Why the Spinning Egg Goes Upright?

What is the best way to tell boiled egg from raw eggs? It is the hard-boiled egg that spins on its side flips upright.

The egg`s elevation means that its centre of gravity moves up. It has been called as ` the paradox of boiled egg, ` as it is paradoxical to the existing physics because the egg is gaining energy. Keith Moffatt of the University of Cambridge, England and Yutaka Shimomura of Keio University solved this mystery into mathematic equation, and announced at British magazine of science Nature on the 28th.

According to the article, it is the friction between the egg and the table that makes spinning eggs upright. The spinning energy creates a horizontal force, pushing the egg upright. There would be no horizontal force on a perfectly smooth table that lacks friction or on a hard rubber table, whose friction is too much.

The research team explains that a twirled raw egg doesn`t rise because its liquid centre soaks up spinning energy from the shell.

As the egg rises, its spinning form is more compact, making it whirl more quickly. It`s like when a turning figure skater speeds up by pulling in his or her arms. According to the research team, the critical spinning speed below which the egg stays horizontal is about ten revolutions per second, which a firm flick of the wrist can make.

puset@donga.com