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

[John Denker <jsd@av8n.com>]

On 12/04/2010 10:54 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?

Of course we deny it, in the general case.

More precisely, we observe that there is no fundamental law that
requires or expresses any cause-and-effect relationship between
stress and strain.

I am quite aware that you can come up with anecdotes and scenarios
where the strain can be inferred from the stress ... but there are
also anecdotes and scenarios where the stress can be inferred from
the strain.  In any case, anecdotes are not data, and scenarios are
not laws.

I have to wonder, where do these questions end?
 -- The notion that Newton's second law expresses a causal relationship:
  F causes ma.
 -- The notion that Hooke's law expresses a causal relationship: stress
  causes strain.
 -- The notion that Ohm's law expresses a causal relationship: V causes IR.
 -- The notion that Hamilton's equation q' = ∂H/∂p expresses a causal
  relationship: q' causes ∂H/∂p.
 -- The notion that Hamilton's other equation p' = -∂H/∂q expresses a
  causal relationship: p' causes -∂H/∂q.

There is no point in asking more questions of this kind.  The answer to
all such questions is the same:  In most cases, especially in mechanics,
the fundamental laws say you cannot have one thing without the other.  
This does *not* mean that one thing causes the other.

The laws of physics must say what happens.  They may or may not say how
it happens.  The fundamental laws rarely if ever say why it happens.