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Simple systems are controlled by one, or a few, parameters. Step on the accelerator, and the car speeds up (other things being equal). Warm the bread dough, and the yeast cells divide quicker (within limits). Increase the speed of a projectile, and it flies farther (ignoring other issues).

Complex systems are controlled by a large number of factors, sometimes so many that building an accurate predictive model is virtually impossible. (Global climate may be one such case.) But often, within a given regime, one parameter clearly dominates. (For the Earth's climate today, carbon dioxide is probably that parameter.)

Consider running — unaided human locomotion. For various overlapping distance ranges I naïvely speculate:

mileslimiting factor
0 - 0.2mechanical force — accelerating the mass of the body
0.1 - 2oxygen intake — getting enough oxidizer to the cells
0.5 - 10waste removal — carrying off lactic acid or other muscle activity byproducts
5 - 30stored energy — metabolizing fuels already present in the body
15+ ...fuel input — absorbing additional sugars via digestive system
20+ ...electrolyte balance — maintaining proper concentrations of Na, K, etc.
30+ ...mechanical breakdown — blistering, bruising, joint damage, etc.
50+ ...brain chemistry — continuing key mental functions

(cf. PushingTheEnvelope (25 Aug 1999), EnvelopePushing (24 Apr 2003), ...)

TopicRunning - TopicScience - Datetag20061016

(correlates: DelicatePower, SafetyFirst, ChivalrousReasoning, ...)