The Flying Circus of Physics, newsletter for February – April 2013

Data pubblicazione: 2-feb-2013

Newsletter February – April 2013, by Jearl Walker

This time I have two unusual examples of swimming.

Syrup swimming

Here is a story that began 300 years ago with Isaac Newton and Christiaan Huygens and which eventually resulted in episodes on the television shows Brianiac and Mythbusters, with a stop along the way at the Ig Noble Awards. In a discussion of hydrodynamics, Newton argued that the viscosity of water will affect the speed of someone swimming though it. That is, if we increase the viscosity, the speed will decrease. Huygens countered that there would be no change in the speed with an increase in viscosity.

I can easily support either contention: Reasonably, a greater viscosity should offer more resistance to the swimmer and thus reduce the speed. Just as reasonably, a greater viscosity should allow the swimmer to pull more firmly against the liquid and thus increase the speed. For the last three centuries the debate has raged. Well, not exactly raged. More likely it was discussed by physicists who were parked in pubs with a few pints in front of them.

In 2004, the story changed because Brian Gettelfinger and E. L. Cussler of the University of Minnesota set up an experiment to answer the question “Will humans swim faster or slower in syrup?” They arranged for ten competitive swimmers and six amateur swimmers to swim through a certain length, first in water, then twice in a water–guar mixture (to increase the viscosity), and then again in water. The density of the mixture was approximately the same as that of water, and so the swimming speed was not affected by the swimmers floating at different levels. The mixture differed from the water only in viscosity.

For all the swimmers, the results were fairly clear: They had the same swimming speed in the higher viscosity as in the lower viscosity.

There are two ways in which an object’s motion through water can be resisted. One is form drag, which has to do with the collision between the object and the water. (If you have ever held your hand outside a moving car, you have felt form drag from the passing air. The drag increases as you rotate your hand from palm down to palm forward.) The other type of resistance is due to the viscosity as the water slides past the object or vice versa. What Gettlefinger and Cussler demonstrated was that form drag is the dominant resistance to a human swimmer, not viscous drag.

For their effort in putting to rest a 300-year-old argument about fluid mechanics, the researchers won the 2005 Ig Noble Prize for Chemistry. Such awards are reportedly given for research that first makes you laugh and then makes you think. (I’ve always wondered if the Ig Noble group tacked on that last part to avoid any litigation.)

More recently two television shows have put swimmers to the test in syrup (or at least syrupy liquids) but the results are certainly more entertaining than scientific. Here is the Mythbusters episode:

http://www.youtube.com/watch?v=V4TEqb-728k&feature=related

And here is the Brianiac episode:

http://www.youtube.com/watch?feature=endscreen&NR=1&v=iagQtBWwZu8

Sand swimming

Certain desert lizards can bury themselves in sand and then swim through it while fully submerged. They might do this to escape from predators or to be safe at night. The mechanics of such sand swimming has caught the attention of a range of scientists and engineers, especially biologists, materials scientists, and scientists researching granular fluids. Some researchers are interested in mimicking the lizards with robots that can swim through granular materials. Not only is the mechanics intriguing but such robots might be used to locate landmines (without disturbing the trigger on the top side). Here is a video of a sandfish lizard, as they are sometimes called:

http://www.youtube.com/watch?v=seOhvl5EO7k

You can see the subsurface locomotion in this next video made with x ray imaging. The sandfish is swimming through a “sand” of plastic spheres.

http://www.youtube.com/watch?v=wzi8NgkHuKU

Here is a video of a robot sand swimmer, seen both on the surface and below the surface (via x ray imaging):

http://www.youtube.com/watch?v=9mwJsGbTkOk

The early explanation for sand swimming was that the lizard propels itself by sending a sinusoidal wave from head to tail, much like a snake oscillates. And as with the snake, the net force is in the forward direction.

However, NMR imaging revealed a clever coordination between the left-right oscillation of the body as the wave moves along it and a front-back swimming motion of the legs. When a leg moves backward, the portion of the body behind it oscillates toward the leg, compacting the sand grains. This action allows the leg to push backward against a solid body of sand and thus propel the lizard forward.

When the leg moves forward, the portion of the body in front of it oscillates away from the leg, leaving the sand grains loosely packed. This action allows the leg to slide through the grains with little resistance and thus with no backward propulsion. The lizard has learned that such loosening occurs only if the oscillation frequency of leg and body exceeds a few times per second. For any smaller frequency, the sand grains would always remain in firm contact with one another, and the lizard would be unable to move.

Archived stories

One of my favorite archived stories has to do with my fascination with tornadoes. Although I grew up in Texas where tornadoes are frequent, I never got the opportunity to see one. To read two stories about them and to find the links to some dramatic videos, go to

http://www.flyingcircusofphysics.com/News/NewsDetail.aspx?NewsID=59

and scroll down to item 2.36.

More videos and photos have been added to the FCP Facebook site (you do not need to be a member):

http://www.facebook.com/pages/Flying-Circus-of-Physics/339329532602?ref=ts