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

[wrobel]

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?

I am sorry but I do not believe that a doctor can be confused by such a childish sophism

 

[wrobel]

I have always thought that an idea of causation is expressed by the Cauchy existence and uniqueness theorem being applied to the ma=F

 

[sophiecentaur]

I am sorry but I do not believe that a doctor can be confused by such a childish sophism

That's a very simplistic / optimistic remark. Iin my experience, many PhDs get their doctorates by just repeating the ideas of their supervisor. Of course, we had some brilliant exceptions but a significant number were disasters waiting to happen; no ability to relate the book to a practical situation. (I'm referring to Physics and EE doctorates)

Cauchy existence and uniqueness theorem

Doesn't that refer to differential equations? DEs are models, applied to the real world. Yes, they work but by making assumptions when the equations are formulated. They are only real to within the limits of final justification through measurement. Has the 'arrow of time' ever been measured to have exceptions in macroscopic objects? Causality is in the same neck of the woods as time symmetry and thermodynamic laws.

 

[jbriggs444]

Doesn't that refer to differential equations? DEs are models, applied to the real world. Yes, they work but by making assumptions when the equations are formulated. They are only real to within the limits of final justification through measurement. Has the 'arrow of time' ever been measured to have exceptions in macroscopic objects? Causality is in the same neck of the woods as time symmetry and thermodynamic laws.

As I recall, your argument in this thread started out as an idea about pedagogy. That it was more helpful to accept student intuitions about cause and effect rather than to bludgeon them into submission about the fact of the matter that the third law is not at all about cause and effect.

Sure. That might be so. Opinions on effective pedagogy can vary.

More recently, it seems that you are making a claim about the physical world. That the real world might possibly enforce third law compliance by some mechanism that involves cause and effect. Even though no mechanism for such has been proposed and no experimental evidence exists or is expected to exist.

Sure. That might be so. And dark matter might be composed of flying spaghetti monsters.

 

[Herman Trivilino]

I am sorry but I do not believe that a doctor can be confused by such a childish sophism

Is this an evidence-based belief?

 

[wrobel]

Is this an evidence-based belief?

We kick undergraduates out of the exams if they don't understand such things

 

[sophiecentaur]

As I recall, your argument in this thread started out as an idea about pedagogy.

That was what attracted me to the thread because contributors seemed to assume that considering cause and effect would have to be a given requirement to do a Physics degree (or even a lower qualification). Let's face it, if it were a trivial matter then it couldn't support the amount of discussion that it generates here and elsewhere.

More recently, it seems that you are making a claim about the physical world.

Not really. I am pointing out that symmetry may or may not be there but that it is hardly relevant until you reach some serious frontiers in Physics (well Post Post Graduate). The claims, in the thread, seem to be that it is a simple requirement to ditch causality or you risk failing to understand what's needed for a career in Physics. My 'technical' comments were made merely to show that it's no simple matter to deal with what is claimed to be a misconception. It's strange how many topics in Physics teaching assume equilibrium or static conditions but it\s not graced with the status that this causality has in this thread.

 

[Herman Trivilino]

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

These research results are, to me, reinforcement of my belief that the terms action and reaction should be all but expunged from the vocabulary used to teach Law III. It is outdated terminology that reinforces the notion that one member of a Law III pair somehow dominates the other.

The language we use should reflect the symmetry of the two Law III forces.

 

[sophiecentaur]

the terms action and reaction should be all but expunged from the vocabulary

The terms were introduced a long time ago and were chosen (probably) to be acceptable to the public at the time. The notion was (and still is) a difficult one as 'another force' suddenly comes out of nowhere. It is obviously a confusing and counterintuitive notion. There are many others which students of Science have to get their heads round and those notions often have an unsatisfactory vocabulary. Replacing any of them with acceptable terms would involve an impossible amount of effort and cooperation between Scientists. We just have to suck it up and always come prepared with a reasonable caveat when they're used in lessons. Deferred gratification was a huge part of Science education in the old days.

I have a feeling that, instead of hitting people with such hard ones early on, we should help students with the advice "it works. the sums give the right answer and you can prove to yourself that it makes sense by doing enough worked examples". At the same time teachers should be as fastidious with their own terminology as they can be.

There are many daft examples - like the fact that The Pauli Exclusion Principle is (as near as dammit) a Force.

 

[jbriggs444]

The terms were introduced a long time ago and were chosen (probably) to be acceptable to the public at the time.

The chosen terms were in Latin. I believe that they were written for the capable learned men of the time:

Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in
se mutuo semper esse æquales et in partes contrarias dirigi

To every action there is always opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

I find the explanation in the second part quite apt and symmetric. The initial phrasing is subject to misinterpretation. The second is straightforward.

The notion was (and still is) a difficult one as 'another force' suddenly comes out of nowhere.

That is your intuition grumbling. Silence it already.

 

[PeroK]

Is this an evidence-based belief?

I think the onus is on you to provide evidence of PhD students who cannot explain how motion is possible given Newton's Third Law.

 

[Herman Trivilino]

The terms were introduced a long time ago and were chosen (probably) to be acceptable to the public at the time.

In the introductory physics classroom they are presented to students as quoted in the Principia. This was written by Newton for his contemporaries.

There are many others which students of Science have to get their heads round and those notions often have an unsatisfactory vocabulary. Replacing any of them with acceptable terms would involve an impossible amount of effort and cooperation between Scientists.

It's sufficient for instructors to use the appropriate vocabulary in their classes, with a nod to the fact that students will encounter other vocabulary in other classes and scenarios.

 

[Herman Trivilino]

I think the onus is on you to provide evidence of PhD students who cannot explain how motion is possible given Newton's Third Law.

Fair enough. I have only one piece of anecdotal evidence that I can specifically recall, but I think I have encountered other documented evidence in the past.

A candidate (with Ph.D. in physics) for a teaching position was instructed that at his job interview he would need to deliver a mock lecture on Law I. He did that, but then went on to discuss Laws II and III. He had a slide in his presentation where he'd written $\vec{F}_{ab}=-\vec{F}_{ba}$ and then made the comment that $-\vec{F}_{ba}$ could be moved to the other side of the equation to demonstrate that the net force is zero. I have no idea how or even if his worldview included the notion that (accelerated) motion couldn't be possible or if he perhaps thought that Law III only applied to equilibrium situations.

Certainly he demonstrated that forces acting on different objects could be summed to find a net force, or perhaps he was unaware that the two forces are indeed exerted on different objects.

 

[jbriggs444]

A candidate (with Ph.D. in physics) for a teaching position was instructed that at his job interview he would need to deliver a mock lecture on Law I. He did that, but then went on to discuss Laws II and III. He had a slide in his presentation where he'd written $\vec{F}_{ab}=-\vec{F}_{ba}$ and then made the comment that $-\vec{F}_{ba}$ could be moved to the other side of the equation to demonstrate that the net force is zero. I have no idea how or even if his worldview included the notion that (accelerated) motion couldn't be possible or if he perhaps thought that Law III only applied to equilibrium situations.

There is an interpretation that the net of all internal forces in a system is zero. It follows that internal forces alone cannot produce an acceleration of the center of mass of a closed system.

Indeed, a modern interpretation of the third law is as a statement of momentum conservation.

But I was not there. I did not see this Ph.D. candidate make his presentation. The interpretation I offer may not be the one the candidate had in mind.

 

[wrobel]

Spoiler: offtop

In my experience problems with the concept of an angular velocity of a rigid body are more frequent at exams than problems with Lagrange equations. The question "bring a definition of the angular velocity" is already dangerous :)

 

[wrobel]

written F→ab=−F→ba a

And that is not all :) $\vec F_{ab}$ is parallel to a line that connects the points $a$ and $b$

 

[A.T.]

denying the validity of the treadmill tests

the disbelief was due to a deep intuition ("it just could never work")

And where did that deep but wrong intuition come from? The stated reason given for the disbelief was usually based on cause-effect thinking:

"... the wheels turn the propeller, the propeller pushes the cart, this turns the wheels ... that's a closed cause-effect loop ... impossible!"

However, what I was referring above was something else: Once DDWFTTW was shown to work on the treadmill, these test were dismissed by contradicting Galilean Invariance, once again based on cause-effect thinking:

"... on the ground it's the moving air that provides the propulsive force/energy, but on the treadmill it's the motor of the treadmill ..."

This confusion is not about DDWFTTW as such, but a result of associating energy flow (which is frame dependent) with cause-effect (which is intuitively assumed to be frame invariant).

This thread about inclined treadmills has nothing to do with DDWFTTW:
https://www.physicsforums.com/threads/work-done-running-on-an-inclined-treadmill.927825/
But the same bogus cause-effect arguments are used to contradict Galilean Invariance. Just search the 2nd page of that thread for "provide" or "providing".

And these misconceptions are quite common. Steve Mould even did a video about them:

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

See above.

 

[A.T.]

To every action there is always opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

I find the explanation in the second part quite apt and symmetric.

Yes, the second part is the actual law. It uses the term "mutual actions", which doesn't elevate either of the forces over the other.

The initial part looks like an attempt to condense the law into a short adage, that unfortunately used the term "reaction", and thus ended up confusing students for centuries.

 

[sophiecentaur]

The chosen terms were in Latin. I believe that they were written for the capable learned men of the time:

In the introductory physics classroom they are presented to students as quoted in the Principia. This was written by Newton for his contemporaries.

And they were an exclusive 'general public' who could actually understand his message. Can anyone assume that they had really thought out the issues of causality? N3 was too brief to include this and a whole paragraph would never have got the message across.

perhaps he was unaware that the two forces are indeed exerted on different objects.

This poor guy at the interview is in a very intimidating PF court now.

And where did that deep but wrong intuition come from? The stated reason given for the disbelief was usually based on cause-effect thinking:

I wouldn't disagree with that, although the equations don't forbid it when followed scrupulously.

But the message I have been sending is nothing to do the apparent paradoxes that are present in 'tricky' situations. My argument here has been against insisting that failure to deal 'correctly' with causality should be a shibboleth which would govern anyone's access to further / higher education? I know you know the accepted view but Physics has advanced very well despite a majority of workers not being too sure about how symmetry works. What's so special about it?

 

[Herman Trivilino]

And they were an exclusive 'general public' who could actually understand his message.

Wow! This thread seems to be degenerating into a quibble. Newton's contemporaries were 17th and 18th century natural philosophers. Not the general public!

My point was and still is that this outdated action-reaction terminology used in the oft-repeated statement of Law III feeds the misconception that the Third Law pair of forces are not symmetrical.

The cause-and-effect confusion arises as a result of the confusion over which object's these forces are exerted on. When a force is exerted on an object the object responds, but this cause and effect is not what Law III is about.

 

[A.T.]

But the message I have been sending is ...

... rather vague. So I addressed a specific point of yours:

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

I gave you examples on how this approach fails and leads to wrong conclusions. You haven't really responded to that, except calling it "tricky":

...nothing to do the apparent paradoxes that are present in 'tricky' situations.

A treadmill is not exactly rocket science. And neither is a cart with a propeller linked to its wheels. Have you considered that they are only "tricky" to those who have some basics misconceptions, which were not properly dispelled in their early physics education?

My argument here has been against insisting that failure to deal 'correctly' with causality should be a shibboleth which would govern anyone's access to further / higher education?

Then you arguing against a straw men.

My point is merely, that in physics education, common misconceptions should be addressed and dispelled early on, by using clear language. Not ignored or even deepened by going along with and catering to misleading formulations.

 

[Herman Trivilino]

The initial part looks like an attempt to condense the law into a short adage, that unfortunately used the term "reaction", and thus ended up confusing students for centuries.

And continues to do so. Elementary education teachers just have students memorize the blurb, "For every action there is an equal but opposite reaction".

As an instructor of college-level introductory physics I first and foremost dispelled the misconceptions this created, and explained that this was likely not well understood by their previous teachers.

I told them that they are likely to encounter the terms action and reaction outside our classroom, but I would not be using them. I of course would repeat this warning a few times throughout the course.

 

[sophiecentaur]

I told them that that are likely to encounter the terms action and reaction outside our classroom, but I would not be using them. I of course would repeat this warning a few times throughout the course.

I hope your confidence is justified with all those students. You delivered the message ok but did 'they all' get it? I guess it would depend on the students' potentials and it would be nice to think that they did.

 

[A.T.]

N3 was too brief to include this and a whole paragraph would never have got the message across.

The problem with Newton's formulation is not that it is too brief, but rather the opposite: He should have skipped the initial prose, and it would have been much clearer:

To every action there is always opposed an equal reaction; or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

 

[Herman Trivilino]

I hope your confidence is justified with all those students. You delivered the message ok but did 'they all' get it? I guess it would depend on the students' potentials and it would be nice to think that they did.

I also presented research-based and other lessons I learned during my professional development activities. Followed up with homework questions. I would rely on test results and discussions to gauge the effectiveness of these teaching strategies.

Of course it's nice to think that they absorbed these lessons, just like it's nice to think that Nature behaves a certain way. But we know that we have to test our thoughts when it comes to Nature's behavior. That's physics. We can and should apply the same rigid process when it comes to physics education.

Edit: I want to further note that students' potentials don't correlate well with their conceptual understandings, although it seems a common misconception that they do. Research shows that students can be good or even superb problem solvers but then perform terribly on conceptual tests. The reasons for that are of course debated.