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Egg Mystery Cracked
Scientists Put the Spin on a Hard-Boiled Phenomenon

Listen to Joe Palca's report.

Video: Joe Palca demonstrates the egg spin.

Get the scientific explanation behind the egg spin.

 Spin a hard-boiled egg on its side and it will flip into a vertical position. Photo: NPR

March 27, 2002 -- With the start of Passover tonight and Easter arriving on Sunday, Americans are likely to purchase more than a billion eggs -- 50 percent more than any other week of the year.

Eggs are also on scientists' minds this week. In the current issue of the journal Nature, researchers explain why a hard-boiled egg, spun rapidly on a table top, will flip over and start spinning like a top. For All Things Considered, NPR's Joe Palca has more.

As a practical matter, it's easy enough to show that if you set a hard-boiled egg on a table, and give it a good spin, it will start off spinning horizontally, then start wobbling, and then start spinning upright. The hard part is explaining why it happens.

 A Hard-Boiled Limerick by Keith Moffatt Place a hard-boiled egg on a table, And spin it as fast as you're able; It will rise on one end, With vectorial blend, Of precession and spin that's quite stable.

"I thought this would be an easy problem, but it turned out to be really very subtle and quite tricky. It took us a long time," says one of the paper's authors, Keith Moffatt, a mathematical physicist at the University of Cambridge in England. He's drawn to problems involving spinning objects. Two years ago he published an article in Nature about the physics of a coin spinning on a table.

Now as Moffatt says, the explanation for why an egg behaves the way it does is tricky. Joe Palca, after a few rounds with Moffatt, explains it this way:

After the egg starts spinning horizontally, two forces -- known as friction and slippage -- combine to get the egg wobbling. As it starts wobbling, some of the horizontal spin gets transferred to a vertical spin. As that happens, the same forces that make a gyroscope spin on its end start making the egg spin on its end.

"The gyroscopic forces must exceed the effect of gravity which is pulling the center of gravity down," explains Moffatt. In other words, as it wobbles on the table, it starts to appear more like a gyroscope, and at some point the gyroscopic forces take over, and it flips up on its axis.

Moffatt says he's not done with spinning objects. He's thinking about working out the mathematics that explains why a raw egg won't spin at all on a table -- another problem, he says, that seems simple until you try to explain it. And maybe after that, he'll attempt an explanation of how an irregular object like a pebble spins on a table.

In Depth

Listen to Joe Palca's report on the physics of spinning coins.

Other Resources

Nature

American Egg Board

More egg trivia