When sand is poured into a pile, like you said, there is a specific
angle which cannot be exceeded. What's more interesting than that,
though, is that no matter how many times you pour the sand or how much
sand you pour, the maximum angle won't change.
The maximum angle that the sand can make with the ground is called
the "angle of repose." One thing that makes a big difference in the
angle of repose is how wet the sand (or other material) is. That's why
when you build a sand castle at the beach, it works best if you use wet
sand. There's currently a lot of research going on trying to understand
the physics of what happens to the physical properties of a granular
material when you add water.
Another thing is that the larger the objects are, the steeper the
angle of repose will be. For example, piled gravel will have a much
steeper angle of repose than piled sand. Also, the amount of friction
provided by the surface underneath the sand pile makes a difference.
Sand piled on a sheet of sandpaper will move differently than sand
piled on a sheet of ice.
So far it seems like it would be pretty straight-forward to
calculate the angle of repose for something. Unfortunately, one of the
things that influences the angle of repose is the shape of the object.
If the grains of sand were perfectly round, they would slide against
each other easily and the angle of repose would be pretty small. But
sand isn't always perfectly round. It tends to be extremely irregular,
and sand from different places will have different shapes. It's
difficult to model how these shapes will interact when piled freely on
top of each other. (For example, you can imagine if you piled a whole
bunch of tetris pieces up - it would be pretty hard to determine how
they would settle if you just dumped them in a pile.)
There are probably other things that influence the angle of repose
- these are just some of the more important ones. There's plenty of
physicists out there making their livings by studying just this sort of
(published on 10/22/2007)