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From: dgary@ecsvax.UUCP (D Gary Grady)
Newsgroups: net.puzzle
Subject: Re: someone asked why weightless ...
Message-ID: <970@ecsvax.UUCP>
Date: Fri, 20-Dec-85 13:05:53 EST
Article-I.D.: ecsvax.970
Posted: Fri Dec 20 13:05:53 1985
Date-Received: Sun, 22-Dec-85 01:29:49 EST
References: <96@decwrl.DEC.COM>
Reply-To: dgary@ecsvax.UUCP (D Gary Grady)
Organization: Duke U Comp Ctr
Lines: 41

I recently posted a puzzlement:

If you're in the space shuttle orbiting 300 miles up, the acceleration
of gravity is very little less than it is at the surface.  If weight =
mass X acceleration, how can you be "weightless"?  As I noted, the
answer to this requires a somewhat more subtle definition than you
normally see in general physics texts.

In article <96@decwrl.DEC.COM> osman@sprite.DEC (Eric, DIGITAL, Burlington Ma.  617 273-7484) writes:
>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.

Centrifugal "force" is a fictitious force (measured in figmentive
newtons, normally called "fig newtons," but that's beside the point).

>Hence the total acceleration for you is zero, and hence you are indeed
>weightless.

But you must be accelerated, otherwise you would go in a straight line!

>p.s.	Does someone remember the details of the "thought" experiment
>	involving being in a windowless elevator, and trying to determine
>	whether	you are accelerating or just plain heavy ?

Well, the result of that thought experiment is called the General Theory
of Relativity which starts by saying (speaking loosely) that you can't
tell a difference because both possibilities amount to the same thing.

Anyway, I'm glad you brought up the elevator (don't say a word,
Henry).  If you're in a falling elevator you're weightless too, aren't
you?  But the gravitational field is still the same, isn't it?

Let me offer a hint:  We need a better definition of weight, and
possibly a better notion of acceleration, to solve the problem.  But no
deep dark physics is needed, just careful thought.
-- 
D Gary Grady
Duke U Comp Center, Durham, NC  27706
(919) 684-3695
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