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From: tan@ihlpg.UUCP (Bill Tanenbaum)
Newsgroups: net.puzzle
Subject: Re: someone asked why weightless 300 miles up in orbit even though gravity is the same (isn't weight still mass * gravity_accelaration?)
Message-ID: <1496@ihlpg.UUCP>
Date: Tue, 24-Dec-85 00:34:42 EST
Article-I.D.: ration?)
Posted: Tue Dec 24 00:34:42 1985
Date-Received: Wed, 25-Dec-85 02:02:02 EST
References: <96@decwrl.DEC.COM>
Organization: AT&T Bell Laboratories
Lines: 25

> [Eric]
> I believe the key to the answer is the word "orbit".  If you're in orbit,
> you're whizzing around the earth just fast enough so that your centrifical
> force OUTWARD matches gravity's force INWARD.
> 
> Hence the total acceleration for you is zero, and hence you are indeed
> weightless.
----------
	Wrong.  Zero acceleration and weightlessness are not the same thing!
The only force on you is gravity [INWARD].  Your acceleration is not zero.
That is why your trajectory is an ellipse and not a straight line.
You are "weightless" because you are in free fall, i.e., not restrained by
non-gravitational forces, just like you would be in the free falling elevator
or airplane.
	You have "weight" on earth only because the non-gravitational
contact force of the earth's surface prevents you from falling freely towards
the center of the earth.  Your total acceleration is zero, but you are NOT
weightless.
	In other words, when you are in free fall, you are accelerating due
to the local gravitational field.  You are weightless because in YOUR OWN
REFERENCE FRAME (not the fixed frame of the earth) there are no gravitational
forces on you.  When you are held back from free fall, as by the surface of
the Earth, the gravitational force manifests itself as "weight".
-- 
Bill Tanenbaum - AT&T Bell Labs - Naperville IL  ihnp4!ihlpg!tan