Driftwood

Adrian Pingstone

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.



REFERENCES

• On Floating Bodies, Archimedes of Syracuse (287 BC – c. 212 BC)
• Specific Gravity, Wikipedia.
• Wood Density, The Engineering Toolbox.

Water Revolutions

Temperature starts revolutions.

Probe any natural phenomena with the 5 Whys. Prepare yourself to leap from the concrete and observable to the speculative and spiritual by the third why.

I keep six honest serving-men
(They taught me all I knew);
Their names are What and Why and When
And How and Where and Who.
— Rudyard Kipling, excerpt from the tale of The Elephant's Child

If you examine any natural phenomena with the 5 Whys, one why is usually down to temperature. Temperature seems to be the engine of flow in much of nature.

Temperature drives change. It starts revolutions.

Fresh Water Lakes

"Who comes to a lake for water and sees the reflection of moon?"
― Rumi

Lake water undergoes a temperature-driven revolution during each seasonal cycle.

During the hottest and the coldest time of the year, lake water temperatures are sharply stratified from top to bottom. On the contrary, during the transitional periods from cold to warm, or from warm to cold, the water temperature in lakes overturns to become well-mixed (i.e., nearly homogeneous from top to bottom).

Seasonal Temperature Profile of a Lake in the Northern Hemisphere

Great Northern Loon

Temperature stratification is experienced by swimmers. During warm months, swimming near the surface, the water is pleasantly warm. Diving down to swim at depth, the water is refreshingly cool.

"When you swim you don't grab hold of the water, because if you do you will sink and drown. Instead you relax, and float."
― Alan Watts

The depth where the water temperature changes most rapidly, is the thermocline depth, or metalimnion.

During the coldest months, lakes might ice over. Ice and snow cover reflect incident solar radiation. A cold strata of denser water will overlie the less dense, warmer water below. Stratification foments ripeness for change.

Sunset over the ice of Lake Superior

Cause and effect of the seasonal overturn of lake water is orchestrated by the inter-related properties of water:

1. Temperature changes density; and
2. Density changes buoyancy.

The stratification that occurs during temperature extremes is inherently unstable, just as the well-mixed water of the transitional seasons is ripe for stratification.

The revolution is ongoing, repetitious, and sublimely beautiful in its simplicity.


REFERENCES

• Just So Stories, 1902, by Rudyard Kipling.
• Thermocline, Wikipedia.
• Lake Ecosystem, Wikipedia.