Re: [Phys-L] causality

[John Denker <jsd@av8n.com>]

On 2/7/19 8:25 PM, Jeffrey Schnick wrote:
> I've been thinking about this example (Newton's 2nd Law) and other
> continuity equations.  I see Newton's second law as a continuity
> equation which I see as being synonymous with a conservation law.
> For some system, say a brick, the force on the brick is the rate at
> which momentum is flowing into the brick and the rate of change of
> momentum of the brick is what we often represent as the mass times
> the acceleration of the brick.  The total momentum of the brick is
> changing right there at the boundary where the incoming momentum is
> crossing the boundary.  Any change in the total amount of momentum
> inside the boundary (between the closed surface of the brick and the
> rest of the universe) is occurring at the exact same instant as the
> momentum crosses the boundary.  I can't see any cause and effect when
> the two things occur simultaneously.  Is the momentum of the brick
> changing because momentum is flowing across the boundary or do we
> judge there to be momentum flowing across the boundary because it is
> decreasing at the boundary on one side of the boundary and increasing
> at the boundary at the other side of the boundary.  Epstein, in his
> book, Relativity Visualized, refers to the statement that force
> causes acceleration as (something like) the great myth of the physics
> community.  

I agree with all that ... except that I would have identified
Newton's *third* law as the one that expresses conservation of
momentum.

This is a big deal IMHO.  Several of the FCI questions that students
find most troublesome become absolutely trivial when rephrased in
terms of conservation of momentum.  Is the momentum transferred from
the large truck to the small car the same as the momentum transferred
to the small car from the large truck?  Duh!

> I confess to using the cause and effect language but I
> don't really think force causes acceleration.

I used to be in the same boat, but I've been training myself
to say the force is "associated" with the acceleration and
vice versa.  A few years ago I went on a rampage and revised
a great many of my web pages to eradicate words that could
be interpreted in terms of causation when that wasn't what
was intended.

I thought the eradication would be difficult, but it really
wasn't.  There are plenty of ways of saying what we mean
without saying things we don't mean.

One problem is that computer languages stab us in the back.
In (say) c++ or even BASIC, a=b means something very different
from b=a.

  It doesn't have to be that way.  ALGOL and Pascal use := as
  the assignment operator.  That's much more logical.