Ancient Egyptians had to pull massive
statues and pyramid stones weighing 2.5 tons on large sleds across the
desert — without any modern
mechanical device. Now, new research
shows how adding a small amount of water to sand significantly reduces
the sliding friction — a clever trick that allowed the Egyptians to cut
the number of workers needed by half.
mechanical device. Now, new research
shows how adding a small amount of water to sand significantly reduces
the sliding friction — a clever trick that allowed the Egyptians to cut
the number of workers needed by half.
To make a good sandcastle, you don’t
use dry sand. By adding water, the grains stick to each other, and your
castle holds its shape. Same thing with sand transportation: Adding water reduces the sliding friction of any object moving over the sand. With the right amount of dampness, water droplets bind the sand grains together.
An international team led by Daniel Bonn from the
of the sled, hindering its movement. And as they added water, both the
force needed to pull the sled and the amount of friction decreased. As
the water made the sand more rigid, the heaps got smaller and smaller
until there was no obstacle forming in front of the moving sled.
Bonn from the
University
of Amsterdam tested the sliding friction of dry and wet sand by pulling a weighted sled across the surface in a tray. With dry sand, a heap would form in front
of the sled, hindering its movement. And as they added water, both the
force needed to pull the sled and the amount of friction decreased. As
the water made the sand more rigid, the heaps got smaller and smaller
until there was no obstacle forming in front of the moving sled.
Their experiments revealed that the
required pulling force decreased proportional to the stiffness of the
sand. When water was added, capillary bridges arose; these small water
droplets act like glue to bind the sand grains together. With the right
amount of water, wet desert sand is about twice as stiff as dry sand,
allowing the sled to glide far more easily.
“I was very surprised by the amount the pulling force could be reduced
— by as much as 50 percent — meaning that the Egyptians needed only
half the men to pull over wet sand as compared to dry,” Bonn tells the Washington Post.
Pictured here is the lab setup: A pile of sand accumulates in front of
the sled when it’s pulled over dry sand (left), but not with wet sand
(right).
— by as much as 50 percent — meaning that the Egyptians needed only
half the men to pull over wet sand as compared to dry,” Bonn tells the Washington Post.
Pictured here is the lab setup: A pile of sand accumulates in front of
the sled when it’s pulled over dry sand (left), but not with wet sand
(right).
high water
contents, the capillary bridges (which used to act like a glue) start
to merge and disappear, and the sliding friction increases again. It’s a
delicate balance. “If you use dry sand, it won’t work as well, but if
the sand is too wet, it won’t work either,” Bonn tells LiveScience.
“There’s an optimum stiffness.” The ideal amount of water falls between 2
and 5 percent of the volume of sand.
The answer had been staring us in the face for a long time. In a wall painting from the tomb of Djehutihotep (schematic above), you can see a worker pouring water on the sand in front
of a sled that’s carrying a colossal statue. The sleds were little more
than large wooden planks with upturned edges. “Egyptologists had been
interpreting the water as part of a purification ritual,” Bonn says,
“and had never sought a scientific explanation.”
Πηγή : iflscience.com for a long time. In a wall painting from the tomb of Djehutihotep (schematic above), you can see a worker pouring water on the sand in front
of a sled that’s carrying a colossal statue. The sleds were little more
than large wooden planks with upturned edges. “Egyptologists had been
interpreting the water as part of a purification ritual,” Bonn says,
“and had never sought a scientific explanation.”
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