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Re: F=m*a, was Thermodynamics



We've circled around this tree many time before. I've always had trouble
with the idea that ma defines a force. Forces cause accelerations but I
don't think you can turn that around and say an acceleration causes a force.
While you can certainly state that a 1 kg mass that accelerates at 1 m/s^2
must be experiencing a 1 N force along the direction of acceleration, you
can also apply that 1 N force to many situations where there will be ZERO
acceleration because it won't be the only force acting. So if I try to
_define_ a 1 N force as that force which accelerates 1 kg at 1 m/s^2, why
can't I also define a 1 N force as that force which suspends a .098 kg mass
at rest near (but not on) the earth's surface? If I define force as F = ma
(really NET F = ma) and then go push down on a table, how am I ever to
determine that force--the acceleration is again zero? [Is this what John is
saying?]

F=ma may flow nicely for memorization purposes, and in some RARE situations
can be used to _measure_ a single force, but more generally it tells us only
the effect of all the forces acting on an object. a = netF/m is, IMO, a
much better way to write and think about the second law.

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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----- Original Message -----
From: "Ludwik Kowalski" <kowalskiL@MAIL.MONTCLAIR.EDU>


Here is a similar (?) situation. If the speed of light in empty
space is DEFINED as 299,792,458 m/s (as in SI since 1983)
then measuring c makes no sense. On the other hand, if we
define distance and time as "what we measure with instruments"
then an experimental determinations of c=d/t makes sense.
Is this a good analogy of the situation encountered in F=m*a?
Ludwik Kowalski