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

[John Denker <jsd@av8n.com>]

On 08/17/2016 01:54 PM, Richard Tarara wrote:

> inertia is part of our language and seemingly ingrained in our 
> physics traditions.

I had what might be called a non-traditional education.  I never ran
across "inertia" in a physics context until long after I was out of
school.

The Feynman lectures use the idiomatic expression "the principle of
inertia" on two occasions to refer to the first law, and otherwise
don't mention inertia at all.  Certainly there is no attempt to
quantify the inertia the way we quantify mass or momentum.

So this is some kind of existence proof, showing that one can get
along just fine without attempting to use (or even define) inertia.

On the other edge of the same sword, I know of one recent textbook
that attempts to define inertia and gets it spectacularly wrong,
defining it in a way that is inconsistent with Einstein's principle
of equivalence, and incompatible with all Eötvös experiments.

All in all, my respect for the "traditional" approach to inertia
is at a low ebb.

The rotational "moment of inertia" is a whole different topic.

> the physics uses of the word inertia resonate to a large extent with 
> our common use of the term can be used to advantage (or not) with 
> students.

In my experience, incoming students have little if any notion of
what inertia is.  I would not rely on it as a starting point.

They don't know about mass or momentum either.  I prefer to teach
two things (i.e. mass and momentum), not three, not mentioning
inertia at all.

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

> in the relativistic realm though when momentum seemingly can't be
> separated into mass and velocity,

Actually it can.  The crucial equations are:

	p = m u
	u = dx / dτ
where
  p is the spacetime momentum,
  u is the spacetime velocity,
  m is the mass, i.e. the invariant mass, and
  τ is the proper time.

It is hard to imagine anything simpler than that.  Super simple.
Super useful.  Independent of whatever reference frame (if any)
you choose.  Obviously upholds the correspondence principle in
the low-speed limit.

The spacetime velocity u is not to be confused with the /reduced/
velocity
	v = dx / dt
where t is the projection of τ onto some reference frame of
your choosing.

Physics (including relativity) is simple from the spacetime point
of view.  If special relativity seems weird or paradoxical, you're
doing it wrong.  Special relativity is the geometry and trigonometry
of spacetime, nothing more, nothing less.
  https://www.av8n.com/physics/spacetime-welcome.htm