Each molecule of oxygen, nitrogen and water, as well as fart smell, scent of lilac or Corona Virus moves with through three dimensions in any possible direction depending upon all the physical forces acting on it. Each molecule of what we call air moves freely.
Heat speeds them up, collisions with other molecules and with bicyclists send it off in another direction, gravity keeps individual molecules from favoring the up direction but with a mass of a few or a few thousand atoms, gravity is not a huge influence.
In describing the motion of these molecules and the forces affecting them, I did not include wind resistance. Together the molecules of air are wind resistance, but they are so small that the forces on them are heat, gravity, and collisions. They move in what is effectively a vacuum.
The effect of trillions of molecules per second smacking into a rider from random directions at varying speeds is completely predictable in its total effect--a surprising and wonderful reality.
If a rider maintains 20 miles per hour in still air, that same rider will reach the same speed at the same effort an hour or a month or a year later. And assuming the same air density, the same effort will achieve the same speed in Belarus, Borneo, Botswana, Bosnia or Belgium.
The random motion of molecules in air has the effect of totally predictable wind resistance.
When the air moves collectively, when there is wind, the effect is exactly, predictably the same. Uncountable trillions of molecules of varying sizes and shapes moving in unpredictable directions with different speeds will cause exactly the same amount of friction on a car, bicycle, runner or airplane everywhere there is air.
Wind resistance is both invisible and unavoidable. When I feel strong, I leave my house and ride with the wind knowing that the exhilaration of riding 25mph in a 20mph tail wind will turn into a 12mph slog on the return leg. When I don’t feel so great, I ride into the wind first and give myself the tailwind at the end of the ride.
So much of the history of science is discovering that reality is not what anyone guessed or expected. Few of the ancient scientists could wrap their minds around the idea of atoms in a vacuum. Even some of the alchemists who provided the first experimental evidence for atoms could believe what they demonstrated.
Before atoms, air was considered a single substance. The discovery of atoms showed air is a complex mixture of molecules. Physicists then showed that the individual molecules of air, moving randomly, together became, in effect, that single substance the ancient scientists believed in. All that randomness taken together is as predictable as the motion of the moon. And at the same time any single molecule can and does move as randomly as a toddler in a room full of shiny toys.
