Re: Flying birds

Date:         24 May 99 01:52:35 
From:         Steve Lacker <no@spam.thanks>
Organization: Applied Research Laboratories - The University of Texas at Austin
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John Gunnar Enebak wrote:
> You fill a cargo plane with 20 tons of birds, and fly 500 knots at
> 10.000 feet with the birds sitting on the floor of the plane.
> Suddenly all the birds start flying around inside the plane. Will the
> plane then loose 20 tons of weight and start a climb?

What a funny mental image.... ;-)

The answer is "no".

The plane, the air it holds, and the birds within constitute a "closed"
system. The amount of mass within the closed system is constant, save
for the fuel being burned and ejected out the back of the engines (and
yes the plane DOES climb as the fuel burns off...)  When the birds start
flying, no mass is added to or removed from the system, therefore its
weight is unchanged.

Put another way (and TREMENDOUSLY over-simplified)- when the birds are
sitting still, their weight is transferred to the plane through their
feet. When they start flying, their wings exert a downward force on the
air under their wings, causing the air to move downward. The downward
moving air will at some point strike the fueslage, and exert a force on
the fuselage exactly equivalent to the weight of the bird.

-- Stephen G. Lacker
slacker at arlut dot utexas dot  edu
sglacker at texas dot net