Can an egg really jump?

Just in time for Easter, Japanese scientists at Keio University have discovered that eggs can 'jump' in the air if you spin them fast enough. Interestingly, this process has important implications for applications such as weather forecasting or structural engineering.

Just in time for Easter, Japanese scientists at Keio University have discovered that eggs can 'jump' in the air if you spin them fast enough. In "Jumpy eggs caught on camera," Nature reports that with a spin rate of 1,800 revolutions per minute, hard-boiled eggs can jump, but not very high. They can rise by a tenth of a millimeter but only for a few thousandths of a second. The team plans now "to spin a raw egg or spheroid with liquid inside to see if it will rise." And it's not only an Easter story: this process has important implications for applications such as weather forecasting or structural engineering.

After two years of work, with a purpose-built steel machine wired up to high-speed cameras, microphones and electronic sensors, a team of Japanese researchers has finally proved that a hard-boiled egg can jump. All it takes, according to Yutaka Shimomura and colleagues of Keio University, is a good spin.
But does it really happen? To check, Shimomura's team had to make a device capable of spinning an egg perfectly, so they could be sure that the effect wasn't due to an upwards motion introduced by a spin done by hand. They also had to hit a spin rate of 1,800 revolutions per minute. So as not to be too messy and to ensure easy measurements, they used an ovoid-shaped metal egg in the experiments.

Below are two images of such a spinning egg, extracted from this page (in Japanese) at Keio University in Kobe, Japan (Credit: Yutaka Shimomura and colleagues).

Jumpy egg: image 1

Jumpy egg: image 2

This research work has been published by the Proceedings of the Royal Society A (Proc. R. Soc. A) under the name "Can a spinning egg really jump?" Here is a link to the abstract.

Simultaneous three-way observation of optical, acoustic and electric properties demonstrates a theoretical prediction that a spinning prolate spheroid can spontaneously lose contact with the table in the course of its rising motion when the contact friction is weak and the spin is large enough. The durations of the first loss of contact are measured for various initial spin rates and for three aspect ratios. The measurements show good agreements with numerical simulations. It is also visually shown that a spinning hard-boiled egg can jump.

But Proc. R. Soc. A also published a short article about this subject, "How do you make an egg jump?" which gives other details about possible applications such as weather forecasting and structural engineering.

The team of scientists has proved that if you spin an egg shaped object (or 'prolate spheroid') fast enough it will make a series of jumps. The research tests the phenomenon that small, random deviations can cause unexpected events. For example, how the small collective vibrations on the London Millennium Footbridge caused the unexpected 'wobble', and how small variations in wind have the ability to affect whole weather systems.
Professor Yutaka Shimomura, who led the research said: "The 'spinning egg problem' is one of those so-called 'toy problems', but provides a real example of a phenomenon where small fluctuations can cause unexpected events. The theory I have proposed came from a theory of turbulent flows. Experimentally testing this problem could potentially enable scientists to find unexpected problems in areas such as atmospheric dynamics and turbulent flows around airplanes or vehicles."

And it took two years to develop the system to validate the theory of the jumping egg.

The team spent around two years developing a machine to test their theories. They needed to spin the objects at speeds of over 1500rpm without causing artificial disturbance. The final design was capable of spinning eggs at up to 2500rpm, though 1500rpm was the necessary speed required to make the aluminium model egg rise up and 'jump'.

After experimenting with aluminum egg shapes and hard-boiled eggs, the team plans "to spin a raw egg or spheroid with liquid inside to see if it will rise."

Before they reach this milestone, you can watch a spinning egg in action on this short video of a spinning hard-boiled egg in the course of the rising motion (8.60 MB).

Sources: Nature, April, 2006; Proceedings of the Royal Society A; and various web sites

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