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This Week's Experiment - #320 Sand Angles
This week's experiment is a result of taking some time off. I spent most of
today working, but I took some time to go to the beach and relax. As I sat
there watching the waves, I was also playing in the sand. It was not
serious playing, just scooping up sand and letting it fall through my
fingers. As I sat there, the sand took more and more of my attention, and
soon I had this week's experiment. You will need:
paper
tape
a plate
2 cups of sand, salt or sugar
Corn chips, potato chips or corn flakes
M&M's or other round candy
Roll a sheet of paper into a tube about two inches in diameter and tape it so
that it does not unroll. Stand this tube upright in the center of a dinner
plate. Carefully pour about a cup of either sand, salt or granulated sugar
into the tube. Slowly lift the tube and watch what happens. The sand
spills out the bottom. OK, so what is so special about that?
The sand formed a cone shaped pile on the plate. Notice the angle of the
slope. If you have a protractor, you might want to measure the angle. Try
adding some more sand, to make a steeper slope. What happens? As you add
more sand, it slides down and the slope stays about the same. You can make
the pile taller, but the slope of the sides stays fairly constant.
That slope is called the angle of repose. If you play with different
materials, you will find that each has its own angle of repose. Some will
form steeper slopes, while others form much flatter slopes. For example, I
poured a pile of corn chips onto the plate and got a nice, tall, fairly steep
angle of repose. I tried the same thing with M&M candies and got a very
flat slope. What is the difference between the two? Their shape, of
course. The rough shape of the corn chips helps them interlock, forming a
nice, steep slope. The rounded shape of the candy lets them slide past each
other, so you get a much flatter angle of repose. No matter how hard you
work, you can't pile the candies to get a nice, steep slope made of M&M's.
Other things also have an impact on the angle of repose. Instead of using
the smooth plate, try stacking the M&M's on a rough textured mat or a piece
of course cloth. It is easier to get them piled into a slope, because the
friction helps keep them in place.
Moisture also plays a part. If you have ever built sand castles, you know
that it is much easier to build tall castles if you use wet sand. The water
holds the grains together to form very steep slopes. On the other hand, too
much water makes it soupy, so that it will not form a slope at all.
All this is interesting, but is it useful? Yes. The angle of repose
explains the shape of sand dunes and snow drifts, but it also tells builders
how steep a slope you can safely build a house on. Sometimes builders
ignore this, which is why you sometimes hear about houses sliding down
hillsides in California. Each type of soil has its own angle of repose.
If you form a slope that is steeper than this critical angle, eventually,
nature will take over and things will shift.
When you finish, be sure to clean up your mess. Take the sand outside.
Since you have handled the corn chips and candy, probably no one else will
want to eat them, so you might as well eat them yourself. After all, you
would not want them to go to waste.
Have a good week.
These experiments are from Robert Krampf - The Happy Scientist
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