The root phenomenon behind N3 is the conservation of linear momentum.
This conservation phenomenon can be modeled as N3 for the case of a
simple two body interaction, viewed within the context of Newtonian
mechanics. When fields are allowed into the interaction, N3 is at
best a clumsy model.
Bob Sciamanda
Physics, Edinboro Univ of PA (Emeritus)
www.winbeam.com/~trebor
trebor@winbeam.com
----- Original Message -----
From: "LaMontagne, Bob" <RLAMONT@providence.edu>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>;
"Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, August 03, 2007 10:30 AM
Subject: 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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