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.
Click
here
to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
