Thursday, April 25, 2002

The Spinning Egg Rises

Scientists theoretically explained the paradoxical behavior of a hard-boiled egg: if it is spun with its axis of symmetry horizontal, this axis will rise from the horizontal to the vertical, raising the center of gravity. -- This is not a joke. See the reference [1]. -- They traced the essential mechanism to the action of the frictional force between the spinning object and the table. Their paper was referred to in a column of a Japanese newspaper [2]. A non-scientist friend of mine read the column, and told me that she wished to experiment. I noticed the original paper before, but did not think to experiment by myself. She had more of a scientist's mind than me.

Being stimulated by her words, I tried to rotate a boiled egg on the floor. It seemed difficult to make the egg rotate fast enough around its axis of symmetry put horizontally to cause rising. Starting from rotation around the axis a little off the vertical, however, I could see the axis rising and becoming just vertical. This is wonderful enough. I heard that the friend had also succeeded in observing the odd motion of the egg. She added that she had been quite thrilled when the axis became vertical.

The scientists also write why a raw egg does not show the same behavior. It is because the angular velocity imparted to the shell diffuses into the fluid interior; this process dissipates most of the initial kinetic energy imparted to the egg, making the remaining energy insufficient for the condition of gyroscopic balance to be established. This is a type of research Torahiko Terada (Japanese physicist, astronomer and essayist. Professor of Tokyo University. 1878 - 1935; see a portrait) would have liked.
  1. H. K. Moffatt and Y. Shimomura, "Spinning eggs -- a paradox resolved," Nature Vol. 416, pp. 385-386 (2002).
  2. Y. Uchiyama, "Self-rising boiled eggs," Asahi-Shimbun, 22 Apr., p. 23, (2002).

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