By wetting sand first, the required pulling force is halved. The Ancient Egyptians used this trick to transport pyramid stones. Physicists from the FOM Foundation and the University of Amsterdam discovered this.
The Ancient Egyptians transported giant blocks of stone across the desert to create impressive structures such as the pyramids. Researchers have wondered for years how the workers were able to do this thousands of years ago “First they wet the desert sand and then dragged a sled over it with large stones on it. Our experiments show that the tensile force is then halved," says Professor of Physics Daniel Bonn (UvA).
He points to a wall drawing from 1800 BC from the tomb of the local Egyptian governor Djehutihotep. It shows, among other things, someone who is pouring water on the sand in front of the sled, on which a large statue is being transported. “The Ancient Egyptians knew these kinds of handy tricks, because it saves so much work,” he says.
Grain size
The researchers conducted a scaling experiment in a laboratory. They dragged a sled with different weights on it over the sand and thus made the discovery. “We also found that the sled was more difficult to move over sand of the same grain size. With sand with different grain sizes, it was actually easier. Egypt has that type of sand and this worked to the advantage of the workers.”
Why does the required pulling force decrease when the sand is wet? The addition of water creates so-called capillary bridges. “These are very small water bridges that ensure that the grains stick together and become hard. They are not there with dry sand and mud.”
Bonn emphasizes that research into the Ancient Egyptians can still yield a lot today. For example, when using granular matter, such as sand, asphalt, concrete and coal. “Its transport is responsible for ten percent of the world's energy consumption.”
North-South line
The physicist mentions the construction of the North-South line as an example. There, too, granular matter was transported in (cement) and out of (mud) the tunnel. “We have conducted research into exactly how sand passes through a pipe. This showed that it takes less energy to remove a little wet sand. You may not expect that, because it gets harder. But that is precisely why it is easier to transport through a tube and that saves half the energy.”
In follow-up research, Bonn wants to look even more specifically at friction, the resistance that arises when two parts pass each other. “We don't really understand friction yet. But it is responsible for thirty percent of energy consumption. Through our research on the Ancient Egyptians, we believe that the softest material determines the friction force and increases the friction surface. If that's true, we don't just understand better how it works. But we can also better design products that come into contact with other objects. Think, for example, of non-stick pans and stronger car tires.”