Sitting on Eggs - No Breaks!
Name: Jaime W.
Date: June 2002
How does a bird sit on an egg and not break it? Now when I asked my
teacher for some hints he said the shape of the egg is a big factor but I
have been unable to find that information anywhere.
Birds do not weigh very much. Have you ever tried to break a chicken egg by
crushing it in your fist? It is surprisingly difficult. If the egg shell
is not cracked, it can withstand quite a bit of force.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
I have just written a manuscript entitled: "The Geometry of Avian Eggs"
which addresses the very question of the shapes of eggs. I hope to have it
published, since very little has been written on the subject, surprisingly.
My study was motivated by a Newton BBS question some months back.
For the purpose of your question you can make the following approximate
computation. Assume the egg is an oblate oval or revolution, about 5 cm
long and 4 cm wide. From the formula for the surface area of an oblate oval
or revolution you can calculate the surface area (your teacher will be able
to help you find the formula in a handbook, or ask your math teacher). The
bird weighs maybe a couple of pounds -- at least for that size egg -- and
assume only a single egg (there are probably more, but a single egg will
provide an upper limit. Assume 1/2 the weight of the bird is supported by
the egg and the other 1/2 is supported by the nest itself.
I have not done the calculation but I think you will find that the pressure
(lbs/ in^2) on the egg will be quite small, even if you do not take into
account that the mother hen may not just "sit" on the egg but supports her
own weight partially with her legs.
Should you want a more detailed analysis, you would have to take into
account that a rounded shape like an egg, or a suspension bridge across a
river distributes the weight in such a way that it is able to support a much
larger load than say a bar or a single pillar or column. This is a much more
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Update: June 2012