Adrian Bejan | Lifetime of Rolling Stones, from Design in Nature
In this video, Adrian Bejan asks why bigger things live longer, a pattern long noticed in animals but never predicted from physics. He argues that life and travel are really the same idea, so a rule that works for animals should work for anything that moves, from vehicles and rivers to winds and rolling stones. Using a single rolling stone that slowly comes to a stop, he shows how its kinetic energy, worn away by friction, sets both how far it travels and how long it lasts. He then asks how many rolls a stone makes before stopping, and uses the constructal law to explain why moving bodies, from eroding rocks to the human-made wheel, evolve toward easier rolling.
Why the bigger live longer. Bejan starts from a trend in zoology, sometimes called animal design, that bigger animals live longer, the same way big dogs outlive tiny ones. His aim is not just to describe this pattern but to predict it, since describing something is not the same as explaining why it must happen.
Life and travel are the same thing. A large river carries water far and lasts a long time, while milk spilled on the kitchen floor goes nowhere and is gone in moments. Because moving and living are tied together, the same rule should hold for animals, vehicles, rivers, winds, and rolling stones alike.
A rolling stone tells the story. A stone hurtling downhill carries kinetic energy that friction slowly wears away until it stops, having covered a certain distance over a certain lifetime. From this single balance, Bejan predicts that more initial speed buys both a longer journey and a longer life.
Lifespan grows with the square root of travel. A surprising result is that lifetime rises only with the square root of the distance covered. As Bejan puts it, moving farther when you are old does not make you live longer.
Why stones become round. Counting how many times a stone rolls before stopping gives a simple answer, few rolls for rough shapes and many more for smooth ones, just as a ball bearing rolls far more freely than a jagged rock. The constructal law predicts that a moving stone keeps changing toward less friction, becoming rounder rather than breaking apart.
The same pull toward easier rolling. This drive toward smoother movement runs from eroding rocks to rivers cutting their channels, and even to the human biped, which Bejan pictures as a wheel with only two spokes. About 10,000 years ago the human-made wheel appeared and evolved from solid disks toward spoked forms, following the very same direction.
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