Re: [Phys-l] [PTSOS] anvil and sledgehammer inertia demo

[John Denker <jsd@av8n.com>]

In the context of air tables and pucks:

On 09/29/2010 11:12 PM, Derek McKenzie wrote:
> > > The vertical forces are balanced.  The horizontal forces are
> > > nearly zero.
>
> Just out of interest, how do you justify the claim that the forces
> are balanced, without chasing your tail?

That's quite a deep question.

The practical answer is that I would not get into that
with an introductory-level student.  The vertical motion
is not interesting, and practically speaking it suffices
to say we are going to focus on the interesting horizontal
motion.

To say the same thing the other way, I would be pleased
but very astonished to see a student start asking questions
about the vertical motion of the puck.

You know and I know that the air table was very cleverly
designed to make the vertical motion be uninteresting.
I could discuss the principles involved, but that would 
be waaay outside the scope of the course.

Perhaps it would be better to not say anything about the
balance of the vertical forces and instead to rely on the
fact that the horizontal motion is _decoupled_ from the
vertical motion.  This decoupling has been known since
Galileo.  In modern terms we understand it as a consequence
of the vector character of the laws of motion.  Linear
independence and all that.