Part of the show Dinosaurs and Fossils - Jurassic Science set in Stone
What you Need
What to do
Hard boil an egg
Lay in on its side on the surface
Spin the egg as fast as you can.
Does anything strange happen?
Try spinning the egg starting off at different angles.
What may happen
If you spin the egg fast enough you will find that it stands up on end. Which end it stands on will depend on the angle you started spinning the egg
Why does it happen?
When you spin the egg you nearly always spin it at a slight angle. It is virtually impossible not to.
As the egg will try to spin around its centre of mass the point that is touching will be moving across the table, so you would expect a friction force between it and the table.
However you can seperate the rotation into mostly a rotation around an axis at right angles to the egg, plus a small rolling rotation around the long axis of the egg.
This actually means that the egg is spinning like a wheel and is actually pushing the egg along, so the frictional force will be acting in the same direction as the contact point is moving pushing the egg along.
If the egg is very flat the contact point and centre of gravity are very close together so it only has to spin very slowly to work, as the egg becomes more upright more of its spin is around the long direction so it rolls faster and still acts as a wheel.
This force is what is driving the change in angle of the egg.
Normally this force would act to twist the egg and make it roll, however the egg is spinning, so the egg acts as a gyroscope. Gyroscopes have a very interesting property.
This means that the friction on the egg (blue arrow) will actually cause the axis of rotation of the egg to move at right angles to this (green arrow) and start to stand upright.
Eventually if the egg was spinning fast enough it will stand upright.
The exact dynamics of this are very complex and were only worked out in 2002 by Keith Moffat from Cambridge Nature 416, 385 - 386 (2002).