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Re: transfer of momentum



On 11/19/2003 10:52 AM, Peter Vajk wrote:

Clarity about forces can (nay, must) be achieved by
identifying in every case _which body is acting on which
other body_ -- cf. N3!!! This preempts any confusion
about "the" force at the A/B boundary.

True as to bodies, but not true as to _locations_ as
I have repeatedl pointed out.

In fluid dynamics, SR, GR, and other situations, one
commonly deals with the momentum of _regions_ not
bodies.

On 11/19/2003 09:56 AM, Bob Sciamanda wrote:

In this light, I object to your making this
conceptual view "mandatory". One man's conceptual
clarification of an equation may be another's
confounding weirdity. Physics preaches no conceptual dogmas.

I did not mean to make anything mandatory in such
an absolute sense, but only in the conditional sense
that _if_ one uses the model _then_ one ought to use
it correctly. Momentum flow through a node, even if
there is no net accumulation of momentum in that node,
remains an indispensible part of our understanding of
conservation of momentum. Tension in a chain remains
a good example of this.

On 11/19/2003 10:36 AM, Chuck Britton wrote:
If I understand John correctly -

He is trying to DOWNplay the equation that most of
> us rattle off as N2, F=ma.

Let's be careful what we say about this. I'm not
trying to make F=ma less useful, nor even to say
it's less useful than you think it is. I'm just
pointing out the momentum transfer is useful
*also* and is in many cases more useful than
F=ma *comparatively* speaking. If you start
out with $5 in your left pocket and somebody
puts $10 in your right pocket, the $5 is still
there and hasn't gotten any smaller in the
absolute sense.

(There is what's known as the logarithmic utility
of money, so the monetary analogy actually
understates the advantage of additional ideas. I
do not think there is logarithmic utility of
ideas; indeed it seems to go the other way: two
ideas have *more* than twice the utility of one.)

There's no "conservation of laws" law that says
you are only allowed to know one dynamical law.

In this area and practically every other area, it
is good to have more than one way to do things.
Front door and fire escape. Belt and suspenders.
If I can solve a problem only one way, I get
worried.