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

[ludwik kowalski <kowalskil@mail.montclair.edu>]

It is often stated that physicists build models of reality. A model  
which states that acceleration is always caused by a net force seems  
to be very useful. That is what Sarma would say about the a=F/m. The  
mass of an object, in a classical model, does not depend on speed. But  
the m must known to calculate the a(t) when a particular F(t) is  
given. Do I interpret him correctly?

Yes, a mathematician can write the second law as F=m*a, or m=F/a. That  
does not contradict the useful model of a physicist--in order to  
accelerate an object one needs a net force. Causality is part of the  
model, it is not part of reality.

Yes, I know that more general models can be, and have been, created.  
All models have limited validity. The simplest model is appropriate in  
teaching introductory physics courses. That is what most authors of  
textbooks do. The assumptions under which simple models are valid are   
usually clearly stated. Learning about more general models, in  
advanced classes, does not mean "unlearning of what has already been  
learnt."

Ludwik
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On Nov 28, 2010, at 8:01 PM, John Denker wrote:

> On 11/28/2010 05:00 PM, D.V.N. Sarma wrote:
> > The whole of physics was built upon the assumption that
> > every effect has a cause and it has come into existence
> > as a consequence of this search.
>
> Thank you for sharing that unsubstantiated and exceedingly
> implausible opinion.
>
> Let me point out that other people have held a different
> opinion, e.g.
>   The present does not seem to me to be an opportune time
>   to enter into the investigation of the cause of the
>   acceleration of natural motion, concerning which various
>   philosophers have produced various opinions .... Such
>   fantasies, and others like them, would have to be examined
>   and resolved, with little gain. For the present, it suffices
>   .... to say that in equal times, equal additions of speed
>   are made.
>         – Galileo Galilei,
>         Discorsi e Dimostrazioni Matematiche intorno à
>         due nuoue scienze Attenenti alla Mecanica & i
>         Movimenti Locali (1638).
>         [page 203 of the National Edition]
>         [translation by Stillman Drake]
>
>   Hypotheses non fingo.
>         – Isaac Newton
>         Philosophiæ Naturalis Principia Mathematica
>         (2nd edition, 1713).
>
> > The whole of physics was built upon the assumption that
> > every effect has a cause and it has come into existence
> > as a consequence of this search.
>
> It would have been better to say that the whole of premodern
> physics /up to a point 400 years ago/ was built on that assumption.
> According to some, including Stillman Drake, Albert Einstein,
> and others, /modern/ science began when people stopped using
> that approach.
>
> > I know that acceleration occurs only after i put my finger to the  
> > body
> > and push it. This determines temporal sequence of force and
> > acceleration.
>
> Absolutely not.  As functions of time, F(t) = ma(t).  It is the
> same t.  The law of motion insists that there is no temporal sequence.
>
> For details on this, see
>  http://www.av8n.com/physics/causation.htm
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Ludwik

http://csam.montclair.edu/~kowalski/life/intro.html