# Why does a spinning egg stand up?

##### March 13, 2018 by Lisa Zyga, Phys.org feature

If a hard-boiled egg is spun fast enough on a table, it will slowly stand upright as it continues to spin. Although the spinning egg rises due to the force of friction between the egg and the table, the full explanation involves a relatively complex set of equations. In an attempt to explain the concepts to a broader audience, physicist Rod Cross at the University of Sydney has sought a simpler explanation of the rise of a spinning egg.

"Spinning have been studied for more than 100 years, but there has not previously been a simple for the rise, either of spinning eggs or the tippe top," Cross told Phys.org. "The essential physics cannot be conveyed to an or to a physics teacher by explaining that an egg rises because the equations predict that it will rise.

"Part of the problem is that there have not been enough experimental measurements to pin down the separate roles of sliding and rolling in causing the egg (or tippe top) to rise and then causing it to stop rising if it is not spun fast enough."

In experiments with a solid aluminum spheroid, Cross demonstrated an important characteristic of the spinning egg, which is that it precesses (i.e., rotates) about two different axes. The obvious rotation is that in the direction which it spins, about the vertical axis. The egg also rotates about a horizontal axis as it stands up on its end.

The new study shows that rotation about the is due to precession, and that the precession itself is due to the horizontal friction force. If the egg starts rolling, then the friction force drops to zero and the egg stops rising.

On the other hand, the precession of the spinning egg about the is caused by the normal force (from the table pushing up on the egg), rather than friction.

The experiments also confirmed another related characteristic of the spinning egg: the faster the egg spins, the more vertical it stands. If the egg spins too slowly, it won't rise at all. Again, the explanation can be traced back to the force of friction, since slow rotation causes the egg to roll instead of slide and stand up.

Cross also found similar explanations for the inversion of a tippe top and the slow fall and rapid precession of spinning coins.

Explore further: Spin pumping effect proven for the first time

More information: Rod Cross. "Why does a spinning egg rise?" European Journal of Physics. DOI: 10.1088/1361-6404/aa997b

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##### antialias_physorg
3.8 / 5 (4) Mar 13, 2018
That seems like one of those nightmarish calculations they had us do in theoretical mechanics. On the face of it they seem rather simple/innocent. But as soon as you get into the details the horror begins (and you quickly figure out why it's almost always easier to solve real-world problems via simulation rather than by solving the state equations directly)
##### mackita
1 / 5 (1) Mar 13, 2018
These toys (1, 2) are also based on the same phenomenon. They were once considered magical...
as soon as you get into the details the horror begins (and you quickly figure out why it's almost always easier to solve real-world problems via simulation rather than by solving the state equations directly
This is also why learning formal math has no good meaning for solving real life problems (which are always by at least one level more complex than anyone can just solve).
##### antialias_physorg
3.7 / 5 (6) Mar 13, 2018
This is also why learning formal math has no good meaning for solving real life problems

You think that simulations aren't based on *very solid* math that needs to be understood before you use them?

Wow...you are so deluded. Taken your meds today, have you?
##### mackita
not rated yet Mar 13, 2018
You think that simulations aren't based on *very solid* math that needs to be understood before you use them
Nope, how did you got into it? And why do you always end with ad hominem insults, despite it's just you who didn't understand and twisted the meaning of my post in a stupid way?

The math used by numeric solvers is very simple - it's actually even simpler than rigorous formulation of problem with differential equations. For example the heat transfer problems are all based on Laplace equation and it's recurrent solving by finite element method is just based on repeated averaging of temperature values of neighboring element. Every little child could handle it.
##### mackita
not rated yet Mar 13, 2018
BTW If you don't know about it, it also answers the question, if someone has to understand the math used by numeric solver before (being able to) using it. Of course you don't have to understand it. For example these simulators can be used without any knowledge of math actually.
##### Whydening Gyre
2.3 / 5 (3) Mar 13, 2018
BTW If you don't know about it, it also answers the question, if someone has to understand the math used by numeric solver before (being able to) using it. Of course you don't have to understand it. For example https://www.youtu...0p8sSfpQ can be used without any knowledge of math actually.

However - someone had to do the math to create the sim, in the first place....
##### mackita
not rated yet Mar 14, 2018
I'm pointing to another aspect: most of formal models in physics are based on rather trivial mass-energy balance.Their formulation in form of differential equation is more complex one, their analytical solving even more. Numerical simulators decompose the differential equations back into formulas, which involve mass-energy balance and solve it iteratively for every small element of the solution. After optimization resulting formulas are often even simpler than that. Apparently the complexity of analytical form is unnecessary here, as the problem can be formulated directly at the elementary level in differential form. Most of the complexity of formal models is required only for luxury, which is the having solution in analytical i.e. functional form for special simplistic cases. Unfortunately, under many situations this solution is already so complex, that for its evaluation some computer must be used anyway, which would wipe out the main advantage of analytical solving of formal models
##### antialias_physorg
5 / 5 (1) Mar 14, 2018
And why do you always end with ad hominem insults,

Because it always boils down to one thing: You're dumb as rocks? There's really no point for you to be anywhere near a science site (or even just a science journalism site like this). The stuff that goes on here is so far over your head it's not even funny. You don't even understand the meaning of the word 'science' - and it show in every post you make.

To use a simulation you have to know what it's based on. Otherwise you run the risk of using the wrong simulation and producing garbage output (a phenomenon not uncommon e.g. in statistical analysis where use of the wrong statistical measures can skew the results). If you are not aware of what a simulation approximates and how well then you won't be able to confidently draw conclusions from the results.

Just throwing a random solver at a problem will get you nothing (well, it *will* get you laughed at - that's for sure)
##### mackita
1 / 5 (1) Mar 14, 2018
You're dumb as rocks? There's really no point for you to be anywhere near a science site
We can just think, how any complex human activity - like the programming - would look like for our animal pets. They wouldn't be able to spot, what their masters are doing, so that from their perspective this activity would look like the motion of ants within their nest: a completely chaotic, meaningful and unpredictable, i.e. downright idiotic.
This is the general consequence of observation higher dimensional system from lower dimensional perspective: the shadow of rod regularly rotating in 3D will look like irregularly moving object when being projected at 2D plane, because part of information will be missing. So I'm quite accustomed to objections which label me stupid without even attempting to understand what I'm saying.
Shlomo Riskin: When you're one step ahead of the crowd you're a genius. When you're two steps ahead, you're a crackpot. This is simply how it works.
##### milnik
not rated yet Mar 14, 2018
The uncooked egg can not last longer and rotate properly. Why? If this is to be solved, then it is much easier to find a solution for boiled eggs. Cooked egg is a non-homogeneous and asymmetrical ellipsoid. It has its center of gravity and, whatever it is, on the ground, the point of the support does not stand on any axis passing through the center of mass. If you eat any rotation. there will be a movement composed of two spins: a rotation around the axis of the axis and a rotation of the center of mass around the point of the support. This occurs on the sinusoidal radius of the center of mass around that point.
##### mackita
1 / 5 (1) Mar 14, 2018
See also tennis racket theorem, which describes the Dzhanibekov effect: rotation of an object around its first and third principal axes is stable, while rotation around its second principal axis (or intermediate axis) is not.﻿ The randomly rotating (rotten?) egg would therefore orient along its longest axis spontaneously (i.e. like the projectile) even at free cosmic state.