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Re: [Phys-l] interaction

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

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of John Clement
Sent: Thu 8/2/2007 11:32 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] interaction



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".


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