless philosophical debates, when that merely technical definition is taken out of context. It's better to to use "interaction" & "inertial" instead here.

ETA: And as @Orodruin points out below, inertial forces exist only in non-inertial frames.

 

[Orodruin]

Centrifugal force is absolutely a very real inertial force in an inertial reference frame.

The centrifugal force is zero in an inertial frame by definition. So no.

 

[jbriggs444]

Cut the string and the object zooms away along a tangential path. Well, cutting the string eliminates the centripetal and centrifugal forces, so of course the object moves off straight forward.

If centripetal and centrifugal forces are equal and opposite and if they both vanish when you cut the string then the object should have been moving in a straight line even before you cut the string.

Indeed, it did have constant velocity in the rotating frame.
However, in the inertial frame, it did not.

 

[ALBAR]

I don't understand why people don't stay in the essentially inertial laboratory reference frame when pondering basic mechanics. I agree that if centrifugal force were applied externally as the centripetal is, the object would go straight (with perhaps some lateral stretching). The centripetal force acts normal to the velocity while the object's matter does what matter always does: it resists any change of its momentum by generating inertial force. Neither force can exist without the other. All forces must have counter force. If you don't believe Newton, try pushing or pulling on nothing. The roles of these forces are cause and effect. Centripetal causes acceleration, simultaneously triggering production of inertial force by Newton's second and third laws. That force is what I boldly tag the word "centrifugal" onto.

 

[jbriggs444]

I don't understand why people don't stay in the essentially inertial laboratory reference frame when pondering basic mechanics. I agree that if centrifugal force were applied externally as the centripetal is, the object would go straight (with perhaps some lateral stretching). The centripetal force acts normal to the velocity while the object's matter does what matter always does: it resists any change of its momentum by generating inertial force. Neither force can exist without the other. All forces must have counter force. If you don't believe Newton, try pushing or pulling on nothing. The roles of these forces are cause and effect. Centripetal causes acceleration, simultaneously triggering production of inertial force by Newton's second and third laws. That force is what I boldly tag the word "centrifugal" onto.

That is the "reactive centrifugal force". Yes, it is a real force. But it is not what is meant by "centrifugal force". The "centrifugal force" arises only in the non-inertial rotating frame.

As you point out, the centrifugal force in the non-inertial sense does not have a corresponding third law reaction force.

 

[Ibix]

The centripetal force acts normal to the velocity

Right.

the object's matter does what matter always does: it resists any change of its momentum by generating inertial force.

That's not what the phrase "inertial force" means, and I don't think there's any concept similar to what you are describing here.

All forces must have counter force.

I assume you're intending to cite Newton's third law here. If so, I don't think it means what you think it means. The centripetal force on the bottle has a third law pair often called the "centrifugal reaction force" which acts on the fridge door. This is not an inertial force and is not the same as the centrifugal force, which is an inertial force and is therefore zero in the inertial frame.

 

[Orodruin]

I don't understand why people don't stay in the essentially inertial laboratory reference frame when pondering basic mechanics. I agree that if centrifugal force were applied externally as the centripetal is, the object would go straight (with perhaps some lateral stretching). The centripetal force acts normal to the velocity while the object's matter does what matter always does: it resists any change of its momentum by generating inertial force. Neither force can exist without the other. All forces must have counter force. If you don't believe Newton, try pushing or pulling on nothing. The roles of these forces are cause and effect. Centripetal causes acceleration, simultaneously triggering production of inertial force by Newton's second and third laws. That force is what I boldly tag the word "centrifugal" onto.

As others have already said, you are confusing "centrifugal force" with "reactive centrifugal force". The actual centrifugal force is defined as the inertial force due to angular velocity and distance from the rotational center in a rotating frame. This force is zero in any inertial frame. You can attach words as you wish, but unless you use standard terminology, you will have a hard time to make yourself understood.

Reactive centrifugal force is not what people generally mean when they say "centrifugal force" as per the above. It is just the third law pair of whatever the centripetal force is and it does not act on the object itself - it is a force from the object on whatever is keeping it in rotational motion.

inertial force by Newton's second and third laws

That's not what an inertial force is. An inertial force is acting on the object itself and has no third-law partner. It is merely due to the effects of using a non-inertial reference frame and is therefore always proportional to the mass of the object.

 

[A.T.]

All forces must have counter force.

Only interaction forces obey Newton's 3 Law. Inertial forces do not, thus momentum is not conserved in non-inertial reference frames.

The roles of these forces are cause and effect.

There is nothing in Newton's 3 Law that allows you to tell which is cause and which effect, so those roles are arbitrary and irrelevant.

production of inertial force by Newton's second and third laws.

Inertial forces are never part of a Newton's 3 Law force pair.

That force is what I boldly tag the word "centrifugal" onto.

You can do that, but by itself it is not sufficient and ambiguous, because there are different types of "centrifugal" froce:

Inertial centrifugal force (exists only in rotating frames, not part of any Newton 3rd force pair):
https://en.wikipedia.org/wiki/Centrifugal_force

Reactive centrifugal force (exists in every frame, forms Newton 3rd pair with centripetal force):
https://en.wikipedia.org/wiki/Reactive_centrifugal_force

See this diagram:

 

[sophiecentaur]

Reactive centrifugal force is not what people generally mean when they say "centrifugal force"

But it's what they feel on fairground rides etc.. That experience trumps the intellectual appreciation of formal Physics. The "doesn't exist" statement that we all got first at school is responsible for a lot of later confusion. Using the term 'frame of reference' is accurate but clouds the whole thing in yet more mystery.

I really sympathise with 'beginners' who read through this thread because they are very likely to leave it none the wiser. It can be a sledge hammer to crack a wallnut.

My friendly Nuthatch has just come onto the feeder outside my window. That has eased my frustration.

 

[A.T.]

Reactive centrifugal force is not what people generally mean when they say "centrifugal force"

But it's what they feel on fairground rides etc.

No, what the rider "feels" is the centripetal force by the ride. The reactive centrifugal force is what the ride "feels".

 

[Orodruin]

But it's what they feel on fairground rides etc

No it isn’t - at least not if you consider them as a whole. That would be the centripetal force. The reactive centrifugal force does not act on them, it is the force they exert on the ride.

If you start making fbds of body parts, then yes, the centripetal force on the head will be the third law pair of the head’s reactive centrifugal force on the neck.

 

[Ibix]

But it's what they feel on fairground rides etc.

Not in general - the reactive centrifugal force is what their body applies to the outer wall of the ride. Unless somebody on the inwards side of you slides into you, you don't feel the reactive centrifugal force.

The only thing you feel, in the strict sense of feel ("something is activating my touch receptors"), is the centripetal force from the floor or walls. In the broader sense of feel ("intuit the presence of"), you feel the centrifugal force (the inertial one, not the reactive one) as an outward g-force in your own rotating frame.

 

[sophiecentaur]

Not in general - the reactive centrifugal force is what their body applies to the outer wall of the ride

It's so easy, when you know the right answer, to assume that the uninitiated interpret things according to accepted Physics. If you ask the average man on the Clapham omnibus what he is 'feeling' he will say that he feels the force of gravity pushing him into his seat. This is the level at which answers to this sort of question should start off - that is if we actually want people to grasp what's what. You can put people right if you start in the right place.
Explaining to me what happens is not helpful to him because you use terms that both of us understand but not he.

 

[Orodruin]

It's so easy, when you know the right answer, to assume that the uninitiated interpret things according to accepted Physics. If you ask the average man on the Clapham omnibus what he is 'feeling' he will say that he feels the force of gravity pushing him into his seat. This is the level at which answers to this sort of question should start off - that is if we actually want people to grasp what's what. You can put people right if you start in the right place.
Explaining to me what happens is not helpful to him because you use terms that both of us understand but not he.

Perpetuating that misconception by saying “indeed, gravity pushes you into your seat” also will not do anything to remedy the understanding.

Furthermore, it is not the equivalent. The equivalent is that centrifugal force pushes you outward, not reactive centrifugal force.

 

[A.T.]

If you ask the average man on the Clapham omnibus what he is 'feeling' he will say that he feels the force of gravity pushing him into his seat.

Nobody in this thread was talking about "feeling" before you started to talk about it. First you claimed that people "feel" the reactive centrifugal force, that doesn't even act on them. Now you switched to "what people believe they feel" and also switched to gravity acting on them.

you use terms that both of us understand

I have my doubts.

 

[sophiecentaur]

Perpetuating that misconception by saying “indeed, gravity pushes you into your seat” also will not do anything to remedy the understanding.

You still don't take my point. Have you ever discussed the issue with the man on the bus? Does he tell you his shoes are pushing him upwards from the pavement or does he tell you his shoes push against the pavement? It's surely not beyond a capable physicist to consider just how strong is the intuitive notion of being forced onto the ground or into the back of the seat of an accelerating car or being thrown forward as you brake etc. etc..
Just telling someone that they are wrong and that they should look at it 'this way' may just possibly not be the best way to help them through this misconception.

“indeed, gravity pushes you into your seat” was not what I wrote.

The feeling is just that for the naive observer and that should be acknowledged before you can hope to point out that it is a misconception. Throwing the naked term 'reference frame' at them is a sure fire way to confuse them if you can't resolve their problem.

Even the term 'centrifugal' is not a totally wrong description. The stone that flies out from a spinning car wheel does, in fact, go further and further away from the centre of the wheel. They can see it happen.

 

[sophiecentaur]

Nobody in this thread was talking about "feeling" before you started to talk about it.

I agree but, if you want to 'correct' someone usefully, it is necessary to reconcile their opinion (based on their experience) with a better model / description. Just telling them they're wrong is probably not the best way.

I may have ignored the 'I' classification of the OP but the original comment / question seems to be championing the naive interpretation of the situation.

 

[Ibix]

If you ask the average man on the Clapham omnibus what he is 'feeling' he will say that he feels the force of gravity pushing him into his seat.

Fair enough, but the analogous concept in rotational motion is the centrifugal force, not the reactive centrifugal force. The former is an inertial force, the latter is the third law pair to the centripetal force. It's the latter which ALBAR (who has resurrected this thread and is not the OP, who was user079622) appears to be invoking incorrectly and unhelpfully and it is that bit that Orodruin, A.T. and I are objecting to.

 

[Orodruin]

You still don't take my point.

And you are still ignoring this:

Furthermore, it is not the equivalent. The equivalent is that centrifugal force pushes you outward, not reactive centrifugal force.

 

[sophiecentaur]

And you are still ignoring this:

Quardature discussions I think. We don't need to be informing each other about the theory; we know it. My point is about how to correct popular misconceptions. Imo, when you genuinely want to help with them you have to be aware where those misconceptions come from and to deal with it. Just stressing the better (proper?) model louder and louder really doesn't help.

I just remember my (otherwise quite impressive) teacher saying there's no such thing as centrifugal force and my switching off about the whole thing. The Physics syllabus didn't deal with reference frames; we did everything (automatically) in the inertial frame. Likewise, in Electricity, we waited till University to discuss electron flow and photons. All three concepts are actually a higher level than most people want to or can deal with, which is obvious from so many posts we can read on PF.

I feel it's not off-topic to mention Piagetian levels of intellectual development. Piaget's work is more than a hundred years old and his observations were made on a small group of upper middle class children. The ages that he states for the various levels are now considered to be way out. It's thought that many adults work on the Concrete level all their lives because appropriate education isn't available to them. It's not always justified to assume people will easily accept new ideas which involve formal operational processing. Let's face it, Marketing works very successfully on this assumption. When trying to give explanations, it's worth while bearing this in mind.

 

[Orodruin]

teacher saying there's no such thing as centrifugal force

Nobody is saying that. It is a very appreciable effect - in a rotating system. It is however zero in an inertial frame. Confusing it with reactive centrifugal force as you were doing is not going to help anyone.

 

[sophiecentaur]

Nobody is saying that.

I think you must have talked to a very limited set of people - or just not been listening. If they haven't been saying it then why are there so many confused people who seem to believe it it? Just because you know the right answer doesn't mean that many people don't. My interest is in why they don't get it. Part of the answer is that reference frames are a very sophisticated idea and you need to appreciate that some people find it very alien.

 

[Orodruin]

I think you must have talked to a very limited set of people - or just not been listening.

I was referring to this thread, which is what was being discussed.

 

[ALBAR]

That is the "reactive centrifugal force". Yes, it is a real force. But it is not what is meant by "centrifugal force". The "centrifugal force" arises only in the non-inertial rotating frame.

As you point out, the centrifugal force in the non-inertial sense does not have a corresponding third law reaction force.

Only interaction forces obey Newton's 3 Law. Inertial forces do not, thus momentum is not conserved in non-inertial reference frames.

There is nothing in Newton's 3 Law that allows you to tell which is cause and which effect, so those roles are arbitrary and irrelevant.

Inertial forces are never part of a Newton's 3 Law force pair.

You can do that, but by itself it is not sufficient and ambiguous, because there a different types of "centrifugal" froce:

Inertial centrifugal force (exists only in rotating frames, not part of any Newton 3rd force pair):
https://en.wikipedia.org/wiki/Centrifugal_force

Reactive centrifugal force (exists in every frame, forms Newton 3rd pair with centripetal force):
https://en.wikipedia.org/wiki/Reactive_centrifugal_force

See this diagram:

View attachment 339623

Sorry about using outmoded terminology. I've been using my dictionary's definition of centrifugal force, a term that entered the language around 1721. It's true that there is nothing in the third law that allows you to tell which force causes change, but the first law does for many cases. It is not irrelevant. For example, leaving rotation for a moment for simplicity, imagine pulling on a wagon. The wagon's reaction force counters your force 100% every instant. Why are you not locked in a tug-of-war with the wagon? Understanding the roles of specific forces helps to solve such dilemmas.

 

[Orodruin]

Why are you not locked in a tug-of-war with the wagon?

The resolution to this has nothing to do with what is action and what is reaction, but everything to do with realising that those forces act on different bodies. This is the source of a common misconception regarding the third law where people misconstrue it as meaning forces always cancel. They clearly do not.

 

[jbriggs444]

It's true that there is nothing in the third law that allows you to tell which force causes change, but the first law does for many cases.

Both forces "cause change" in the sense that they embody a momentum flow into the targetted body. If another force somewhere else "prevents change" then what of it?

The second law dictates how forces "cause change". The net force results in an acceleration.

 

[ALBAR]

Only interaction forces obey Newton's 3 Law. Inertial forces do not, thus momentum is not conserved in non-inertial reference frames.

There is nothing in Newton's 3 Law that allows you to tell which is cause and which effect, so those roles are arbitrary and irrelevant.

Inertial forces are never part of a Newton's 3 Law force pair.

You can do that, but by itself it is not sufficient and ambiguous, because there a different types of "centrifugal" froce:

Inertial centrifugal force (exists only in rotating frames, not part of any Newton 3rd force pair):
https://en.wikipedia.org/wiki/Centrifugal_force

Reactive centrifugal force (exists in every frame, forms Newton 3rd pair with centripetal force):
https://en.wikipedia.org/wiki/Reactive_centrifugal_force

See this diagram:

View attachment 339623

I agree that opposing and equal forces do not always cancel. Squeezing something in a vice proves that. Let's do another thought experiment. Tie two powerful magnets slightly apart, north pole facing south pole. Simultaneously release them. They accelerate until they collide. Yes, there are two bodies, but each is captive of those perplexing third law pairs! Does it seem like either one should be moving?

 

[jbriggs444]

I agree that opposing and equal forces do not always cancel. Squeezing something in a vice proves that. Let's do another thought experiment. Tie two powerful magnets slightly apart, north pole facing south pole. Simultaneously release them. They accelerate until they collide. Yes, there are two bodies, but each is captive of those perplexing third law pairs! Does it seem like either one should be moving?

Please be selective and non-duplicative with the quoting. [Edit after problem was corrected]

A discussion of stresses and deformations has little place in a thread about centrifugal force. The sort of thought experiments that you have proposed in this thread are purely speculative and have no place in these forums.

A thought experiment is properly used to explore the consequences of a set of well defined postulates. They are not properly used to explore personal intuitions. That would be cartoon physics.

 

[ALBAR]

Please be selective and non-duplicative with the quoting.

A discussion of stresses and deformations has little place in a thread about centrifugal force. The sort of thought experiments that you have proposed in this thread are purely speculative and have no place in these forums.

A thought experiment is properly used to explore the consequences of a set of well defined postulates. They are not properly used to explore personal intuitions. That would be cartoon physics.

That was my reply to another science adviser, Orodruin, I believe.

 

[Orodruin]

Yes, there are two bodies, but each is captive of those perplexing third law pairs! Does it seem like either one should be moving?

This to me indicates that you are indeed suffering from the very common third law means no motion misconception. Yes! They both should be moving! The forces from either on the other are indeed equal in magnitude but opposite in direction - but they act on different bodies.