Living in a physical world VII. Gravity and life on the ground.

Unless some energy-demanding process counteracts its effect, gravity inevitably makes aerial life descend. For terrestrial life, gravity acts less obviously, less immediately, and less consistently. Sometimes it matters; sometimes other agencies eclipse its effects. Sometimes it acts as impediment or nuisance; sometimes it plays a crucial positive role. In short, gravity has more diverse consequences and has elicited a wider range of biological devices for organisms that live on the ground. For one thing, much more depends on the distinction between gravity, thus weight, and inertia, thus mass. Steadily lift an object, and you work against gravity; pull downward, and you enlist gravity’s assistance. Sliding an object steadily sideways may entail no irreduceable resistance, but the frictional force you do feel still comes from gravity, from the press of the object against the substratum. But accelerate an object, and you work against its mass. Big Neanderthal thrusting spears put gravity to use, working best with heavy bodies that leaned forward over well-planted feet to get sufficient purchase on the ground. Lighter, thrown spears depended more on inertial mass – a running body, in effect no purchase at all, could aid a launch. Similarly, a lighter person has to lean further outward when opening a substantial door. The lesser weight needs to be more effectively applied to produce the sideways force that will accelerate the mass of the door. Muscularity is a secondary matter. For another, organisms consist of both solids and liquids. In practice the two phases of matter face gravity in slightly different guises that reflect the difference between compressive stress and hydrostatic pressure. Both variables have dimensions of force per unit area, but in a specific direction for stress while omnidirectional for pressure. Stack solid bricks ever higher (with pads between to ensure uniform force transfer), and eventually the lowest will crush. That crushing point is reached when the compressive strength of brick, or force-resistance relative to cross section, of about 20 MPa (or MN m –2 ), equals the weight of the column relative to cross section. If made of bricks whose density is 2000 kg m –3 , the column will be about 1000 m high. Taper changes the picture – a column tapering upward can extend farther; one expanding upward will not reach as far. With similar reasoning, Weisskopf (1975) estimated the maximum height of a mountain as 10 km, about 10% higher than our present highest; in his analysis, plastic flow rather than crushing set the limit, so taper mattered little. Extend a pipe of liquid water upward in the air, and the pipe eventually bursts at ground level. The column of water extending upward stresses (in the sense above) the material of the pipe, but it does so in proportion only to the height of the column – cross section and contained volume have no direct relevance. The pressure difference, ∆p, across the walls of the pipe will be the product of the liquid’s density, ρ, gravitational acceleration, g, and the column’s height, h, in the familiar equation for both manometry and conversions of pressure units: