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

[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.

 

[berkeman]

Thread closed temporarily for Moderation and cleanup...

 

[Mark44]

I agree that opposing and equal forces do not always cancel. Squeezing something in a vice proves that.

No it doesn't. The force that the vise (not vice, that's completely different) exerts on the object is equal to the force that the object exerts on the vise, but in the opposite direction.

Please be selective and non-duplicative with the quoting.

Fixed.
Thread is still closed for moderation and cleanup.

 

[berkeman]

Okay, after the cleanup and a Mentor discussion, the thread is reopened provisionally.

 

[A.T.]

Yes, there are two bodies, but each is captive of those perplexing third law pairs!

Only one force of a 3rd law pair acts on a body. The other force of that 3rd law pair always acts on another body.

Does it seem like either one should be moving?

How they are moving is a matter of reference frame and of what other forces act on them (2nd law). For the 3rd law motion is irrelevant.

 

[A.T.]

Just stressing the better (proper?) model louder and louder really doesn't help.

But stressing wrong ideas does help? I don't see how you are helping anybody, by being/acting just as confused as the person you are trying to help, in the name of "picking them up where they are".

Stating the correct model is the basis. Making it more intuitive is a matter of going through many examples, and applying it, rather than "explaining it in the right way".

 

[sophiecentaur]

But stressing wrong ideas does help?

Is that what I wrote? I am suggesting that acknowledging that the 'wrong' ideas exist and pointing out why will give an opportunity to reconcile the right and the wrong. When the people who 'know' cannot see how the people are thinking who "don't know" then they are not likely to point them in the best direction for revised thinking. You may be forgetting that questions like the ones in this thread are very common and they must be arising for a reason - not just ignorance.

Putting oneself in the shoes of another person can often allow to help them effectively. People who don't get that could maybe limit their (accurate and well informed) responses to the better informed threads in which they can contribute on a more level playing field; a win-win situation. I suspect that idea may be in quadrature with people who are arguing against me here.

 

[A.T.]

Putting oneself in the shoes of another person can often allow to help them effectively.

The problem with your posts is that it's not clear if you are putting yourself into the shoes of a confused person, or if you are confused yourself. Maybe you can make it more clear when you are merely channeling some common misconception.

 

[sophiecentaur]

The problem with your posts is that it's not clear if you are putting yourself into the shoes of a confused person, or if you are confused yourself.

Oh! Could you quote what I wrote that could give that impression? Maybe I was too subtle?

 

[rcgldr]

Only one force of a 3rd law pair acts on a body. The other force of that 3rd law pair always acts on another body.

It may help with a simple example, like the spinning space station example posted earlier. The "floor" of the space station exerts a centripetal force on the astronaut, and the reaction to that centripetal acceleration results in a reactive centrifugal force exerted onto the "floor" of the space station.

As explained earlier, this is a different scenario than the glass sliding to the outside of an opening door, or a bead sliding outwards on a rotating rod, which is due to the fact that once the velocity is non-zero, the direction of force is inwards of the direction of velocity, resulting in a curved path, but not curved enough to prevent outwards movement relative to the axis of rotation of the door | bead. For the bead sliding on a rod, there could be an initial linear impulse, and then the angular velocity of the rod adjusted so that it no longer exerts any force on the bead, in which case the bead moves in a linear path and outwards on the rod. The rod would asymptotically approach the direction of the initial linear impulse.

 

[ALBAR]

Only one force of a 3rd law pair acts on a body. The other force of that 3rd law pair always acts on another body.

How they are moving is a matter of reference frame and of what other forces act on them (2nd law). For the 3rd law motion is irrelevant.

Do separate bodies such as earth and moon exert reaction forces on each other? I believe that the reactions of two isolated bodies to gravitational attraction, either pulling them together or holding them in orbit, stay local at the masses that generate them rather than act or react on other mass. Reaction forces can't be exerted over empty distance as gravity can. Reaction forces pair up in opposition, reacting to each other when objects make contact as in a collision or are physically joined, as by a rope, or are parts of a single object. Example: A twirled baton is thrown up in the air. The centrifugal force (or whatever the modern term) of each of its halves is linked to the other half and serves as its centripetal force. These two force mutually support each other because they act on the same object, the baton.

 

[jbriggs444]

Do separate bodies such as earth and moon exert reaction forces on each other?

Yes. In the Newtonian model, gravity is a force-at-a-distance. The gravitational force of moon on earth is the third law reaction force that goes with the gravitational force of earth on moon.

In the model of General Relativity, gravity is not a force at all and there is no non-locality problem. But let us not go there.

Reaction forces pair up in opposition, reacting to each other when objects make contact as in a collision or are physically joined, as by a rope, or are parts of a single object. Example: A twirled baton is thrown up in the air. The centrifugal force (or whatever the modern term) of each of its halves is linked to the other half and serves as its centripetal force.

No.

 

[Orodruin]

Do separate bodies such as earth and moon exert reaction forces on each other?

Yes. The gravitational force from the Earth on the Moon is the third law pair of that from the Moon on the Earth.

I believe that the reactions of two isolated bodies to gravitational attraction, either pulling them together or holding them in orbit, stay local at the masses that generate them rather than act or react on other mass. Reaction forces can't be exerted over empty distance as gravity can.

This is nonsense. (My emphasis)
What you are describing here are contact forces - not action/reaction pairs in general.

Reaction forces pair up in opposition, reacting to each other when objects make contact as in a collision or are physically joined, as by a rope, or are parts of a single object. Example: A twirled baton is thrown up in the air. The centrifugal force (or whatever the modern term) of each of its halves is linked to the other half and serves as its centripetal force. These two force mutually support each other because they act on the same object, the baton.

Wrong. They do not act on the same object in the classicsl mechanics sense of the word. One acts on one half of the baton and the other on the other half. You chose to split the baton as far as making FBDs go and then the forces act on different parts.

The baton as a whole has internal forces and strains, but those are irrelevant as far as a FBD for the entire baton would be concerned.

 

[ALBAR]

Yes. In the Newtonian model, gravity is a force-at-a-distance. The gravitational force of moon on earth is the third law reaction force that goes with the gravitational force of earth on moon.

In the model of General Relativity, gravity is not a force at all and there is no non-locality problem. But let us not go there.

No.

I don't believe that force of gravity should be designated as reactive. The first law clarifies the roles of forces, but is hardly needed to explain what is instinctive knowledge. Release a mass in a gravitational field and everybody knows what happens and why. Could any case of cause and effect--action and reaction--be more obvious? But set this issue aside. If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

 

[Ibix]

If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

The gravitational force of A on B and the gravitational force of B on A are a third law pair, which is why the joint center of mass of the two bodies doesn't move. You don't need two more forces.

 

[Orodruin]

I don't believe that force of gravity should be designated as reactive. The first law clarifies the roles of forces, but is hardly needed to explain what is instinctive knowledge. Release a mass in a gravitational field and everybody knows what happens and why. Could any case of cause and effect--action and reaction--be more obvious? But set this issue aside. If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

Again, you are confusing action/reaction with contact forces.

 

[jbriggs444]

Could any case of cause and effect--action and reaction

Action/reaction forces are NOT cause and effect. The third law is a statement that interaction forces come in pairs. Neither force is designated as cause. Neither force is designated as effect. It is correlation, not causation.

"the mutual actions of two bodies upon each other are always equal, and directed to contrary parts"

 

[A.T.]

I don't believe that force of gravity should be designated as reactive.

Gravity is a good example why the "action/reaction" terminology is bad and confusing.

All that Newton's 3rd says is that there are equal but opposite forces, and that also applies to Newtonian gravity as well: Each mass exerts a force on the other mass, and those two forces are equal but opposite.

 

[ALBAR]

The gravitational force of A on B and the gravitational force of B on A are a third law pair, which is why the joint center of mass of the two bodies doesn't move. You don't need two more forces.

You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.

 

[A.T.]

You mean only one force exists at each body?

In the inertial frame of reference, yes.

Can't be! No force can be exerted without resistance. Try pushing on nothing.

That's called inertia, and is represented by the mass of a body, not by some resistive force.

 

[PeroK]

You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.

You mean motion is impossible? Can't be. A force can result in acceleration! Try kicking a football.