Re: Apparent weight

[Tim Folkerts <Tim.Folkerts@valpo.edu>]

I am trying to interpret Newton's Laws for a beginnig student, and I 
recognize that my view may not be compatible with relativistic 
interpretations, but at a general physics level I think it works.

> 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"?

The piece of Pt-Ir in Paris is 1 kg by definition.

Find a frame where the mass is at rest and stays at rest (possible 
because we believe in  N1).  Now do something to it to make it 
accelerate, say put it on the end of a stretched spring. The quantity 
ma is DEFINED as the force you applied.

I can do the same something to other masses. If they accelerate the 
same amount, they are also 1 kg.  If they accelerate twice as much, 
they are 0.5 kg....

If I do a different something to the original 1 kg and it accelerates 
twice as much as before, then the new for is twice as big as the old 
force.

I think this is all self-consistent and uniquely defined.  

> 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.

Perhaps I'm missing something, but as I understand you, the existance 
of a "closed system" (CS) implies that N3 must be true - it is simply a 
device for grouping forces.  Since N3 also implies conservation of 
momentum (CoM), is CoM a trival consequence of the existance of CS's?

Personally, I don't think so.  I think CoM is a very well established 
_experimental_ result.  By extension, I view N3 as a well established 
_experimental_ result.  Perhaps we should say that CS's exist because  
of CoM???

As I think about it, maybe it is N1 & N3 that are related, not N1 & N2 
as some people like to think.

But I have a quiz to grade, and no more time right now to ponder this 
more deeply at the moment.



---   Tim Folkerts


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