Re: Newton's second law

[Bob Sciamanda <trebor@VELOCITY.NET>]

Amen, John!
I think it needs to be emphasized that N2 is a statement about INTERACTIONS
(Forces).  It says that interactions produce momentum changes.  The converse
is dangerous:  Momentum "changes" concocted by merely considering more or
less than the mass originally under consideration are not evidence of
interaction forces.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.velocity.net/~trebor/
trebor@velocity.net
----- Original Message -----
From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, November 07, 2003 12:19 PM
Subject: Re: Newton's second law


> > It is very dangerous to expand N2 into F = mdv/dt + v dm dt.
>
> I'd reiterate that it is far worse than merely "dangerous."  Except
> for those times when it is trivially correct (i.e., when dm/dt = 0),
> it is almost always dead wrong.  And in those very few cases where it
> is neither trivial nor dead wrong, it is right for the wrong reason.
>
> --
> John Mallinckrodt        mailto:ajm@csupomona.edu
> Cal Poly Pomona          http://www.csupomona.edu/~ajm