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25 August 2019 21:47

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Subject: Precession of an egg shell
Question: I have a question regarding precession of spinning tops. The physics tells us that if we take a symmetrical top such as a hardboiled egg and spin it about its axis of symmetry it will precess as a gyroscope, and so it does. The spin axis follows a cone.

However, if we take an eggshell (or better a plastic object of similar shape) and spin it, it does not precess. Instead it spins in the upright position, its spin axis remains motionless perpendicular to the Earth.

This happens even if I spin it at an angle between my hands and drop it on the table. It does not precess at the angle at which I launched it but adopts the upright position. Furthermore, if I lay it on a table (with its axis of symmetry horizontally) and give it a spin, it uprights itself. It does not want to spin in any other position but with its axis of symmetry upright.

Any explanations? (Preferably in terms of mathematics and physics.)
Date: 31 January 2005
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Thats really easy to answer. The hollow egg is a 'better' gyroscope than a solid egg.
The ideal gyroscope has a heavy outer rim with a thin as possible center.

If you made some wooden eggs and drilled different sized wholes in each one down the centre, you would find a optimum size of whole. This assumes they all spin at the same speed.

Note: Ones with small wholes will have more torque (because of the energy stored) but less precessional torque because it will have 'dead' wieght. The 'dead' wieght is acting against the rest of the mass.

Hope this makes sense.

You may also find that because the empty egg is lighter you can spin it faster. Hence it
becomes better still.

You can do the maths for yourself on the maths page.

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Answer: Victor Geere - 21/02/2005 09:30:13
I don't think this is precession. The axis is vertical so there is no nett force perpendicular to the axis that would make it precess at a 90 degree angle to that force. As it slows down and starts to fall over it might seem like it is precessing, just like the solid egg, that has a less effective core.

If you spin it at an angle on the table it seems to pick itself up because it spins faster at the equator of the egg. When a part of the egg away from the egg's axis touches the table this bounces off the table and seems to pick the egg up.

If you could spin the egg with a horisontal axis with one tip of the axis resting on an eifel tower model it would precess around the eifel tower model because of the nett effect of gravity that makes it fall horisontally (because of precession) around the eifel tower. This will work with both eggs, and more effectively with the hollow one.

My humble opinion.

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Answer: Victor Geere - 21/02/2005 09:30:32
I don't think this is precession. The axis is vertical so there is no nett force perpendicular to the axis that would make it precess at a 90 degree angle to that force. As it slows down and starts to fall over it might seem like it is precessing, just like the solid egg, that has a less effective core.

If you spin it at an angle on the table it seems to pick itself up because it spins faster at the equator of the egg. When a part of the egg away from the egg's axis touches the table this bounces off the table and seems to pick the egg up.

If you could spin the egg with a horisontal axis with one tip of the axis resting on an eifel tower model it would precess around the eifel tower model because of the nett effect of gravity that makes it fall horisontally (because of precession) around the eifel tower. This will work with both eggs, and more effectively with the hollow one.

My humble opinion.

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Answer: Jim Warchol - 06/09/2005 19:21:32
You guys are over thinking this situation.

A function of a gyro sensor is based on the mass and rotational speed of the "spinning mass". The boiled egg would have a greater mass and if spun at the same speed (as the hollow egg) it would be a more sensative device.

Anyway, the answer lies in the distribution of mass. Your egg was probably boiled on its side. So the mass was unevenly distibuted; this is a type of "mass unbalance". If you boil an egg on its bottom it will probably spin upright also. I'm doing it now will a solid somewhat egg shape toy.

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Answer: David Peters - 28/09/2005 10:12:08
Hello all,

I have been reading this on this site for awhile and thought I would through my 2 cents worth in on this thread.

I think what happens is that a normal gyroscope is spinning on a point and when it is leant over it is still spinning on that point. With the egg because of the shape you try to spin it on its side you are actually trying to rotate it around its own circumfrence therefor not spinning around the point directly on the rotation axis.

Do this make sense?

David

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