Re: [Phys-L] causation

[John Denker <jsd@av8n.com>]

On 2/9/19 12:03 PM, Alex. F. Burr via Phys-l wrote:

> From the point of view of non-professional physics, I can apply a 
> force and get an acceleration.

OK.

> I cannot apply an acceleration and get a force.

Actually you can do that, too.

In an ordinary centrifuge, you control the rotation rate
and therefore control the acceleration.  Given the mass
and the acceleration, you can infer the force.

Most of the time, in the context of the basic laws of
mechanics, when a student (or anybody else) asks
"why xxx" the best response is not "because yyy".
Instead, it is better to interpret it as "how do we
know xxx" or "how could we have predicted xxx".

Galileo famously divorced physics from philosophy:

> > 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.

That is considered Day One of modern science.  Newton, having
gone to school on Galileo, made the same point more tersely.
When asked /why/ gravity behaved as it did, he said

> > "Hypotheses non fingo."

To repeat:
 ++ "How can we predict the trajectory" is a scientific question.
 -- "Why does it move" is not.

===============

On 2/9/19 8:09 AM, Steve Spicklemire via Phys-l wrote:

> If I throw a ball in the air, I feel it’s reasonable to say that
> something “causes” it to come down.
Most of the time when I throw a ball, it goes /up/ for a while
before it comes down.  What "causes" it to go up?

Also, consider a pendulum.  Neglect damping for a moment.  It
goes down, and then it goes up again, then down again ........
Similarly, consider planetary orbits.  They are a well-known
example with very low damping.

I vote with Galileo on this one.
  The basic laws of mechanics say what happens.
  They may or may not say how it happens.
  The almost never say why it happens.

===============

Very commonly, this is taught wrongly in science classes, at
every level from third grade on up.  In accordance with government
standards, students are taught to say a ball rolls downhill
"because" the energy is lower at the bottom.  This is BS.  In
the absence of friction, the ball will roll downhill and then
roll up the hill on the other side of the valley.  If you want
it to roll down and /stay/ down, you need friction, and you
need to explain spontaneity, irreversibility, equilibrium, etc.
in terms of /entropy/ not energy.  (You can construct situations
where the energy of one thing is a proxy for the entropy of
something else, but this is definitely not the general case.)