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

[GiorgioPastore]

Inertial force, because it's proportional to the object's inertia (mass).

Using inertia in place of mass is another confusing thing for beginners. Moreover, historically, the term "vis inertiae" is different. Newton conceived of "inertia" as "the innate force possessed by an object which resists changes in motion",

Among "pseudoforce" and "fictitious," there is an important difference: "fictitious" sounds like something that does not exist. Unfortunately, nobody could convince a meteorologist that Coriolis force does not play a role in oceanic and atmospheric circulation. "Pseudoforce" conveys the correct idea that it behaves like a force (it determines the acceleration of a body in a non-inertial reference frame), although it doesn't share all the properties of a force (there is no action-reaction pair).

 

[sophiecentaur]

And then the other becomes a 'reaction', which in common uses of those words happens after the 'action'. But that's not the case for a 3rd Law force pair, where the forces act simultaneously.

I think you are just choosing to put it that way. In any practical case, as a force is applied then the speed of sound in the system will prevent anything instantaneous happening. Depending on your referenced frame, the two forces the choice is arbitrary and the terms Cause and Effect are interchangeable. and be considered as instantaneous. But is this relevant and can you think of a better alternative way to describe the message in N3?

Unfortunately, nobody could convince a meteorologist that Coriolis force does not play a role in oceanic and atmospheric circulation.

Is that a problem? If it quacks like a duck then it's a duck and you can hardly expect a meteorologist to take on board the Philosophy of Physics. You are implying everyone should consider every situation with deep thorkus and ask questions like Monty Python's "What do we mean by mean."

 

[GiorgioPastore]

It is not a problem for me. It is a problem for people who translate "fictitious forces" as "illusion" without further explanation. The problem exists, as evidenced by the original post in this thread.

 

[PeroK]

It is not a problem for me. It is a problem for people who translate "fictitious forces" as "illusion" without further explanation. The problem exists, as evidenced by the original post in this thread.

The problem exists for people who don't grasp the concept. Illusory forces would do just as well.

Science is not about what words we use to name something.

 

[GiorgioPastore]

Opinions, like definitions, are not right or wrong. They can be useful or useless. From my experience in teaching Physics, I can say that names, although they do not modify Science, do play a role in the learning process. Some names do not help to grasp the concept.
Who, staying at the normal meaning of words, would use something illusory? Still, Coriolis and other pseudoforces are routinely used in the equations of motion, playing the same formal role as forces arising from interactions.

By the way, the non-neutrality of language and names in the learning process is well established in Physics Education Research. Probably the most cited is the case of heat (see, for instance, https://pubs.aip.org/aip/acp/articl...59/Concerning-Scientific-Discourse-about-Heat ). Therefore, if there are alternatives among the traditional names, I'll prefer the less ambiguous one.

 

[A.T.]

the terms Cause and Effect are interchangeable.

The terms "cause" and "effect" are not freely interchangeable in most contexts. That's why their use for N3, where they are indeed interchangeable, is misleading.

can you think of a better alternative way to describe the message in N3?

Interaction forces between two objects always come in pairs of equal but opposite forces, each acting on a different object.

 

[PeroK]

Therefore, if there are alternatives among the traditional names, I'll prefer the less ambiguous one.

I accept the general point, but I don't accept that "inertial" or "pseudoforce" make things any clearer than "fictitious" or "illusory". I would say that "fictitious" is the word that would be best known and understood by high-school students. It's as good as any, IMO.

Moreover, how do we cope with something like the Doppler effect? Or, anything that is named after a person? That has no physical meaning whatsover. There's no linguistic road to physics mastery, as it were.

 

[sophiecentaur]

The terms "cause" and "effect" are not freely interchangeable in most contexts.

In the frame of the total CM they are. Which one hits first? If you're in a car collision, which is the cause?

 

[A.T.]

In the frame of the total CM they are.

The forces in Newton's 3rd Law are frame invariant, so it makes no sense to assign cause and effect to them based on frame of reference.

which is the cause?

Why do we need to know? If cause and effect cannot be uniquely assigned, if one cannot justify why it's this way and not the other way around, then don't talk about cause and effect at all.

 

[sophiecentaur]

Why do we need to know?

Only for convenience. So we can start putting signs into the equations. I know that's equally arbitrary but it usually makes thing easier. Doing without the usual terms is just wearing a hair shirt for n good reason, imo.

 

[A.T.]

Doing without the usual terms is just wearing a hair shirt for n good reason, imo.

No idea what you are talking about. The formulation of Newton's 3rd Law in post #36 doesn't mention action/reaction or cause/effect. Just what you actually needed in order to apply the law.

 

[sophiecentaur]

No idea what you are talking about.

Ditto. Did you actually mean post #36?

I'm talking about regular people using regular methods of solving and discussing mechanical problems. One usually deals with independent and dependent variables (= cause and effect). I can't think of a straightforward situation where an observation doesn't assume or use cause and effect ideas. Maybe you personally don't need to approach things that way but most discussions, in my experience, use the ideas of cause and effect to get messages across. The symmetry is there, of course and there will usually be an alternative way.

 

[A.T.]

Maybe you personally don't need to approach things that way ...

When applying Newton's 3rd Law, there is objectively no need to assign cause and effect, because it's completely inconsequential, and thus should be cut by Occam's razor.

 

[PeroK]

When applying Newton's 3rd Law, there is objectively no need to assign cause and effect, because it's completely inconsequential, and thus should be cut by Occam's razor.

Cause and effect, if it has any meaning at all, is more general than Newton's third law. It's part of a wider understanding of how one event causes another.

Even Occam must have cut himself shaving once or twice.

 

[A.T.]

Cause and effect, if it has any meaning at all, is more general than Newton's third law. It's part of a wider understanding of how one event causes another.

And those events have a temporal ordering where the cause precedes the effect, while the forces in Newton's 3rd are simultaneous.

 

[PeroK]

And those events have a temporal ordering where the cause precedes the effect, while the forces in Newton's 3rd are simultaneous.

I'm not convinced that cause and effect implies a temporal ordering. Google AI, for what it's worth. is happy with the idea that cause and effect can be simultaneous:

The law of cause and effect in physics is best represented by
Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. This demonstrates that interactions are reciprocal: if object A exerts a force on object B (the cause), object B simultaneously exerts an equal and opposite force on object A (the effect). This principle is the bedrock of classical mechanics, providing a predictable framework for understanding how forces and interactions work.


Newton's Third Law of Motion

• Action: When one object exerts a force on a second object.
• Reaction: The second object exerts a force back on the first object that is equal in magnitude and opposite in direction.
• Example: When you push on a wall (action), the wall pushes back on you with equal force (reaction).

...
To be clear, I'm not claiming that this is correct. What I do claim is that your own personal preferences do not have a special place in the teaching of physics education. You have your own ideas, of course, but they are not necessarily universally accepted. Nor are mine.

 

[jbriggs444]

I'm not convinced that cause and effect implies a temporal ordering. Google AI, for what it's worth. is happy with the idea that cause and effect can be simultaneous:

The law of cause and effect in physics is best represented by
Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.

Correlation is not the same as causation.

I would say that it is possible for some state of affairs to "cause" a Newton's third law interaction to take place. But it would not be correct to say that one side of the interaction "causes" the other. This is, of course, an opinion about language rather than an opinion about physics.

 

[PeroK]

Correlation is not the same as causation.

I would say that it is possible for some state of affairs to "cause" an Newton's third law interaction to take place. But it would not be correct to say that one side of the interaction "causes" the other. This is, of course, an opinion about language rather than an opinion about physics.

That's nearer to what I would say. But, if someone wants to say pushing a wall and the wall pushing back is cause and effect, then I'm not going to argue.

Until someone fully defines cause and effect (and establishes the condition for temporal ordering, or not), I tend to see the term as not fully defined.

 

[sophiecentaur]

and thus should be cut by Occam's razor.

Yebbut not everybody wakes up every morning with Occam's razor in their hand.

 

[A.T.]

I'm not convinced that cause and effect implies a temporal ordering.

If cause and effect doesn't imply temporal ordering when applied to Newton's 3rd Law, then what does it imply at all in terms of physics? If it's completely inconsequential in the context of Newton's 3rd Law, then it should be cut by Occam's razor.

 

[sophiecentaur]

If it's completely inconsequential in the context of Newton's 3rd Law, then it should be cut by Occam's razor.

Logically you could argue that you are correct but does your approach actually help understanding or discussion. It often makes things more difficult and my two words "hair shirt" apply. Reductionism for the sake of tidiness alone can get in the way of teaching and learning because we are all human and less tidy than you may want.

 

[A.T.]

It often makes things more difficult

How does dropping the arbitrary and irrelevant cause & effect assignment make applying Newton's 3rd Law more difficult? If anything, it makes things simpler, because no time is ever wasted on wondering how to identify the cause vs. effect in a given scenario.

What exactly is gained by pretending that the cause & effect assignment has any relevance for Newton's 3rd Law, and thus wasting the student's time on figuring out that it actually doesn't?

 

[Herman Trivilino]

And then the other becomes a 'reaction', which in common uses of those words happens after the 'action'. But that's not the case for a 3rd Law force pair, where the forces act simultaneously.

It also invokes the notion that the action is the cause of the reaction, elevating the status of the action above that of the reaction. These vocabulary choices matter in the introductory physics classroom. Students often carry them into their higher education and eventual profession. True for both physicists and those who major in something other than physics.

The famous author and provider of in-service professional development for physics teachers recommends the vocabulary Third-Law pairs of forces. And emphasizes that forces are interactions between objects so that the Third Law layers on to that concept, emphasizing that the interaction is in all ways symmetrical.

There are lots of ways to demonstrate this concept to students and it's essential that the students see them actually performed. Talking about them is not sufficient. Showing videos of them robs the student of experiencing them and in many cases having that kinestetic experience.

 

[wrobel]

I have always been wondering how such a banal thing can generate endless discussions.

 

[A.T.]

It also invokes the notion that the action is the cause of the reaction, elevating the status of the action above that of the reaction. These vocabulary choices matter in the introductory physics classroom. Students often carry them into their higher education and eventual profession. True for both physicists and those who major in something other than physics.

Exactly. Misguided notions about basic laws, sometimes stemming from misleading formulations, often stand in the way of analyzing problems correctly.

One example is failure to accept and apply Galilean Invariance, based on the wrong intuition that it makes a difference whether 'A pushes B' or 'B pushes A', stemming from the misleading formulation of Newton's 3rd Law.

Also, when feedback loops are involved, naive linear cause-effect intuitions often fail. See the endless internet discussions about DDWFTTW (directly downwind faster than the wind), where even engineers and physics professors argued that it would be perpetual motion, after they ran the cause-effect-loop in their heads.

 

[sophiecentaur]

What exactly is gained by pretending that the cause & effect assignment has any relevance for Newton's 3rd Law,

It also invokes the notion that the action is the cause of the reaction, elevating the status of the action above that of the reaction.

This thread is like other discussions of 'Work done by and work done on'. Imo there's little point in arguing one way or another at all times. 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? There is no reason to avoid cause and effect just because it's not strictly necessary.
it's the equivalent of choosing arrow directions on a free body diagram. In fact, in that case, you have to do that before you try even to make a start. Some choices will introduce confusion - if you try to start with an unfamiliar choice.
The flow of causality intuitively follows the flow of energy so where is the problem? These things will resolve themselves during the analysis.

 

[jbriggs444]

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.

The notion of causality that I subscribe to is one where the relation is anti-symmetric. That is, if A causes B then B does not cause A.

By contrast, in Newton's third law the two members of the force pair are symmetric.

 

[sophiecentaur]

The famous author and provider of in-service professional development for physics teachers recommends the vocabulary

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

 

[sophiecentaur]

This intuition is a non-starter.

Not exactly a starter rather than a non-starter. It would certainly need to be resolved somewhere down the line but, in a world where causality is used all the time to 'explain' situations (outside pure Physics), why not at least acknowledge at least an arbitrary preference when solving a problem or explaining a process?

That is, if A causes B then B does not cause A.

The problem is that the maths we use does not make this clear. Simply re-arranging an equation can look to the user as if cause and effect have been reversed. There is an implied 'arrow' through many equations / experiences that can only give a meaningful answer when we start with a cause. 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?

 

[jbriggs444]

The problem is that the maths we use does not make this clear. Simply re-arranging an equation can look to the user as if cause and effect have been reversed. There is an implied 'arrow' through many equations / experiences that can only give a meaningful answer when we start with a cause.

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

It is attractive to imagine ourselves in the driver's seat "causing" the rocket to burn hotter by pressing on the accelerator. But that idea does help us solve any equations.

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?

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