Re: Why is matter scalable on Earth but weightless in space?

Date: Mon Jan 29 11:02:50 2007
Posted By: Bryan Dunne, Instructor, Astronomy, University of Illinois
Area of science: Physics
ID: 1164659684.Ph
Message:
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Lots to sort out here...

1) The "negative pressure" the Supernova Legacy Team talked about is what
we commonly refer to as "dark energy".  The idea is that the vacuum of
space always contains some small amount of energy, even if there are no
particles in that part of space.  Although this energy has never been
observed (it is an exceedingly small amount of energy per volume),
scientists hypothesize that it exists to explain an interesting fact about
the expansion of the Universe - it is accelerating (getting faster with
time).  Without this dark energy, we would expect the expansion to be
slowing down due to the Universe's self-gravity.

So, how does an exceeding small amount of energy per volume accelerate the
Universe?  Well, there's a lot of empty space between the galaxies, so over
vast distances, those bits of dark energy add up!  However, on smaller
scales - planets, stars, individual galaxies, the effect of dark energy is
overwhelmed by the forces that hold us together: the nuclear forces,
electromagnetism, and gravity.

2) There is nothing magical about being inside the Earth's atmosphere that
gives things weight.  Gravity makes an apple fall from a tree and keeps the
Moon orbiting the Earth.  What we feel as weight is actually the reaction
force of the ground (or whatever surface we are in contact with) pushing
upwards against us to counteract the force due to gravity (Newton's 3rd Law).

Think about it this way: 1) The Earth's gravity pulls on us towards the
Earth's center.  2) We exert an equal force on the ground.  3) The ground
pushes back with an equal and opposite force.  That is what we feel as
weight.

So why do the astronauts experience "weightlessness" while in orbit?  Its
not because there is no gravitational force.  The force of the Earth's
gravity on the astronauts is only about 3% less in orbit than it is on the
ground.  The real reason is that they and their spacecraft are in free-fall
together.  There is no relative force between the walls of the spaceship
and the astronauts inside it.  Therefore the astronauts feel as if they
have no weight.

Actually, NASA has an airplane, called the KC-135 Reduced Gravity Aircraft
(also known as the 'Vomit Comet'), that flies in 6 mile-long arcs that
mimic free-fall while descending.  This allows those onboard to experience
brief periods of "weightlessness" (about 25 sec long), while still being
inside the Earth's atmosphere.

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