Re: [Phys-l] Equations (causal relationship)

[John Denker <jsd@av8n.com>]

Michael Edmiston wrote:
> Each individual force can be measured in many circumstances, 
> especially the zero acceleration zero velocity case.  But I cannot 
> imagine any way to measure the individual accelerations ...

That is a failure of imagination.  It has nothing to do with the
actual physics.

I thought John M. addressed this point perfectly well.  There are
lots of cases where the acceleration is the controlled variable, and
the force may be calculated therefrom.

To add to the list of examples:  Consider a pre-Newtonian description
of the motion of the moon.  The sun is fixed;  the earth circles around
the sun;  the moon circles around the earth.  Therefore the moon's
path is an epicycloid.  Given the two radii and the two times, we know
the acceleration of the moon.  This is very nicely described as the
sum of two accelerations.

For that matter, from a modern-physics point of view, practically every
nontrivial acceleration measured in the HS physics lab (or intro-level
college physics lab) is the sum of two terms:
 a) The acceleration of the object relative to the lab frame, plus
 b) The absolute acceleration of the lab itself, relative to any
  freely-falling frame.  This is how general relativity tells us
  to look at things.

> I am an experimentalist, not a theorist. 

That's a lame excuse.

I'm an experimentalist too, and I've got the scars to prove it.  That
doesn't prevent me from understanding the principles involved.

Despite repeated requests, I have not seen any experimental *or* theoretical
evidence that forces cause accelerations and not vice versa.  If you're
an experimentalist, show me the experiment that proves F causes ma and
not vice versa.  I've seen lots of experiments that support the idea that
F equals ma ... but not F causes ma.