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



On 11/28/2010 08:34 PM, LaMontagne, Bob wrote:

When you push an object with your finger there is a long history of
thoughts and actions that culminate with you placing your finger on
the object and an acceleration of that object.

So, it all comes down to history and intent, then ... my intent,
when I place my finger on the object. So let me ask some partly-
rhetorical questions:

1) What if I *intended* to subject the particle to a definite
acceleration (and then measure the force)? That's easy to do,
operationally. Would I then need a new equation of motion,
perhaps ma=F?

2) How does the particle know what I intended?

3) In practice, how do I account for history and intent? What
is the equation? Do I divide by history and multiply by intent?

4) What experiment could I do to distinguish between F=ma and ma=F?

These are partly rhetorical, in the sense that I do not expect
answers. I've been asking the same questions for years without
any meaningful answers. Meanwhile the questions are partly non-
rhetorical, in the sense that I'm trying to keep an open mind
about this, and if anybody has any actual objective evidence I'm
willing to listen.

I think a poll of most people would result in an interpretation of
the word "caused" in this case to be that the finger caused the
object to move

Oh, please. Are you really going to establish the fundamental
laws of physics based on a poll? Maybe you could save some money
by using the same poll to settle the question of evolution. You
might as well ask about climate change while you're at it.


On 11/28/2010 07:26 PM, ludwik kowalski wrote:
It is often stated that physicists build models of reality. A model
which states that acceleration is always caused by a net force
seems to be very useful. That is what Sarma would say about the
a=F/m. The mass of an object, in a classical model, does not depend
on speed. But the m must known to calculate the a(t) when a
particular F(t) is given. Do I interpret him correctly?

Yes, a mathematician can write the second law as F=m*a, or m=F/a.
That does not contradict the useful model of a physicist--in order
to accelerate an object one needs a net force. Causality is part
of the model, it is not part of reality.

Yes, I know that more general models can be, and have been,
created. All models have limited validity. The simplest model is
appropriate in teaching introductory physics courses.

My head is spinning.

The *simplest* thing is to say nothing about cause and effect.

If a student brings up the issue, you can quote Galileo and
Newton. The laws of physics say what happens. They may or may
not say how it happens. The fundamental laws rarely if ever
say /why/ it happens. That's what sets physics apart from
metaphysics, philosophy, and shamanism.

Yes, a mathematician can write the second law as F=m*a, or m=F/a.

Actually a mathematician would not write that, unless we are
guaranteed that a is nonzero. More to the point would be
writing ma=F as a synonym for F=ma.

What are you saying here? Are you saying that a mathematician
would write that, but you would not? Why not? It seems you
perfectly well know that ma=F and F=ma, so why not just say so?

It's true that a mathematician would write ma=F and interpret it
as being absolutely synonymous with F=ma. A mathematician would
write that, and so would any sixth-grader! It is taught in 6th
grade (and sometimes earlier) that equality is reflexive, symmetric,
and transitive.

That is both simple and true. Are you really going to spend your
class-time to teach the kids a new notion of equality that is much
less simple and much less true? Something you admittedly know is
"not part of reality"? Now it's my turn to ask "Why" .... Why
would you want to do such a thing?

Talking about cause and effect in this way sets science back
400 years.

Learning about more general models, in advanced classes, does not
mean "unlearning of what has already been learnt."

What makes you say that? In this case, the next class will very
much revolve around unlearning everything you have said about
cause and effect and its relation to F=ma.

On behalf of the folks who have to teach the next class, I beg
you: It would be much better to say nothing than to say that
F causes ma.

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

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.
– G. Galilei (op cit)

I have not as yet been able to discover the reason for these
properties of gravity from phenomena, and I do not feign hypotheses.
For whatever is not deduced from the phenomena must be called a
hypothesis; and hypotheses, whether metaphysical or physical, or
based on occult qualities, or mechanical, have no place in
experimental philosophy.
– I. Newton (op cit)