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

[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.

 

[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.