Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] interaction




On Aug 4, 2007, at 7:32 PM, John Denker wrote:

The OP asked about "interaction". That was in the context of N3, and
how to teach N3.

We need to go beyond (or behind) the literal question and address the
/idea/ behind the question.



On 08/04/2007 01:47 PM, Bob Sciamanda wrote:

N3 envisions only interactions between two, otherwise isolated bodies.
N3
is at most clumsy in multi-entity interactions. The more fundamental
model of momentum conservation applies to all these situations - N3 is
a
corollary for the special case of an otherwise isolated two-body
interaction.

That's exactly where we need to go.

1) We all know what the fundamental physics is. The fundamental idea
is conservation of momentum.

2) We know how to state "conservation of momentum" in ways that simultaneously
simple, clear, and rigorous.

3) To my mind, the third law of motion _is_ conservation of momentum. If
Newton didn't state it that way, that's Newton's problem, not my problem.

4) In particular, if we make statements about the force-pairs that arise in
two-body interactions, then it should be obvious that such statements are
only approximations that apply to /bodies/ (not including fields) and
considering bodies only two at a time (not three or more).

I have nothing against approximate and simplified statements, but there is
no point in litigating the technicalities of such statements.

If a student asks a question that cannot easily be answered in terms of
force pairs, we shouldn't feel obliged to answer it in such terms. There
comes a point where the only reasonable course is to transform the question
into a question about momentum, and answer the modified question. You
can't carve a beautiful sculpture with a dull chisel.

5) Please don't tell me that sequencing requires N3 to be expressed as forces
before conservation of momentum can be derived. You can sorta maybe do it
that way, but you definitely don't /need/ to do it that way.

It is certainly possible, and IMHO easier, just to assert conservation of
momentum as a primary and fundamental fact.

This is related to (but not identical to) the fact that you can assert
conservation of energy on Day One, and do quite a lot of physics with
that ... long, long before any notion of "force" is discussed.

Energy is primary and fundamental. Energy is conserved.
Momentum is primary and fundamental. Momentum is conserved.

====================

To repeat:
-- The idea behind N3 is conservation of momentum. When in doubt, answer
the question in terms of conservation of momentum.
-- If the wording of this-or-that version of N3 is not synonymous with
conservation of momentum, there is a problem with the wording.



On 08/04/2007 01:46 PM, LaMontagne, Bob wrote:
.... we
never talk of an object exerting a force on the field in which it is
imbedded.

We routinely talk about a particle transferring momentum to the field.

This is just one example among many where the momentum idea makes
headaches go away.

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

What Bob and John are saying becomes clearer if F=m*a is written as F=dp/dt. Yes, I know that I am just repeating what others wrote many times. Two equal and opposite forces imply two equal and opposite dp. The total p, of the entire closed system, does not change.

_______________________________________________________
Ludwik Kowalski, a retired physicist
5 Horizon Road, apt. 2702, Fort Lee, NJ, 07024, USA
Also an amateur journalist at http://csam.montclair.edu/~kowalski/cf/