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Podcast TranscriptThe Naked Scientists: Science Radio & Science Podcasts |
| 6th May 2006 | < Previous Show | Next Show > | |||||||||||
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How to tell if your egg is raw
Find out whether your egg is raw or not without having to break it open.
What you need
| A fresh egg |
| A hard boiled egg |
| A finger |
What to Do
Spin an egg on a table,
Stop it quickly with your finger, and then release it.
Try with the other egg.
Is there any difference?
What may Happen
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When you stop the hard boiled egg it just stops. | When you touch the raw egg it stops, but if you let go again quickly it will start moving again. So you can tell it's raw. |
What is going on?
The centre of a raw egg is quite liquid so even if you stop the shell moving the centre will keep going for a short time.
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When the egg is spinning the centre of the egg is moving with the outside. | If you stop the egg quickly, you only stop the shell of the egg so the liquid centre will keep on moving. | If you let go the friction between the centre and the shell will start the shell moving again. |
A hard boiled egg on the other hand is solid so if the shell is stopped so is the centre.
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The hard boiled egg is solid so moves as one piece. | So when it stops it all stops. |
Why was the raw egg so difficult to start spinning?
In the same way that it is hard to stop the egg because the centre is liquid, it is hard to start it moving quickly. So even if you spin the shell very quickly the centre will not be moving, so it the shell will quickly be slowed by the friction between it and the centre.
Make an Egg Stand on End.
Could you make an egg lift up on it's end and stay there? Find out how in this experiment.
What you need
| A hard boiled egg |
| A surface, like a worksurface or piece of wood. |
What to Do
Hard boil an egg
Lay in on it's side on the surface
Spin the egg as fast as you can.
Does anything strange happen?
What may Happen
If you spin the egg fast enough you will find that it stands up on end.
What is going on?
Because of the shape of the egg, the point of contact with the ground is not directly above the centre of mass of the egg (this is the average position of all the mass of the object, and is the point that the object would spin around naturally.
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Because the egg is spinning around its centre of mass which is not above the point of contact, the contact point gets dragged around the surface. The friction from this (blue) applies a force on the egg. | View from the wide end of the egg. The friction applies a twisting force to the egg |
Why does this force cause the egg to stand on end?
The egg is spinning so it acts as a gyroscope. Gyroscopes have a very interesting property.
| If you apply a twist to a gyroscope as gravity does in the case to the left (blue arrow) the gyroscope actually moves at right angles to this along the green arrow. If the gyroscope is standing up as in this picture this means that it will keep going around in a circle rather than falling over. |
This means that the friction on the egg (blue arrow) wil actually cause the egg to rotate 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 quite complex and were only worked out in 2002 by Keith Moffat from Cambridge.
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