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Re: Apparent weight




I rather like N3 and think it has a much more important role than
simply a "computational device" as Jack Uretsky proposed. My
interpretations of the three laws would go something like:


N1: There exist inertial frames. The test for an inertial frame is that
an object that is not acted upon by a force is not accelerated.
Comment: This leaves the notion of force undefined. It also insures
that N1 is independent of N2.

I like this one. It won't do to have N1 a subset of N2.


N2: F=dp/dt
Comment: The 4 forces of nature are given. N2 then permits one to
measure mass (definition of mass). Otherwise, N2 is empty because
force and mass are defined in terms of each other.

I would call N2 a definition of force. Mass can be defined
independently by a specific chunk of matter. Acceleration is defined
by length, time and calculus.


N3: Any part of a system may be analyzed in isolation, using N2, replacing the
remainder of the system by the forces exerted by that remainder.
Comment: N3 is just a computational device.

I see N3 as the most important - 180 degrees from a "computational
device"! It is exactly the same as conservation of momentum. It is the
one law that CAN NOT be a definition. There is no reason to believe a
priori that there must be a second, opposite force. It is an
experimental result that tells us something new about the universe.

I would go so far as to say that N3 is the FIRST real PHYSICS that we
typically teach our students. A competent mathematician could
"derive" kinematics from Euclidean geometry & calculus. N2 is a
definition (of force in my view, or of mass ) and hence is not
falsifiable. N3 finally gives us something to check!



--- Dr. Folkerts


********************************************************
Timothy J. Folkerts Tim.Folkerts@valpo.edu
Dept. of Physics & Astronomy 219-464-6634
Valparaiso University
Valparaiso, IN 46383