...as originally posted to, and subsequently removed from, the
Wikipedia article on Weightlessness:
===========
Criticism of the terms "Zero Gravity" and "Microgravity"
It is important to note, as stated at the beginning of this article,
that there is plenty of gravity pulling on a spacecraft in orbit around
the Earth. Gravity is the very reason why the spacecraft is orbiting. Therefore it is totally inaccurate to say that astronauts are
experiencing "zero gravity" or "microgravity". What orbiting astronauts experience is zero-g, a measure of acceleration relative to their
spacecraft, which results in weightlessness. But zero-g is not "zero gravity". If there were "zero gravity" or "microgravity", their
spacecraft would not be pulled into an orbit around the Earth. It would
go in a straight line.
As a thought experiment, imagine a spacecraft that had the ability to
rise up to orbital altitude by going straight up like a helicopter and
hover over one spot on the Earth. The astronauts inside would not
experience weightlessness. Their ride inside this hovering spacecraft
would be similar to riding an elevator up an incredibly tall building
and stopping at the top floor. While hovering above Earth's atmosphere,
their weight would be very close to what they weigh on the surface of
the Earth, even as a space shuttle goes zinging by them. So astronauts
in a hovering spacecraft are being pulled by strong gravity just as
space shuttle astronauts are pulled by strong gravity. The difference
between them is that the orbiting shuttle is freely being pulled toward
the center of the Earth. The lack of relative acceleration between the orbiting shuttle and its astronauts inside (who are also being freely
pulled toward the center of the Earth) result in them being weightless.
But the hovering spacecraft (as with an elevator at the top of an
incredibly tall building) is not freely falling. The pull of gravity it
is experiencing is being opposed by the hovering force. This force gets transfered to the astronauts within (along with everything else within
the spacecraft) resulting in weight. This example illustrates the fact
that there is plenty of gravity out in space. If you were to take any
object that is orbiting the Earth and stop it dead in its track and
then release it, the Earth's gravity would pull it straight down back
toward the Earth's surface.
To use confused terms like "zero gravity" and "microgravity" is to
mistake the general concept of acceleration for the concept of gravity. "Zero-g" and "micro-g" are perfectly accurate terms referring to the
lack of acceleration (in the frame of reference of the spacecraft) that
cause weightlessness, even while gravity is strongly pulling the
trajectory of that spacecraft into an orbit.
The specific point of confusion is that "g" does not mean "gravity".
The designator "g" is an arbitrary scale of acceleration not to be
confused with gravity itself. "Zero-g" means zero acceleration, not
zero gravity. "1-g" is the acceleration experienced on the surface of
the Earth due to gravity, but it is not gravity itself. This scale is
widely used because it is easy to relate to from common experience of acceleration due to gravity. But any other scale of acceleration can be
used to describe the condition of weightlessness. It could be described
using a scale that has nothing to do with Earth's gravity. Similarly, a distance can be measured in feet as well as meters, where a meter has
nothing to do with the length of a human foot. For a weightless
astronaut to say that they are in zero gravity is the same type of
error as saying that an object that has a length of 0.3048 meters is identically one human foot in flesh and blood. "1 foot" is an arbitrary
scale for measuring length that was (at some point in history) based on
a person's foot, but not to be confused with an actual human foot.
"1-g" is an arbitrary scale for measuring acceleration that is based on gravity, but not to be confused with actual gravity. A zero-g
environment is also a zero-meters/second^2 environment and a zero-feet/second^2 environment. Any arbitrary scale of acceleration can
be used, and none of them have any exclusive relationship to gravity.
Another illustration of this type of mistake is when people erroneously
speak of a jet pilot blacking out as a result of "gravity-induced Loss
Of Consciousness". The proper term is g-induced Loss Of Consciousness.
It is the acceleration produced by their maneuvers that is the culprit
for g-LOC. It is clearly "g-induced" and not "gravity-induced", because gravity obviously remained constant at 1-g the whole time for the
pilot. Likewise, the purpose of NASA's " Reduced Gravity Aircraft" is
not to reduce gravity, but rather to fly in a parabolic arc that brings relative acceleration to zero. "g" is reduced while gravity stays
essentially the same. So clearly it is possible to experience zero-g
without going into space. Any aircraft can do this by pushing it over
into a parabolic arc. Even any car that hits a bump fast enough to
leave the ground will experience zero-g for the time that the wheels
are not in contact with the road. The easiest way to experience zero-g
is to bend your legs and jump off the ground. For the time that you are
in the air, you are experiencing weightlessness. The difference with astronauts is that the experience is not momentary because their
spacecraft is continually getting pulled toward the Earth. It is
possible for non-astronauts to experience longer durations of
weightlessness by cliff diving, bungee jumping, freefall parachuting, barrelling over a waterfalls or more safely by riding many types of
modern amusement park rides that put the occupant in a freefall. What
is common for the astronauts as well as these other examples is that it
is not gravity that is changing, but rather the acceleration in their
falling frame of reference goes to zero-g.
As it stands today, NASA itself is one of the biggest promoters of the erroneous terms "zero gravity" and "microgravity" (along with the
similarly erroneous term "reduced gravity"). Astronauts themselves have
been quoted as having experienced "no gravity" while in space. Surely
they are aware that there was plenty of gravity throughout every orbit
they made, with gravity being the very thing that pulled them into an
orbit.
===========
Here is the version of the Wikipedia section as subsequently tweaked by
other editors: http://en.wikipedia.org/w/index.php?title=Weightlessness&diff=prev&oldid=93613581
~ CT
As it stands today, NASA itself is one of the biggest promoters of the erroneous terms "zero gravity" and "microgravity" (along with the
similarly erroneous term "reduced gravity"). Astronauts themselves have been quoted as having experienced "no gravity" while in space. Surely
they are aware that there was plenty of gravity throughout every orbit
they made, with gravity being the very thing that pulled them into an orbit.
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