Re: [Phys-l] F causes a

[John Denker <jsd@av8n.com>]

Stefan Jeglinski asked:

> So how would you assess the following example?
>
> Someone too far distant for you to see throws the block at you. All 
> you know is suddenly a flying block hits you in the face. In your 
> vernacular (which I would say is far more simplistic than the 
> discussion that has been going on), you feel a force as the block 
> hits your face. In fact, the Jim Green I've read for years here would 
> claim that only a force could cause (oops there's that word) his 
> tooth to have been knocked out. Would you not say that the 
> (de)acceleration of the block as it hit you is what caused the force 
> that knocked your tooth out? If not, where did that force come from?

This is getting repetitious.  As stated back on the 27th of April,
it is important to distinguish:
  Equation
  Causation
  Definition
  Calculation

Physics observation and theory tell us that there is an *equation*
between F and ma.  This is true and useful in an astonishingly
wide range of common conditions.

*Sometimes* there is also a chain of causation that leads via
F to ma ... and sometimes the reverse.  You can't prove one
direction of causation by selecting examples that support
that viewpoint;  too many counterexamples exist.

*Sometimes* it is expedient to define force in terms of ma ...
and sometimes not.  (PSSC didn't.)

*Sometimes* it is expedient to calculate F in terms of ma, and
sometimes the reverse.  You can't prove one direction by selecting
examples that support that viewpoint;  too many counterexamples
exist.

Remember to distinguish:
  Equation
  Causation
  Definition
  Calculation

==========

It is not at all obvious that "feeling" is caused by force.  The
last time I looked, the nerves were more accurately described as
strain detectors, not stress detectors.

Also, how do you know that the breakage of the tooth was "caused"
by stress and not strain?

If one is looking for causes, one might ask why the brick was
thrown ... but that is not a physics question.  Similarly, one
might ask why you were standing downrange at the annual brick-
throwing contest :-).

To sum up, neither this example nor the many examples that
preceded it provide any appreciable support to the notion that
"F causes ma (and not vice versa)".