If you put a boiled egg on the table and spin it quickly, and your method is correct, you will see a strange phenomenon. When the egg rotates, it will stand up slowly. This looks cool, but it's also strange.

This is a strange phenomenon, and it was not until 2002 that physicists discovered the reason-a very complicated mathematical explanation. Now, a physicist has put forward a simple explanation for boiled eggs standing in rotation.

Rod Cross of the University of Sydney explained: "Scientists have been studying spinning eggs for more than 100 years, but there is no simple explanation for this phenomenon. Basic physics cannot be explained to an undergraduate or physics teacher, because the equation predicts that the egg will stand up, so it stands up. "

Part of the problem is that there are not enough experimental measurements to determine the independent effects of sliding and rolling friction in the process of eggs gradually standing upright or stopping due to insufficient speed.

It turns out that what actually happens is that the egg rotates around two axes, not one.

Put an egg on the table, and the egg will fall to one side, with the slightly pointed end slightly upward. If you give it a slow rotation speed, it will rotate around an axis that runs through the egg, that is, the short axis perpendicular to the desktop.

Although an object usually rotates around its own center of mass, due to the shape of the egg, its center of mass does not coincide with the point where the egg starts to rotate, that is, the point where it contacts the table.

The difference between the center of mass and the contact point causes the egg to start shaking, and then makes it stand gradually, and the angle of the short axis begins to tilt during the rotation.

The angle change of the short axis is called precession, and precession is a term used to describe the direction change of the rotating shaft. The precession of the short axis is the result of normal reaction (the reaction between the contact point of the table and the egg).

However, you can see that if you watch the video below carefully, when the egg rotates around its short axis, it will also rotate around a longer, almost horizontal axis at a very slow speed.

This rotation around the long axis is due to the precession of the short axis, and the angle of this second axis changes with the rotation of the egg. At the same time, friction will also make the long axis of the egg precess after rotation.

Cross pointed out that friction plays an important role in changing the rotation angle of eggs, which will lead to the blunt end rising and the eggs will "stand up".

Image source: modified from rod cross/ European Journal of Physics

But when the egg starts to shake, it overcomes the braking of friction and keeps it upright. This makes the shorter axis, that is, the first axis, become the most important rotation axis again. In cross's new essay, he found that the equations used to describe the precession of an egg upright and a flip gyro are the same in form.

His experiment also confirmed on his website that the faster the rotation, the straighter the egg stands, and if the rotation is too slow, it will not produce enough friction.

If you want to try to spin an egg at home, first make sure it is cooked-because the center of mass of raw eggs will change with the flow of liquid.

His paper was published in the journal European Physics.