Re: [Phys-l] definitions ... purely operational, or not

["LaMontagne, Bob" <RLAMONT@providence.edu>]

Are you seriously suggesting that gravity stops when leaving the earth's atmosphere? Have some of my students been right all along?

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker [jsd@av8n.com]
Sent: Wednesday, November 10, 2010 5:09 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] definitions ... purely operational, or not

On 11/09/2010 08:10 PM, LaMontagne, Bob wrote:
> But there definitely is gravity! - otherwise the space shuttle would
> drift off tangentially - that is what we are trying to get students
> to understand - just no local free fall acceleration.

AFAICT the phrase "free-fall acceleration" is synonymous with
"gravity".  Therefore:
 -- If you are trying to say there is gravity but no local
  gravity, there are much clearer ways of saying it.
 -- If you are trying to draw the distinction between gravity
  and free-fall acceleration, I have no idea what you mean ...
  and I doubt any other folks (including students) do either.

Also:  As for the shuttle not flying off tangentially, students
are aware that it doesn't fly off tangentially, but they are also
aware that the astronauts are weightless to first order in the
frame of the shuttle.  Merely reminding them of one well-known
fact or the other does not /explain/ the facts.  The goal is to
get them to understand both facts at the same, to understand how
the facts tie together with each other and with everything else
we know.

Constructive suggestion:  To explain how straight lines that are
initially parallel can converge toward each other, refer to the
lines of longitude (or other great circles) on a globe.  Also
  http://www.av8n.com/physics/geodesics.htm#fig-darts

=========

In the context of:
> > >  By the way, the folks at NASA and astronauts
> > > increasingly use the term "microgravity" in an effort to clear this up.

On 11/09/2010 07:52 PM, Hugh Haskell wrote:

> > And that, IMO is the worst offense of all.

Why is it offensive to tell the truth?

In the frame comoving with the space station, the gravity is zero
to first order and nonzero to second order.  When an otherwise-large
first-order term is absent and all that remains is a much smaller
second-order term, saying "micro" is an improvement over saying
"zero".
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