Sand
Slope
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
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.