I Is calling fictitious forces "not real" just about terminology?

[Herman Trivilino]

My point is that most things in life involve considering cause and effect. Why bend over backwards to avoid this in the case of of N3?

The only bending over backwards is when uttering the phrase "action-reaction" instead of "Third-Law pair of forces". The former is a translation of a 17th century text. Who in the modern era uses "action" to refer to a force?

There is no reason to avoid cause and effect just because it's not strictly necessary.

There's no cause and effect here! That's the whole point.

These things will resolve themselves during the analysis.

Research shows otherwise. Students who are capable of solving chapter-end and test problems are overwhelming unable to answer questions involving the concepts of the Third Law. In other words, many students after successfully completing a course will be left with severe misconceptions. I've seen it when interviewing teaching applicants with a Ph.D. in physics.

One example of a misconception: If for every force there's a equal-but-opposite force how can there ever be a nonzero net force?

 

[Herman Trivilino]

I wonder whether that 'author' is in fact the product of a bit of AI composition.

Well, the first edition of the book where I took the "composition" from was published in 1990. I've got a later copy of it on my bookshelf. I've attended presentations by the author and spoken to him personally.

Edit: Sorry. I just realized I'd forgotten to state his name. Arnold B.Arons. University of Washington for many years. He's deceased.

 

[PeroK]

Arnold B.Arons. University of Washington for many years. He's deceased.

And who are we to argue with the late Arnold B Arons?

 

[PeroK]

Research shows otherwise. Students who are capable of solving chapter-end and test problems are overwhelming unable to answer questions involving the concepts of the Third Law. In other words, many students after successfully completing a course will be left with severe misconceptions. I've seen it when interviewing teaching applicants with a Ph.D. in physics.

One example of a misconception: If for every force there's a equal-but-opposite force how can there ever be a nonzero net force?

Now we have reached the nub of the matter. Unless we use your preferred terminolgy and look at physics your preferred way, and share your idiosyncracies, then we don't understand it at all and have the most basic misconceptions!

 

[sophiecentaur]

Research shows otherwise.

It's not a matter of "research". it's a matter of seeing what happens every time you analyse a system of forces. You can assign any direction to the force vectors you choose and their signs and directions will fall out of the calculation. That's what I was meaning.

 

[Herman Trivilino]

Now we have reached the nub of the matter. Unless we use your preferred terminolgy and look at physics your preferred way, and share your idiosyncracies, then we don't understand it at all and have the most basic misconceptions!

Not at all. I'm offering an opinion, not a mandate.

I didn't mean to imply that all students are left with these misconceptions, or that my suggested way is better than another. Just to point out that the phrase "action-reaction" has its problems.

 

[Herman Trivilino]

And who are we to argue with the late Arnold B Arons?

We're here to discuss different opinions, not to argue from authority. I apologize if I came off as doing the latter.

 

[A.T.]

There is no reason to avoid cause and effect just because it's not strictly necessary.

There is lots of reasons. Beyond Occam's razor, it leads to misguided notions, which stand in the way of proper analysis. See the examples in post #55.

it's the equivalent of choosing arrow directions on a free body diagram.

No, it's not equivalent:
- Sign conventions / arrow direction definitions are required to do the math.
- The action / reaction assignment in N3 is irrelevant for the math.

The flow of causality intuitively follows the flow of energy so where is the problem?

Explained by @jbriggs444 in post #60.

These things will resolve themselves during the analysis.

How does action/reaction in N3 resolve itself during the analysis? It doesn't even enter a correctly done quantitative analysis. At most it just confuses and prevents or delays a correct analysis.

 

[PeroK]

There is lots of reasons. Beyond Occam's razor, it leads to misguided notions, which stand in the way of proper analysis. See the examples in post #55.

Can you prove that the arguments about DDWFTTW are between those who understand N3 as you do and those who do not understand it?

It sounds like a bogus argument to me.

 

[A.T.]

It sounds like a bogus argument to me.

Then stick to the Occam's razor reason.

 

[Herman Trivilino]

It's only when the rocket engine has been lit that any acceleration can happen. The reverse description "If we see the space ship accelerate then there has to be a force acting" has to involve an 'if'. Doesn't that imply some directionality?

I guess I don't get it. I can write equivalent versions of your two phrases, placing "if" in the other one ...

“If the rocketship is accelerating then the rocket engine is lit”

“There has to be a force acting on the space ship to make it accelerate”

 

[A.T.]

I disagree. The equations are about correlation, not causation.

It is not a question of what causes what. It is a question of what we know and what we want to find out.

Sean Carroll discusses this in other contexts in his recent podcast around 1:54:00
https://www.preposterousuniverse.com/podcast/2025/11/17/ama-november-2025/

The transcript link is below the player. From the transcript:

1:54:02.2 SC: Calvin Firth says, from the Friedmann equation, the curvature of the universe can be related to its energy contents. Omega K equals one minus omega naught. Is there a causal relationship between curvature and energy contents? Does one determine the other or can we only say that they are related?

1:54:17.2 SC: This is a great question because people see equations in Physics and they talk about them in certain ways. You know, we say that an electron has a charge and that charge causes an electric field around the electron. And we can relate them. There's Gauss's law. So if you integrate up the electric field around a sphere around the electron, that's equal to the charge. Okay? But could you equally well say that the electric field causes the charge? And the answer is yes, you could. And likewise, in the Friedmann equation, you relate the curvature of space to the amount of matter. Does one cause the other? Not in any way that we would ordinarily associate to the word cause.

 

[sophiecentaur]

I can write equivalent versions of your two phrases, placing "if" in the other one

Absolutely true. But we do not think and experience life in equations. There is an 'arrow' of causality as there is an arrow of time which, whatever the philosophy says, it affects our thought processes and assumptions. Is there really any point in denying this when we make models of our World?

In the lab we can do experiments in which we can choose which variable is the independent one and which is the dependent one. Stretching a wire is a good example and there is a good reason why we measure the force (stress) needed to produce an extension )strain; increasing the hanging weight will stretch and break the wire to fast for us to analyse actually what goes on at the end. We would all(?) agree which is the cause of this occurrence but the experiment has to be carried out the other way round (with an implied "if"). In School, we do it the other way round, with springs and rubber bands, I would suggest that it's done this way because it makes sense to an early learner.

In astronomical observations we can't tinker with the variables and we can only ever look at effects and decide on what 'law' was involved and what the cause must have been. I would be interested to find examples where one observer has concluded the inverse of another observer about cause and effect - and if / when it has been resolved.

Why do people have a problem in acknowledging this 'arrow'? Does the existence of equations dictate that there is no natural order in things or is it just a need for 'tidiness' in our thought models? (A left brain right brain situation.)

 

[Herman Trivilino]

Stretching a wire is a good example and there is a good reason why we measure the force (stress) needed to produce an extension )strain; increasing the hanging weight will stretch and break the wire to fast for us to analyse actually what goes on at the end.

The force exerted on the wire is the cause of the wire stretching. But this is not an example of the Third Law. This is: The hanging weight exerts a force on the wire, and the wire exerts a force on the hanging weight. Neither of these is the cause of of the other. Neither one precedes the other in time. Either one can be considered the "action" and either one can be considered the "reaction". The two forces are perfectly symmetrical.

Earth's gravity is the cause of the force exerted by the hanging weight, and the deformation of the wire is the cause of the force exerted by the wire.

The use of the terminology action and reaction to describe forces is outdated. It's not used to refer to forces in any other context I can think of. It's 17th and 18th century terminology and has caused many a 20th and 21st century student to have the misconception that there is an asymmetry, that somehow the action precedes the reaction, and that the action is the cause of the reaction.

Another example would be a book lying flat on a horizontal table top. The book exerts a force on the table and the table exerts a force on the book. Neither is the cause of the other. The force exerted by the book is caused by Earth's gravity and the force exerted by the table is caused by the deformation of the table.

 

[sophiecentaur]

has caused many a 20th and 21st century student to have the misconception that there is an asymmetry

There is no asymmetry in the equations but, in our brains there is hundreds of thousands of years of evolution in which we developed a notion of cause and effect. Moreover, I can only think of situations in which cause and effect are actually not identifiable. Those situations are during the analysis of a situation in which maths (of some kind) has been invoked. Not surprisingly, 'students' are confused because those situations are difficult in any case and they have trouble choosing the directions of arrows and the signs they (should) use when answering problem questions. If their teacher goes to the trouble of telling them "it doesn't matter and it will all come out in the wash" that calms them down.

Frankly,forcing the idea of symmetry in these matters at any time before advanced Physics just piles on the angst for them.

 

[A.T.]

forcing the idea of symmetry in these matters

It's not the symmetry that requires forcing, because Newton's 3rd is already symmetrical. It's the fake asymmetry that is forced into it by misleading language.

 

[sophiecentaur]

. It's the fake asymmetry that is forced into it by misleading language.

"misleading"? Only difficult to reconcile with the maths. As with "the Sabbath", maths was made for Man and not Man for the Equation. If you insist on us changing all of our views of the world to fit the maths we would have to consider all the Physics of the past hundred years in any statement we make about anything.

an only ever look at effects and decide on what 'law' was involved and what the cause must have been. I would be interested to find examples where one observer has concluded the inverse of another observer about cause and effect - and if / when it has been resolved.

Any comments? (You have to quote something where the 'if' is not used or implied.)

 

[A.T.]

"misleading"? Only difficult to reconcile with the maths.

If your preferred language is difficult to reconcile with the maths, than it's obviously you who is forcing something into Newton's 3rd Law, that isn't actually there. Accusing others of forcing symmetry, into a law that is already symmetrical, is rather silly.

 

[Herman Trivilino]

There is no asymmetry in the equations but,

There is no asymmetry in the physics. One can introduce Law III with no reference to any equations. Hold a bathroom scale against a wall and push on it. It tells you how hard you are pushing. Repeat with the scale flipped over and it tells you how hard the wall is pushing.

in our brains there is hundreds of thousands of years of evolution in which we developed a notion of cause and effect.

And lots of other misconceptions, too.

If millenia of evolution tells you that the scale readings in the above experiment are different then you have been misled. Are you suggesting we just sweep that under the rug?

Frankly,forcing the idea of symmetry in these matters at any time before advanced Physics just piles on the angst for them.

We don't wait until more advanced classes to address them. In fact, research shows that these misconceptions can persist beyond advanced courses.

 

[sophiecentaur]

Accusing others of forcing symmetry, into a law

But aren't you doing just that by demanding symmetry into our basic thoughts, rather than introducing symmetry into an equation - for the purpose of calculations. It doesn't imply anything about the rest of our World.

And lots of other misconceptions, too.

The general public and even educated people (pardon the elitist description) live their whole lives based on cause and effect. It's only the fringe cognoscenti who introduce this symmetry to describe certain processes ( but not 'processes' because that very word implies change in time; the word should be 'situations' perhaps). Use of N3 is not actually affected by assuming symmetry or not; an equation doesn't in itself necessarily attribute a cause or an effect. Physics is always supported by experiment and observation of change. So my thesis is that symmetry is merely an artifice to allow calculations. We use symmetry but why make it an article of faith? As soon as we launch into maths, we introduce alternative interpretations and answers and we choose which result we think makes sense on the basis of our reality.

One can introduce Law III with no reference to any equations. Hold a bathroom scale against a wall and push on it. It tells you how hard you are pushing. Repeat with the scale flipped over and it tells you how hard the wall is pushing.

That involves an equation.

We don't wait until more advanced classes to address them.

When would you introduce this very sophisticated idea? I'm just trying to imagine how a class of year 10 students (14 yrs) would receive what you have been saying? Most kids get the commutative law ok but that's 'inside' arithmetic and algebra. In real situations it hardly occurs to them until you point it out.

is rather silly.

Possibly you are missing my point.

 

[A.T.]

... demanding symmetry into our basic thoughts

Nobody is doing that. You can have thoughts about elves and unicorns when applying Newton's 3rd Law, if you like. But physics teaching should not confuse students with superfluous stuff that has nothing to do with that law.

 

[sophiecentaur]

You can have thoughts about elves and unicorns

I smell a straw man there. Use a car crash as an example
Give that question to a student and tell them to ignore cause and effect.

 

[jbriggs444]

I smell a straw man there. Use a car crash as an example
Give that question to a student and tell them to ignore cause and effect.

One does not tell the student that cause and effect do not exist. One tells the student that there is no causal relationship in the third law.

One does not [always] teach in an exercise. One asks the question to test knowledge or to exercise skills. We might reasonably ask questions to probe whether the student is improperly using cause and effect reasoning.

Let us try some questions.

"A car is moving toward a sturdy barrier that is firmly anchored into the ground. The car crashes into the barrier and comes to a stop.

Which comes first? The force of car on barrier? Or the force of barrier on car?
Which is larger in magnitude? The force of car on barrier? Or the force of barrier on car?
Does either force cause the other?

Suppose that the car masses 1000 kg and is initially moving at 10 m/s. Suppose further that the collision takes 100 milliseconds. Assume that no other forces on the car during the collision are significant.

What is the magnitude of the [time] average force of barrier on car for those 100 milliseconds?
"

There is a "cause" for the collision. That cause is the state of affairs that exists with the car in motion and the barrier firmly anchored in the car's path. More indirectly the root cause was the fact that we were asked to pose a question involving a car crash. This caused us to posit a state of affairs where a crash would ensue.

 

[Herman Trivilino]

But aren't you doing just that by demanding symmetry into our basic thoughts, rather than introducing symmetry into an equation - for the purpose of calculations. It doesn't imply anything about the rest of our World.

It's not a demand. It's a generalization based on observation.

That involves an equation.

It involves taking two readings on a bathroom scale and observing that the two readings are the same regardless of the orientation of the scale. You can quibble all you want about that being an equation, you're just arguing semantics.

When would you introduce this very sophisticated idea?

It's not sophisticated. It's a simple idea. We're just addressing the misconception that the "action" somehow precedes the "reaction" when in fact they are (within newtonian physics) simultaneous. One therefore cannot be the cause of the other.

An understanding of Laws I and II are sophisticated. That doesn't stop us from trying to get students to understand them. It would be neglect to allow misconceptions to dominate an understanding of these ideas, sweep it all under the rug, and hope that it will get corrected in later courses. Particularly in view of the fact that the research shows these misconceptions to persist in and beyond those advanced courses. The place to address misconceptions about newtonian physics is when newtonian physics is introduced.

Possibly you are missing my point.

I am definitely missing your point.

You seem to be separating the math that's used to describe physical laws from the laws themselves, when in fact the math is just a part of the physical laws.

 

[robphy]

Possibly interesting:
"Causal reasoning in understanding Newton’s third law"
Cheng Chen, Lei Bao, Joseph C. Fritchman, and Hemin Ma
Phys. Rev. Phys. Educ. Res. 17, 010128 – Published 21 April, 2021 https://doi.org/10.1103/PhysRevPhysEducRes.17.010128

 

[sophiecentaur]

It's not a demand. It's a generalization based on observation.

We didn't evolve with the capability of confirming symmetry or not in our observations. In a hundred years or so, you can't expect the intuitive response to anything to take it into account. It's a totally academic appreciation. So it's not surprising that intuition assumes causality.

you're just arguing semantics.

Imo, it's much more important than semantics. It's only in thought experiments that simultaneity can be assumed. The alternative is the steady state equilibrium condition, after a long settling time or a turning value condition. In practice, there is always a change with time. There are many paradoxes about what happens to the energy during an interaction. Two capacitors cannot share the same energy as one initially charged capacitor; the calculations have to include some asymmetry.

Possibly interesting:
"Causal reasoning in understanding Newton’s third law"
Cheng Chen, Lei Bao, Joseph C. Fritchman, and Hemin Ma
Phys. Rev. Phys. Educ. Res. 17, 010128 – Published 21 April, 2021 https://doi.org/10.1103/PhysRevPhysEducRes.17.010128

Interesting. Thanks. The paper seems to imply that causality is a majority point of view. The problem with a study like this is that what students 'learn' is always going to be biased towards the formal thought processes that have been taught. There's no totally reliable result here unless we are dealing with students with absolutely no prior education. I have used the word 'faith' several times in this thread and I think it's a very relevant factor which all of us should be aware of.

This thread hasn't got far to resolve things without introducing the word "misconception" which has assumed that one side of the argument is automatically right about this. There are many true misconceptions but, in a case like this, the ideas cross between real world and maths and we're down (only) to faith strong 'education', rather than misconceptions..

We can either say symmetry is there or we can say that it only applies to a certain level and that there is no absolute evidence one way or another. The presence of a mathematical equation proves nothing about the real world.
Physicists no longer say that photons are massless; instead they put a maximum possible mass. This is a similar idea but we don't argue about it these days.

 

[A.T.]

I smell a straw man there.

Yeah, it's right here, in your next sentence:

Use a car crash as an example. Give that question to a student and tell them to ignore cause and effect

Like in most of your posts here, you fail to justify forcing cause & effect into Newton's 3rd Law, and instead resort to pretending sombody wants to abolish cause & effect in general, so you can argue against that strawman.

 

[A.T.]

When would you introduce this very sophisticated idea?

Symmetry is the lack of hierarchy that asymmetry entails. It's one thing less to think about, thus simpler than asymmetry, rather than more sophisticated.

 

[A.T.]

The flow of causality intuitively follows the flow of energy so where is the problem?

This intuition is a non-starter. Energy depends on reference frame. Causality (to the extent that we have a viable definition to go on) is an invariant.

And it's exactly this mismatch between the frame-dependent energy flow, and the intuitively assumed frame-invariant causality, that often leads to implicit contradictions of Galilean Invariance.

We saw it in the DDWFTTW debates, where many people were denying the validity of the treadmill tests, based on the difference in what provides the energy. And more recently in this treadmill thread:
https://www.physicsforums.com/threads/work-done-running-on-an-inclined-treadmill.927825/

 

[sophiecentaur]

denying the validity of the treadmill tests,

Yes I read some of those discussions but the reason for the disbelief was due to a deep intuition ("it just could never work") and failure to go through all the appropriate steps- including chasing 'signs' and sticking to them.

This is a typical result of trying to establish what's really happening and using mathematical axioms to draw a conclusion when what's being sought after is interpretation (it makes no difference to the results of your sums, which way you think about it). This is all high end stuff and it's just not suitable beginners material. If you wouldn't hit a three year old with Newton's Laws of motion then there will be an age (and ability level) before which it's not appropriate to insist on the implications that this thread. One step at a time.