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5 December 2013 11:26 am ,
Vol. 342 ,
At age 30, Dutch biologist Freek Vonk has built up a respectable career as a snake scientist. But in his home country,...
Since arriving on the island of Guam in the 1940s, the brown tree snake ( Boiga irregularis ) has extirpated native...
An animal rights group known as the Nonhuman Rights Project filed lawsuits in three New York courts this week in an...
Researchers have been hot on the trail of the elusive Denisovans, a type of ancient human known only by their DNA and...
Thousands of scientists in the Russian Academy of Sciences (RAS) are about to lose their jobs as a result of the...
Dyslexia, a learning disability that hinders reading, hasn't been associated with deficits in vision, hearing, or...
Exotic, elusive, and dangerous, snakes have fascinated humankind for millennia. They can be hard to find, yet their...
Researchers have sequenced and analyzed the first two snake genomes, which represent two evolutionary extremes. The...
- 5 December 2013 11:26 am , Vol. 342 , #6163
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Spinning Coin Could Solve Swirling Debate
20 April 2000 6:00 pm
A two-bit, tabletop experiment may help settle a decades-old controversy in fluid dynamics. Spin a coin and it will wobble faster and faster as it tips over, until it abruptly stops. Now, a mathematician says he knows why the motion ends with a rattling flourish, and his explanation, reported 20 April in Nature, may provide an example of a type of fluid flow whose very existence is debated.
The 18th century French mathematician Leonhard Euler was the first to study disks rolling on surfaces, and mathematicians have long known that as a rolling disk tips over, the point of contact whizzes around faster and faster. But no one had explained precisely how the disk loses its energy and why it suddenly poops out, instead of lingering in motion like a pendulum gradually coming to rest. Then 2 years ago, Keith Moffatt, a fluid dynamicist at Cambridge University in the United Kingdom, spotted a toy called "Euler's disk," a heftier version of the spinning coin, in a mail-order catalogue while shopping for his grandchildren's Christmas presents. He ordered one and decided to figure out how it works.
Moffatt realized that as the disk spins nearly horizontally, it squeezes air between itself and the table. The disk stirs this air, much as a kid stirs the filling of a sandwich-style cookie by twisting its two faces. The flowing air soaks up the disk's energy, Moffatt hypothesized, and causes it to list even more. Indeed, he calculated that at a definite time, the disk should become horizontal and the point of contact should go around infinitely fast. In reality, nature doesn't allow that. Rather, Moffatt says, the coin's edge loses its grip on the table surface a split second earlier, and the coin suddenly slides down flat.
But the fact that the equations say the disk should move infinitely fast at a definite time may help explain other phenomena, says Hassan Aref, a physicist at the University of Illinois, Urbana-Champaign. Experts have argued for decades whether such infinities--called finite time singularities--arise from the complicated equations of fluid dynamics. If singularities do exist, they may be the seeds for violent phenomena such as turbulence in the atmosphere or eruptions on the sun's surface, even if, as with the disk, nature always finds a way to dodge the infinite at the last instant. "The emergence of the singularity may not just appear in an executive's toy," Aref says. "It may be something that appears in more complicated systems, too."