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Re: [Phys-l] Interaction


On 3 Aug 2007, at 09:00, Jeffry 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.

I think the common understanding of the word "interaction" (see any dictionary) suffices for physical purposes. certainly that is the way I use the term. It is possibly the least confusing of words used in two vocabularies, technical and common parlance. Contrast "energy" and "power".

I consider Thomas Moore's introductory calculus-based physics textbook
Six Ideas that Shaped Physics (see
<http://www.physics.pomona.edu/sixideas/> )
to be an excellent example of the incorporation of PER into introductory
physics-among other things: he incorporates modern physics and
thermodynamics throughout the book, he treats energy conservation as
such rather than as constancy of energy,

?

he uses the symbol F with an
appropriate subscript to designate the magnitude of every force,

Admirable! I wish I had had the courage to do that when I was teaching. One can get away with NOT saying "Let W be the gravitational force acting on the abject and F the applied force. Let N be the normal force exerted by the table on the object and f be the frictional force exerted on the object by the table." (All are vectors, of course.) Any teacher who thinks that is not daunting to the beginning student has a chronic empathy deficiency.

All the F's can go on the FBD - nothing else. Simple. Whoever said "Avoid subscripts" was not testing his rule for effectiveness in teaching. I never allowed student to write a non-subscripted symbol to represent a torque. A torque must always be defined relative to some point in space, and that point, however obvious it may be, should be recognized with a subscript.

he uses
spacetime diagrams, and he carefully states the realm of applicability
of every equation statement of a principle of physics. I looked to
that book to see how Thomas Moore used the word interaction and it
raised some questions in my mind.

Thomas Moore defines an interaction as "a physical relationship between
two objects that, in the absence of other interactions, changes the
motion of each." Later, he states Newton's third law as: "When objects
A and B interact, the force the interaction exerts on A is equal in
magnitude and opposite in direction to the force that it exerts on B."
In the definition, an interaction is a relationship between two objects
but in application (in many places throughout the book) the interaction
is a third entity and it is that third entity that exerts the forces on
objects that are said to be interacting with each other. Can a
relationship be an entity that exerts forces?

I guess it could, if one chose to define it that way, but what PER says that's a good thing to do?

Moore's definition of an interaction does not satisfy me. It is so narrow that it appears "object" means "point mass" and "interaction" means "mutual force". In common usage "interactions" refer to processes that change the states of extended particles by having them exchange angular impulses or by raising their temperatures in the case of inelastic interactions and in many other ways. The objects could even exchange mass and charge in some interactions. It is also the case that some interactions between two objects can leave one of the objects unchanged, for example catalysis, or the transmission of a STD.

It is unclear to me how introducing a new entity, the interaction, makes mutual forces clearer.

Leigh