Physics Photo of the Week

April 16, 2010

Running on Water
These Canada Geese were enjoying a balmy February day in 2009 at the Conservation Pond near the Red Barn on the Warren Wilson Farm.  Suddenly they all decided that they should vacate the premises and took off running on the water until they reached a speed sufficient for flight.  Such water running is common among aquatic birds.  There is even a lizard, called the "Jesus Lizard" that runs on water to escape predators.  (See a video here).

How does water support such running?  The same phenomenon enables water skiing and speedboat planing.  With rapid application of the forces for brief times - a gooses foot touches the water only briefly, the speeding water ski is in contact with a given segment of water for only a about 0.02 seconds - the inertial mass of the water prevents the water from achieving much acceleration, so the equal and opposite force keeps the running bird or the water skier from sinking. 

The full explanation may require a more sophisticated explanation such as non-Newtonian properties of fluids.  A thick slurry of cornstarch and water is an extreme example.  If you press quickly on the cornstarch/water slurry, the fluid behaves like a solid, but small even pressure allows the fluid to flow.  See the Youtube video of people running on a pool filled with a cornstarch slurry, but sinking if they stop running.  Such non-Newtonian properties of water are usually negligible, but the small non-Newtonian effects may be mostly responsible for running on water!



Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu. 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.


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