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Re: [Phys-l] interaction

This question has mixed an advanced model with the initial model. First the
students need to have the model that all forces are due to interactions with
another physical object. Once this model is established then the field
model can be introduced later. The interaction model really helps students
when they are doing problems which generally involve things one could do in
the lab.

Fields must come after interactions. Fields are at a much higher level
because they involve things that you can not see, but interactions involve
looking for physical objects that one can see, or at least know are present.
One can still build understanding using N3L and interactions, but it may be
more difficult.

Of course the real problem comes in if you mix in relativity. But when
generalizing to a better model, one can talk about interactions between the
fields and an object, and between the fields and the agents.

Learning physics is like learning to dance. First you crawl, then you walk,
then your run, then you can learn to dance. If you skip a stage, it can
make the later learning difficult or impossible. Interactions is a very
powerful way to help the students make the necessary connections. While it
is true that some students can very easily learn physics without it, they
often generate an interior model of interactions, but are not necessarily
aware of it.

If you skip to the field model, many students will never acquire an
understanding of N3L, and will still tend to make up mythical forces such as
the force of motion. Fields tend to be mysterious objects which fit in
nicely with the idea that you can have forces created by things other than
physical objects. In elementary physics all forces are ultimately caused by
physical objects. Notice that causation is not necessarily used in a strict
mathematical sense here.

Incidentally at the AAPT conference there was a very interesting session
which had a paper showing that young children understand causation very well
and early on, but do not understand mechanistic explanations well. This
actually starts with babies as young as a few months old. This type of
cognition continues into adulthood. So that the idea that forces cause
acceleration is natural to people, but the abstract relationship between F
and a are very unnatural to most people. Now there is evidence for why I
say that using a causal connection is a powerful way of generating the
understanding, but the abstract formula is not.

John M. Clement
Houston, TX


May I pose a question that a student might ask if presented with this
interaction approach? Consider the sun and one of the planets. In the
usual approach to fields at the introductory level, we consider the planet
to interact not directly with the sun, but rather with the local
gravitational field it is imbedded in. In the sense of interactions
obeying NTN 3rd, why is the force on the sun toward the planet considered
the other interaction force? The planets are interacting with the fields,
not interacting with each other directly (action at a distance.) I think
this interaction approach gets a little fuzzy here.

Bob at PC


As used in various curricula, the word interaction is fairly precisely
used.
The word force pair is VERY misleading, because students then think that
any
force pair is according to Newton's third law. I recall that Karplus had
a
text that emphasizes interactions, and Minds on Physics uses it
extensively.

The trick is to introduce the idea of interactions before Newton's second
law. The idea is that there is a single interaction. Eventually the idea
of making the distinction between interaction forces, and "balancing"
forces
needs to be made clear. Balancing forces are forces on the same object
which might balance. Notice that balancing forces can also come in pairs,
but might be only one force or even many. So many situations have pairs
of
forces in balance that students will then balancing pairs as interaction
pairs. A book on a table has 2 important balancing forces, normal and
gravitational (ignoring buoyancy), and students often think of them as
being
NTN 3rd law pairs, which will be made even worse by the term "force pair"

Basically an interaction as used in reformed curricula could be any NTN
3rd
law situation. The distinction is also made between contact forces and
non-contact forces where initially the only example of the latter is
gravitational forces. Students have to learn to identify forces by
looking
for contact. The term is designed to try to force students to identify
forces when there are 2 interacting objects. Also it is very helpful in
preventing students from adding mythical forces such as the force of
mortion, because they must always identify the physical object that
produces
the force.

Actually the term interaction is not an "official" physics term, but is
rather used by specific curricula. It might eventually become a standard
term with time. But of course there are a number of examples of terms
which
do not have "standard" definitions and vary from author to author. My
favorite example here is "weight".

John M. Clement
Houston, TX



On 08/02/2007 12:53 PM, Jeffrey Schnick wrote:
Information indicating that if you talk more about interactions than
about one object exerting a force on another then students will gain a
better understanding of Newton's Third Law has convinced me that I
should use the word "interaction" more often in my introductory
physics
course. As such, I want to make sure that I have a clear
understanding
of how physicists use the word.

That's a good clear question, but there is not going to be
a comparably clear answer.

"Interaction" is a word physicists use when we don't want
to be precise, so looking for a precise meaning is futile
and/or a step in the wrong direction.

For example, when we say "Coulomb interaction" you don't
know whether that refers to the electrostatic force and/or
the electrostatic energy and/or the static electric field.
You can't write an equation
Interaction = .....
in contrast to the more-specific things mentioned above:
Force = ....
Energy = ....
Field = ....

Interaction is a useful catch-all term.

In the context of the third law, if you mean "force pair"
it might be best to say "force pair".


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l