[Phys-l] FW: The "why" questions

[Al Bachman <bachman_28@msn.com>]

Al Bachman


 


From: bachman_28@msn.com
To: betwys1@sbcglobal.net
Subject: RE: [Phys-l] The "why" questions
Date: Tue, 7 Dec 2010 01:01:50 +0000




   You can think of this as a question of approximations.
 
   The first order approximation is that objects are rigid bodies.
If analyzed in too much detail you get infinite speeds of propagation,
and other paradoxes.
   
   In the next order, one considers objects as collections of smaller bodies,
subject to internal forces. Newton's second law becomes (Fext)net = M a(of the CM),
Now there are internal degrees of freedom, finite speed of propagation ,etc.
 
Al Bachman


 
> Date: Mon, 6 Dec 2010 11:58:02 -0600
> From: betwys1@sbcglobal.net
> To: phys-l@carnot.physics.buffalo.edu
> Subject: Re: [Phys-l] The "why" questions
>
> On 12/4/2010 11:54 AM, D.V.N. Sarma wrote:
> > Let us consider a body subject to two opposing forces
> >
> > 1. If the two forces are equal the net force is zero. But the condition
> > of the body is not the same as that of body on which no forces at all
> > are acting.
> > The body will be strained.
> > 2 If the forces are not equal there will be a net unbalanced force
> > and there will be acceleration. But the condition of the body is not the
> > same as that of the body under the action of a single force of the same
> > magnitude. The strains in both the cases will be different.
> >
> > It is incomprehensible to me why we object to the cause effect relation
> > between force and acceleration when we have no objection to the cause
> > and effect relationship between force and strain.
> >
> > Or do we deny the cause and effect relationship between force and strain?
> >
> > regards,
> > Sarma.
> It is entirely possible that you are chasing a chimera, with me alongside.
>
> But since you postulate a force on a mass with some spatial extension, 
> opposed by an equal force; then I must consider the transmission time of 
> that force through the mass until an unyielding
> force opposing this transmitted force appears: after which event, a 
> force is transmitted back to the originating force.
> The initial unbalanced force gives some compression, the opposing force 
> appears after some delta t transmission time, and the initial force is 
> in turn balanced after a further delta t for the return traversal.
>
> This does not appear entirely fanciful. One can propose instrumenting a 
> rod with a strain gage at each end.
> The experiment avoids easy conclusions involving the CofM concept, as 
> it seems to me.
> It speaks against the too ready acceptance of this article of faith: "If 
> two opposing forces are equal, the net force is zero."
>
> Brian W
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