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From: rdp@teddy.UUCP
Newsgroups: net.physics
Subject: Re: Bottled birds?
Message-ID: <1217@teddy.UUCP>
Date: Wed, 28-Aug-85 15:57:45 EDT
Article-I.D.: teddy.1217
Posted: Wed Aug 28 15:57:45 1985
Date-Received: Sat, 31-Aug-85 05:03:18 EDT
References: <457@petfe.UUCP>
Reply-To: rdp@teddy.UUCP (Richard D. Pierce)
Organization: GenRad, Inc., Concord, Mass.
Lines: 40
Summary: 

In article <457@petfe.UUCP> bobp@petfe.UUCP (Bob Philhower) writes:
><<>>
>OK, all you bird-lovers (P?); anyone want to comment on this situation?
>
>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?
>

A similar puzzler was given in an old Scientific American (maybe 20
years ago). Seems there was this guy with a panel truck filled with
pigeons. He comes to a rickety old bridge, gets out, and starts
banging on the side of the truck. A local farmer is watching this
spectacle and asks the driver why he is doing this. The driver responds
"Because I don't think the bridge will support both my truck and the
birds, so I want to get the birds flying around while I drive over".

The question is, will the birds flying around make any difference. The answer
is, in an airtight, or nearly airtight container (truck, bottle, etc.) there
will be no difference. Why? because the birds exert a downwards force equal
to their mass times the acceleration of gravity. This force is the same
whether they are sitting or flying. In the former case, that force is
counteracted by the bed of the truck(or the bottom of the bottle). In the
latter case, the counter-acting force is supplied by the air, and in turn, by
the bed of the truck (etc.). There will be no difference in average
weight (I say AVERAGE, because there may be slight differences when the birds
move up or down).

>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?
>
>We've tried this, but after squeezing a robin through the bottle neck,
>it doesn't seem to want to fly.
>

Try bees! Or mayonnaise jars for the bird!