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From: cjh@petsd.UUCP (Chris Henrich)
Newsgroups: net.physics
Subject: Re: Bottled birds?
Message-ID: <629@petsd.UUCP>
Date: Tue, 27-Aug-85 18:14:55 EDT
Article-I.D.: petsd.629
Posted: Tue Aug 27 18:14:55 1985
Date-Received: Fri, 30-Aug-85 07:23:16 EDT
References: <457@petfe.UUCP>
Reply-To: cjh@petsd.UUCP (PUT YOUR NAME HERE)
Organization: Perkin-Elmer DSG, Tinton Falls, N.J.
Lines: 42
Summary: Letting the bird out makes a difference

[]
In article <457@petfe.UUCP> bobp@petfe.UUCP (Bob Philhower) writes:
>Given:  A bottle, (of arbitrary size), containing one (1) bird 
>(of arbitrary species).  The bottle is placed on a garden-variety
>bathroom scale (arbitrarily accurate), and the weight of the system
>is measured with the bird sitting on the bottom of the *closed*
>bottle.  Now the bird decides to fly around inside the bottle, and
>the weight is read again.  Any weight difference?
>
>Now, what if the bottle cap (massless, of course) is removed; again,
>any weight difference?  What happens as the bird flies to the top of,
>and eventually out of, the bottle?

	While the bird is flying around in the bottle, it has
to move its wings in such a way that there is more pressure on
their bottom surfaces than on their tops.  A side effect is
that air currents are created which cause increased pressure
on the bottom of the bottle.  Thus the bird's weight affects
the scale exactly as if it was sitting on the bottom of the
bottle.  
	When the air inside the bottle is no longer isolated
from the air outside, the forces do not necessarily balance.
It seems likely to me, however, that little change or none
will be caused by just opening the top of the bottle.  
	When the bird flies out of the bottle, the downward
push that it exerts on the air will not register on the scale,
but rather will act on the floor or other surroundings.
So the weight reading decreases.
	This is not exactly intuitive, because the pattern of
forces is complex.  But if you *really* *believe* that
Newton's third law applies to each little piece of the complex
pattern, then you can expect it to apply to the aggregate of
them.

Regards,
Chris

--
Full-Name:  Christopher J. Henrich
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