Stirring Sand, part 2

In part one, I left you with a challenge. We put a little sand into a glass of water and stirred it. Instead of moving to the outside edge, as we might expect, the sand gathered in the center of the glass. I left you with the challenge of telling me why.

Congratulations! Several of you got the right answer, and several more got it at least partially right.

So, why does the sand do that? Lets investigate by stirring the water again. As you stir the water, notice what happens to its surface. The water rises along the edges of the glass, and a depression forms in the center. The faster you stir, the higher it gets at the edges, and the lower the depression gets in the center.

This change in the surface is caused by inertia. The moving water will try to continue moving in the same direction, forcing it against the side of the glass. That increases the water pressure there, and forces the water up the side of the glass. The highest water pressure will be along the sides of the glass, and the lowest water pressure will be in the center.

Keep stirring, but look at the sand instead of the water. While you are stirring, the sand is suspended in the water, and is flung outwards, as you would expect. It is only when the grains of sand fall to the bottom that they move towards the center.

Now, think about what is different at the bottom of the glass. The bottom of the glass, right? That water is touching the solid, stationary surface of the bottom of the glass, instead of being surrounded by moving liquid. The water tends to stick to that stationary surface, producing a layer water at the very bottom of the glass that spins more slowly than the rest of the water.

Now we have a layer of water at the very bottom that is spinning slower. Because it is moving slower, it has less inertia pushing it outwards, which gives us a thin layer of water with lower pressure along the bottom.

So we have high pressure along the sides of the glass, except at the very bottom, which has low pressure. The water flows from high pressure to lower pressure, moving inward along the bottom towards the center, carrying any sand that is resting on the bottom with it.

Once it reaches the center, the water moves upwards to the low pressure area in the center of the glass. The sand is left behind, piled up in the center.

I'll leave you with something to think about. What could you do to cause the sand to move from the center and form a ring around the outside of the bottom?

Have a wonder-filled week.

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