Published online: 20 September 2004; | doi:10.1038/news040920-2
Swimming in syrup is as easy as water
You can swim just as fast in a pool of gloop.
It's a question that has taxed generations of the finest minds in physics: do humans swim slower in syrup than in water? And since you ask, the answer's no. Scientists have filled a swimming pool with a syrupy mixture and proved it.
"What appealed was the bizarreness of the idea," says Edward Cussler of the University of Minnesota, Minneapolis, who led the experiment. It's a question that also fascinated his student Brian Gettelfinger, a competitive swimmer who narrowly missed out on a place at this summer's Olympic Games in Athens.
Cussler and Gettelfinger took more than 300 kilograms of guar gum, an edible thickening agent found in salad dressings, ice cream and shampoo, and dumped it into a 25-metre swimming pool, creating a gloopy liquid twice as thick as water. "It looked like snot," says Cussler.
The pair then asked 16 volunteers, a mix of both competitive and recreational swimmers, to swim in a regular pool and in the guar syrup. Whatever strokes they used, the swimmers' times differed by no more than 4%, with neither water nor syrup producing consistently faster times, the researchers report in the American Institute of Chemical Engineers Journal1.
The most troublesome part of the experiment was getting permission to do it in the first place. Cussler and Gettelfinger had to obtain 22 separate kinds of approval, including persuading the local authorities that it was okay to put their syrup down the drain afterwards.
But it was worth the hassle, Cussler says, not least because his quest for an answer made him something of a celebrity on campus. "The whole university was arguing about it," he recalls. "It was absolutely hilarious."
But while it might sound like a trivial question, the principle is actually fundamental. Isaac Newton and his contemporary Christiaan Huygens argued the toss over it back in the 17th century while Newton was writing his Principia Mathematica, which sets out many of the laws of physics. Newton thought that an object's speed through a fluid would depend on its viscosity, whereas Huygens thought it would not. In the end, Newton included both versions in his text.
Hamstrung by their lack of access to guar gum or competitive swimmers, Newton's and Huygens' work was mainly theoretical. Cussler's demonstration shows that Huygens was right, at least for human-sized projectiles.
The reason, explains Cussler, is that while you experience more "viscous drag" (basically friction from your movement through the fluid) as the water gets thicker, you generate more forwards force from every stroke. The two effects cancel each other out.
That's not always the case. Below a certain threshold of speed and size, viscous drag becomes the dominant force, making gloopy fluids are more difficult to swim through. Had Cussler done his experiment on swimming bacteria instead of humans, he would have recorded much slower times in syrup than in water.
But for humans, speed depends not on what you swim in, but on what shape you are. Once the effects on thrust and friction have been cancelled out, the predominant force that remains is 'form drag'. This is due to the frontal area presented by a body - try running with a large newspaper held in front of you and see how much more difficult it is.
So the perfect swimmer, whether in water or syrup, has powerful muscles but a narrow frontal profile. "The best swimmer should have the body of a snake and the arms of a gorilla," recommends Cussler.
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