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If not trapped in an eddy, driftwood will cover great distances traveling with the current of moving water.
Why does wood float and driftwood drift?
Archimedes wrote on parchment in his treatise On Floating Bodies,
Any floating object displaces its own weight of fluid.
— Archimedes of Syracuse
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| On Floating Bodies |
Specific gravity is the ratio of the density of a substance to the density of a reference substance.Most wood floats on water because most wood is less dense than water.
— Wikipedia
The ratio of the density of white ash to the density of water is less than one, or about 0.7, so white ash floats.
Balsa is a particularly buoyant wood with a specific gravity of 0.16. Balsa has a density of 160 kg/m3. For comparison, the density of water is 1000 kg/m3. Balsa wood rides high on the water.
Thought Experiments
| Wood |
It floats!A small amount of water spills into the pie tin. If you weighed the spilled water, it would equal the weight of the wood. The wood displaces its own weight. Wood is less dense than water. |
Denser woods like box wood or ebony will sink because they are denser than water.
Rocks and minerals are denser than water so they will sink. Rocks are too dense to displace their own weight in water.
| Rock |
It sinks!A small amount of water spills into the pie tin. If you weighed the spilled water, it would be less than the weight of the rock. The rock is unable to displace its own weight. Rocks are more dense than water. |
Driftwood that had traveled from places unknown to the place I was standing was having a temporary layover (video below) on the Mississippi River. The driftwood was trapped between a submerged rock and the sandy shoreline as it was buffeted by wave cycles.
REFERENCES
- On Floating Bodies, Archimedes of Syracuse (287 BC – c. 212 BC)
- Specific Gravity, Wikipedia.
- Wood Density, The Engineering Toolbox.
