🚀 go-pugleaf

  ... followups reduced significantly ...


meron@cars3.uchicago.edu wrote:
}...
} >Mati, this is certainly a sound, but rather myoptic view. From
} >fundamental phenomina, isn't mass the character that assignes momentum?
} 
} No, why?  Even within classical physics momentum isn't uniquely
} associated with mass. 

Fred McGalliard <frederick.b.mcgalliard@boeing.com> writes:
>
>Interesting. Classical momentum is not associated with mass you say? 

 He said uniquely.  You can get the same momentum with a different mass
 in classical physics.  That "v" dependence, the fact that momentum is 
 associated with motion, is what makes momentum not invariant while 
 mass is -- both classically and in special relativity. 

>If
>E=M is used to assign mass to a photon, even this rather stretched case
>fits the classical. 

 Not at all, which is why it is such a bad idea.  You do not get 
 KE = p^2/2m when you use that for a photon.   You get into all 
 sorts of trouble, and worst of all you abandon the idea that most 
 other sciences (chemistry, biology, non-relativistic physics, 
 engineering, medicine) take for granted: that mass is invariant. 

>I don't know of any simple phenomina where momentum and mass
>are not uniquely associated. 

 I don't know of any where they are.  You can have mass with zero 
 momentum, you can have different masses with the same momentum, 
 the same mass with a different momentum, and when you shift to 
 the "center of mass" frame to analyze a billiards problem, you 
 have to change the momenta and energies but _not_ the masses. 
 (All in reference to non-relativistic classical mechanics.) 

>Granted rest mass and momentum are only
>uniquely associated if you add relativistic mass increases, but if you
>deal with the rest mass as a traped potential energy, then you get the
>momentum proportional to energy and velocity and don't need to do any
>relativistic stuff at all, though this is hardly classical since the
>"relativistic mass" is buried in the momentum.

 But it is quite incorrect to conclude that you just have to use the
 "relativistic mass" in classical mechanics to get relativity.  Please 
 read the FAQ sections on this subject. 


} Energy is energy and mass is mass and the two aren't the same, though
} they're related.

>A good philosophical point. 

 And a good basis for drawing a distinction. 

>You got Math theory? They are not
>orthagonal. They are parallel, and they represent at most a constant
>factor difference. 

 They are not parallel _nor_ perpendicular:

   ( E )^2 = ( mc^2 )^2 + ( pc )^2 

 Look familiar? 

>The concept of rest mass is just the mass where the momentum goes
>through zero. Works great for objects that don't contain too many
>photons. 

 It also works there.  It is a powerful and general result. 

-- 
 James A. Carr   <jac@scri.fsu.edu>     | Commercial e-mail is _NOT_ 
    http://www.scri.fsu.edu/~jac/       | desired to this or any address 
 Supercomputer Computations Res. Inst.  | that resolves to my account 
 Florida State, Tallahassee FL 32306    | for any reason at any time.