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Re: [Phys-l] Weightless



On 11/20/2006 03:38 PM, Richard Tarara wrote:
I don't interpret what I feel in a jet aircraft accelerating forwards as
weight as John seems to imply below. I feel pushed back into my chair.

That is a worthwhile line of inquiry ... but we need to take one
more step down that road.

If you are in an airplane with your eyes closed, starting in
ordinary straight-and-level flight, I very much doubt whether
you can tell the difference between:
-- accelerating straight ahead (at constant pitch attitude), and
-- pitching up and climbing (at constant airspeed).

In either case, the result is a component of g that "pushes" you
back into your seat.

The two cases are different in principle; for instance, there is
a second-order difference in the magnitude of g in the two cases.
But in practice, people are sensitive to the first-order change
in the direction of g and not sensitive to the second-order change
in the magnitude of g.

I'm talking about g as defined relative to a frame comoving with
the aircraft, which is appropriate since you're not supposed to
be looking outside. As always I define g in terms of a freely
falling test particle moving relative to the chosen frame.
http://www.av8n.com/physics/weight.htm

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

I find it useful to define the terms
-- N-down (Newtonian down, directed toward the center of the
earth), and
-- E-down (Einsteinian down, as defined by a freely-falling
particle).


In aviation, the equivalence principle is not some hoity-toity
abstraction. It is a fact of everyday life. And death. Over
the years a lot of people have bet their life that they could
tell which way was N-down ... and lost.

As one example among many, a particularly infamous example:
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001212X19354&key=1

When pilots don't have sufficient outside visual references, they
fly by reference to instruments: gyroscopic instruments.

The astute reader will be wondering at this point, how does the
gyroscope in the artificial-horizon instrument know which way
is N-down? The equivalence principle says that the centrifugal
field is locally indistinguishable from the gravitational field,
so how does the gyroscope figure out the difference? Is this
instrument somehow exempt from the laws of physics???

The answer is that the instrument makes a _non-local_ measurement!
It spends about 5 minutes figuring it out, and then it remembers
the result. It's really quite ingenious.
http://www.av8n.com/how/htm/motion.html#sec-attitude-indicator

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


On 11/20/2006 01:26 PM, John Mallinckrodt wrote:

Standard disclaimer: I'm not promoting the idea that we teach
students general relativity.

Agreed.

I do not expect student pilots to read Misner/Thorne/Wheeler.

I just think there's room within our
professional ranks for a little more appreciation of the deep and
beautiful message conveyed by the principle of equivalence.

I agree with that as far as it goes, but I would go much farther.
I require student pilots to understand the equivalence principle,
at least to the extent that they understand that they /cannot/
perceive the difference between N-down and E-down.
a) It's not that tricky, and
b) their lives depend on it.