## Saturday, March 28, 2015

### Driftwood

Wood and water make agreeable travel companions.

If not trapped in an eddy, driftwood will cover great distances traveling with the current of moving water.

A flat rock skipped across water eventually sinks. Wood floats.

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
 On Floating Bodies
On Floating Bodies is the first recorded work on hydrostatics. Archimedes found the buoyancy of a solid was determined by its form and its specific gravity.
Specific gravity is the ratio of the density of a substance to the density of a reference substance.
— Wikipedia
Most wood floats on water because most wood is less dense than water.

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 Place a full glass of water in a pie tin. Place a small block of wood in the water. 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 Place a full glass of water in a pie tin. Place a small rock in the water. 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.

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