Physics Photo of the Week
October
17, 2008
Deadly Trap
Don't
fall into this pit, you will never get out. There lurks a vicious
"monster" hidden in the bottom. The sides of the pit consist of
loose "rocks" that are constructed at the Angle of Repose - the
steepest angle that dry aggregates can maintain under the influence of
gravity. The rubble at the angle of repose is highly
unstable. Any animal that crawls onto the slope of the pit may
trigger an "avalanche"; the avalanche will carry the victim to the base
of the pit where the hungry "monster" will quickly devour it.
Actually the "monster" is an insect larva called an antlion
- ants are its common prey that fall into the trap. This trap was
about 3 cm diameter underneath a spruce tree. This trap is deadly
only if you are an insect or arthropod walking around on the
ground. The antlion living in the bottom of the trap builds the
trap by burrowing through the sandy soil and flipping the grains of
dirt or sand out of the pit while burrowing deeper in a circular
fashion. As the grains fall on the sides of the area they
distribute themselves in a pile that slopes at the angle of
repose. Sand banks at the angle of repose are very
unstable. An unsuspecting ant walking on the slopes at
such an angle can trigger the avalanche - the ant falls to the bottom
of the pit, gets trapped by sand falling on top of it, making easy prey
for the jaws of the antlion larva lurking in the bottom.
The same angle of repose is seen is piles of sand or gravel
when the sand/gravel spills out of the end of a conveyor in a sand mining
operation. In road construction where engineers build
berms of fill to elevate the road over the surrounding terrain, the
sides of the banks must be less steep than the angle of
repose, or the road bed will be highly unstable leading to disastrous results.
Talus
at the base of mountains and cliffs - formed from the accumulation of
rocks and boulders falling off the top of the cliffs during thousands
of years - forms a slope just smaller than the angle of repose.
The photo at left shows the talus at Lake Louise, Banff National Park,
Canada. (Look for future physics photos featuring Lake Louise).
There will be no Physics Photo of the Week next week (October 24) due
to Warren Wilson College fall vacation. The next Physics Photo of
the Week will be published on October 31, 2008
Physics
Photo of the
Week is
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos feature an 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:
