Catenary
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In physics and geometry, a catenary is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends in a uniform gravitational field.
A chain hanging from points forms a catenary.
Freely-hanging overhead power lines also form a catenary (most prominently visible with high-voltage lines, and with some imperfection near to the insulators).
The silk on a spider's web forming multiple elastic catenaries.
Antoni Gaudí's catenary model at Casa Milà
Minimal surface
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In mathematics, a minimal surface is a surface that locally minimizes its area. This is equivalent to having zero mean curvature.
Circus tent approximates a minimal surface.