Re: Apparent weight

["JACK L. URETSKY (C) 1996; HEP DIV., ARGONNE NATIONAL LAB, ARGONNE, IL 60439" <JLU@hep.anl.gov>]

Hi all-
	Reply to Tim Folkerts, who sayeth (in part):
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> 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!
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N2:  I don't understand.  If N2 defines force, how do you measure mass?
What do you do to compare other chunks of matter with your "specific chunk"?

N3:  Again I don't understand.  In a closed system there can be no external
forces.  To identify a force that is acting I must isolate a part of the
system.  N3 guarantees that in the process of isolating, I don't leave any
unbalanced force that would result in a net force acting on the closed system.
				Regards,
					Jack