Science | Science Experiment of the Week 11 | 320 - Sand Angles

Angle of Repose, Slope, Friction, Moisture, Texture - 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.

This Week's Experiment - #320 Sand Angles

These experiments are from Robert Krampf - The Happy Scientist

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.

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