snip
I'm sure there are lots more references with accurate physics. Hey,
maybe even *NASA* has an accurate webpage on this. I'll check there
and let you know if I find something good.


~ CT

I've found a NASA webpage that comes very close to being accurate:

http://science.nasa.gov/newhome/head...ro-g.plane.htm

Quote:

"What astronauts experience in space isn't really zero-gravity. NASA
scientists call it microgravity or low-g, but it's really free fall or
weightlessness."

I see this quote above to be totally accurate. But later in the page,
there is this fatal error:

"NASA scientists call this microgravity... The term is apt since
Albert Einstein said that acceleration caused by gravity is equivalent
to any other push."

The principle is about _mass_ equivalence, not acceleration
equivalence.

Say that you spin a ball at the end of a tether. It is completely
bogus (vice "apt") to say that you are *increasing gravity* on that
ball. Just the same, it is bogus to say that you are *decreasing
gravity* on a ball that you drop from a tower. There is nothing
"micro" about gravity in a freefall toward the Earth.


~ CT



Full article:
__________________________________________________

Temporary weightlessness

Engineers and scientists experience about 20 to 30 seconds of
weightlessness during each parabola aboard NASA's KC-135 aircraft, an
effective and inexpensive means of testing experiments before they go
to space. Because everything floats, test equipment must be bolted or
taped to the deck, as with the apparatus here for testing liquid
cages.

What astronauts experience in space isn't really zero-gravity. NASA
scientists call it microgravity or low-g, but it's really free fall or
weightlessness.

Gravity goes to the edges of the universe -- it's why planets circle
the sun, stars clump together to form galaxies, and Space Shuttles
stay in orbit.

So what is happening on a spacecraft or when Kornfeld and Antar run
experiments on the KC-135 (as seen at top)?

As a spacecraft orbits a planet, it really falls endlessly in a circle
(or ellipse) that is a delicate balance between the satellite's
forward motion and the planet's gravitational pull. Because everything
is falling together, nothing has weight.

Well, almost no weight. Unless an object is at the precise center of a
satellite's mass, it will try to pull ahead or fall back into a slight
different orbit. And that means that the object will experience a
small amount of acceleration against a wall. And even at the Shuttle's
altitude, a trace of atmosphere is left and gently drags on the
Shuttle which will cause an object to drift inside the Shuttle.

NASA scientists call this microgravity since usually it is equivalent
to about 1/1,000th or less of one Earth gravity (the range depends on
the location in the spacecraft and other factors). The term is apt
since Albert Einstein said that acceleration caused by gravity is
equivalent to any other push.

Free fall can be duplicated, briefly, on Earth, by dropping an object.
Like falling off a cliff, it's not the first step that gets you, or
the long trip down, but the stop at the end.

NASA has drop tubes in which molten droplets of material fall for
about 2 to 3 seconds before hitting a bucket of oil to capture them
safely and cool them off.

For larger experiments, or to train astronauts, NASA uses a KC-135, a
military tanker version of the Boeing 707 jetliner. The pilots guide
these jets on carefully designed parabolic trajectories that resemble
a roller coaster ride.

At the top, the pilot throttles back and noses over, letting the plane
dive to give everyone about 20 to 30 seconds of free fall (actually,
it varies between 0.01 g to 0.001 g; it's not nearly as good or as
long as being in orbit). They do this 40 times on each mission, so
they get about 13 minutes of microgravity time -- at a personal price.

People riding the NASA KC-135 often get extremely sick doing this.
That's why the plane is also called The Vomit Comet.

The things you do for science!

__________________