Re: [Phys-L] inertia and the tablecloth demo

[John Denker <jsd@av8n.com>]

On 08/18/2016 04:58 PM, Derek McKenzie wrote:

> The pedagogical issue with this, however, is that whereas everybody 
> knows what you mean by "push/pull", almost no student will know what
> you mean by "inertia". This places you in the somewhat ridiculous
> position of having to explain what the synonym of a thing means when
> you could just as easily explain the thing itself!

Yes, that sums up my opinion on the matter.  I see no need to mention
"inertia" at all, let alone define it or explain it.

By the same token, I see no need to mention "equal and opposite
reaction" or electrical "condensers".  In the very unlikely event
that some student asks about such a thing, I say don't worry about
it.  It's just an archaic way of talking about the third law, or
about capacitors.

The foregoing are rather unusual easy cases, insofar as we have
two names for the same concept.

   In contrast:  More commonly we have nastier situations where
   there are multiple concepts masquerading behind a single name.
   "Heat" is an infamous example.  There are several different
   things it could mean, each of which would make sense on its
   own, but lumping them all together results in chaos.
      https://www.av8n.com/physics/thermo/heat.html

We could do things in the other order, i.e. define mass as we
do now, and then by the way define inertia as equal to mass.
That strikes me as an unnecessary step in the wrong direction,
because not everybody agrees that that's the right definition,
as Rick Tarara and others have pointed out.

On 08/18/2016 08:28 PM, Joseph Bellina wrote:

> It seems to me that in spite of our understanding there is still an 
> element of mystery associated with the way objects continue to move 
> in the absence of a force, so we need a cause

That's true as stated ... but that doesn't mean we should let
the inmates run the asylum.  We know what the misunderstanding
looks like, but that doesn't mean we should spend time on it.
There is tons of research showing that mentioning a misconception
is as likely to reinforce it as to dispel it.

> I'm not saying inertia is really a cause because we know in the
> Newtonian context we do not need one but in our instinctive
> aristotlean framework we think we do so we give it a name inertia. It
> then becomes part of the conversation even though it is meaningless
> and confusion results

In the immortal words of Smith & Dale (circa 1920):
  SMITH: Doctor, it hurts when I do /this/.
  DALE:  Don't do that.

That is to say, the aforementioned confusion seems easily avoidable.

Galileo's great insight was to declare the frictionless situation
to be /normal/, not requiring an explanation.  Such a declaration
is not a physical fact.  The value of the declaration comes from 
convenience, i.e. from the power and simplicity of the resulting
physical laws.

Incoming students have no rational basis for preferring Galileo's
approach to any other approach.

I tell students:
   I want you to think for yourselves.  I don't want you to take
   something as a fact just because I said so, or because Galileo
   said so.  However sometimes I do ask you to trust me enough to
   accept something as a temporary /hypothesis/, so that we can
   explore its consequences.

   The hypothesis for today says that Galileo was right.  Let's see
   where this takes us.  Then you decide for yourselves whether it
   leads to a nice consistent picture of how the universe works.

Yes, it is possible to argue about the "cause" of continued motion.
However, Galileo rejected all such arguments as being unhelpful
and unnecessary.  This marked Day One of modern science, namely
the separation of physics from metaphysics and philosophy.

On 08/18/2016 08:42 PM, Anthony Lapinski wrote:

> As my college physics teacher often said, Physics explains how, not
> why.

Exactly so.  The teacher was channeling Galileo.
   https://www.av8n.com/physics/causation.htm

Newton went to school on Galileo, and summed up this point by
saying Hypotheses non fingo.

In summary:  Two wrongs don't make a right.  IMHO mentioning
"inertia" as the "cause" of continued motion is two steps in
the wrong direction.