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Re: Weightlessness



Actually, I think Marilyn's initial response addresses the question raised;
it is her additional explanations that confuse things.

The question is:
Earth's gravity holds spaceships in orbit, but the things inside them are
weightless and float around. Why doesn't gravity have an effect inside?

Marilyn's first sentence is:
Actually, it does<-the same way it affects the spacecraft itself.

The questioner acknowledges that gravity "holds" the spaceships in orbit.
The question is why things inside float around if, as one would expect, they
are also under the influence of gravity. The answer has to do with
perspective. The things inside are also in orbit and would be in orbit
whether the spaceship is present or not. In other words, the things inside
"float" in the same orbit as the spaceship. It is only from the perspective
of someone on the spaceship that it appears as though gravity has been
"turned off".

Now, if you want to explain why gravity is needed to "hold" the spaceship in
orbit...well, that is another question (which JC answers).

----------------------------------------------------------
| Robert Cohen Department of Physics |
| East Stroudsburg University |
| rcohen@po-box.esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
----------------------------------------------------------

-----Original Message-----
From: John Clement [mailto:clement@HAL-PC.ORG]
Sent: Sunday, May 20, 2001 4:28 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Weightlessness


Unfortunately this is typical of many "popular" explanations of physical
concepts. I have heard astronauts at NASA giving similar explanations so
this is not at all unusual. A better explanation is often difficult to make
within a few words, without causing more misconceptions in the reader. I
will attempt one:

[snip]



The letter and answer below came from Marilyn vos Savant's column
in today's
Parade Magazine. I thought some of you might find Marilyn's answer as
edifying as I do.


Earth's gravity holds spaceships in orbit, but the things inside them are
weightless and float around. Why doesn't gravity have an effect inside?
<-Jerry Mapes, city unknown

Actually, it does<-the same way it affects the spacecraft itself. When a
space shuttle is orbiting the Earth, the sum of the ³downward"
(gravitational) force and the "forward'' (inertial) force of the
moving ship
and its contents nearly equals zero. So both the ship and its contents are
in freefall, which makes everything weightless. They stay in orbit while
"falling'' (being pulled toward the Earth) because the inertial force
(centrifugal force, in this case) of the moving vehicle is
radial<-away from
the Earth. A similar principle applies to the planets: All are in glorious
free-fall while orbiting within the gravitational field of the Sun.