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.sophiecentaur said: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.sophiecentaur said:But it's what they feel on fairground rides etc.
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.Ibix said:Not in general - the reactive centrifugal force is what their body applies to the outer wall of the ride
Perpetuating that misconception by saying “indeed, gravity pushes you into your seat” also will not do anything to remedy the understanding.sophiecentaur said: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.
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.sophiecentaur said: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.
I have my doubts.sophiecentaur said:you use terms that both of us understand
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..Orodruin said:Perpetuating that misconception by saying “indeed, gravity pushes you into your seat” also will not do anything to remedy the understanding.
“indeed, gravity pushes you into your seat” was not what I wrote.
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.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.
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.A.T. said:Nobody in this thread was talking about "feeling" before you started to talk about it.
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.sophiecentaur said: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.
And you are still ignoring this:sophiecentaur said:You still don't take my point.
Orodruin said:Furthermore, it is not the equivalent. The equivalent is that centrifugal force pushes you outward, not reactive centrifugal force.
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.Orodruin said:And you are still ignoring this:
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 said:teacher saying there's no such thing as centrifugal force
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 said:Nobody is saying that.
I was referring to this thread, which is what was being discussed.sophiecentaur said:I think you must have talked to a very limited set of people - or just not been listening.
jbriggs444 said: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.
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.A.T. said: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
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.ALBAR said:Why are you not locked in a tug-of-war with the wagon?
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?ALBAR said: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.
A.T. said: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
ALBAR said: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?
That was my reply to another science adviser, Orodruin, I believe.jbriggs444 said: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.
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.ALBAR said:Yes, there are two bodies, but each is captive of those perplexing third law pairs! Does it seem like either one should be moving?
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.ALBAR said:I agree that opposing and equal forces do not always cancel. Squeezing something in a vice proves that.
Fixed.jbriggs444 said:Please be selective and non-duplicative with the quoting.
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.ALBAR said:Yes, there are two bodies, but each is captive of those perplexing third law pairs!
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.ALBAR said:Does it seem like either one should be moving?
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".sophiecentaur said:Just stressing the better (proper?) model louder and louder really doesn't 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.A.T. said:But stressing wrong ideas does help?
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 said:Putting oneself in the shoes of another person can often allow to help them effectively.
Oh! Could you quote what I wrote that could give that impression? Maybe I was too subtle?A.T. said: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.
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.A.T. said: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.
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.A.T. said: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.
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.ALBAR said:Do separate bodies such as earth and moon exert reaction forces on each other?
No.ALBAR said: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.
Yes. The gravitational force from the Earth on the Moon is the third law pair of that from the Moon on the Earth.ALBAR said:Do separate bodies such as earth and moon exert reaction forces on each other?
This is nonsense. (My emphasis)ALBAR said: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.
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.ALBAR said: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.
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?jbriggs444 said: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.
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.ALBAR said: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.ALBAR said: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?
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.ALBAR said:Could any case of cause and effect--action and reaction
Gravity is a good example why the "action/reaction" terminology is bad and confusing.ALBAR said:I don't believe that force of gravity should be designated as reactive.
You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.Ibix said: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.
In the inertial frame of reference, yes.ALBAR said:You mean only one force exists at each body?
That's called inertia, and is represented by the mass of a body, not by some resistive force.ALBAR said: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.ALBAR said:You mean only one force exists at each body? Can't be! No force can be exerted without resistance. Try pushing on nothing.
Do you think the other planet is massless and has no inertia?ALBAR said:No force can be exerted without resistance.
This is exactly why we need to differentiate cause from effect. The football must push back 100% at every instant force is applied, but it undergoes acceleration as an effect of the force being applied. The acceleration triggers the resistive force per the second law resulting in a symmetric pair of opposing forces.PeroK said:You mean motion is impossible? Can't be. A force can result in acceleration! Try kicking a football.
Correlation, not causation. Whether the ball strikes your foot or your foot strikes the ball, a strain arises, a stress arises and a pair of forces arises.ALBAR said:The acceleration triggers the resistive force per the second law
But your foot accelerates, so that must be the effect and the ball the cause!ALBAR said:The acceleration triggers the resistive force per the second law resulting in a symmetric pair of opposing forces.
Whaaaat? You now have three forces?? Going completely classical, there's a force acting on the CM of Earth - just like a piece of string and a ball whirling round . The string pulls against the ball and the ball pulls against the string. Also the string pulls on your hand as you pull it. That's N3 happening at each end of the string. Connect the ball directly to your hand (throw away the string) and the direct force is the same as the tension in the string. The gravitational attraction between Earth and Moon is the equivalent to the tension in the string.ALBAR said:If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?
We are getting into collision physics now. Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.Ibix said:But your foot accelerates, so that must be the effect and the ball the cause!
The decelleration of the foot (times the mass of the foot) matches the contact force of ball on foot. That's Newton's second law.ALBAR said:We are getting into collision physics now. Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.
Whether the speed to the foot increases or decreases is just a matter of the chosen reference frame. If your "causation logic" depends on that arbitrary choice, then it is equally arbitrary, and not physically relevant.ALBAR said:Actually the foot decelerates ....
The foot decelerates because the ball exerts a force on it. That's Newton's second law.ALBAR said:Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.
Please, PLEASE let's stay in the good old Newtonian inertial reference frame when discussing the most basic principles of mechanics.A.T. said:Whether the speed to the foot increases or decreases is just a matter of the chosen reference frame. If your "causation logic" depends on that arbitrary choice, then it is equally arbitrary, and not physically relevant.
