Physics

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Video

Materials

• 1 empty toilet paper roll
• 1 rubber band
• 1 rod (for example, the handle of a kitchen utensil)
• Toilet paper
• Salt

Explanation

When you press with the stick, you subject the salt to a force. This force doesn't "propagate" as you might think, that is, in a straight column to the bottom of the toilet roll where the toilet paper should break. Instead, the force spreads from salt grain to salt grain, and even to the sides of the toilet roll. Imagine a salt grain that the rod directly pushes on. This salt grain then pushes on the salt grains around it, which in turn push on the salt grains around them, and so on ... In the end the pressure reaches all sides of the toilet roll, as well as the entire bottom - instead of only a small area of the toilet paper, which would have torn it.

This toilet roll is like a silo. A silo is a cylindrical building, often seen at farms, for storing bulk goods, such as grains. If a silo is loaded with grainy material, such as salt or corn kernels, the fact that a force from above largely spreads to the walls leads to a strange effect. The pressure on the bottom plate in the silo doesn't increase linearly if the silo is filled with more grain. Thus, filling with twice as much grain doesn't mean that the pressure on the bottom plate becomes twice as great. And after filling with enough grain, the pressure on the bottom plate doesn't increase at all anymore! Then it's just the strength of the walls you have to worry about. This so-called saturation pressure depends, among other things, on the diameter of the silo and the type of grain.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you replace the salt with corn kernels?
• What happens if you pour in less salt?
• What happens if you pour in more salt (use a paper towel roll)?
• What happens if you use a pointy rod?
• Can you find the saturation pressure for your "silo"?

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