Hanging cables and spider threads

TL;DR

This paper explores the mathematical modeling of hanging cables and spider threads, detailing how to determine their shape, tension, and parameters through nonlinear equations and iterative numerical methods.

Contribution

It provides a comprehensive method to compute the shape and tension of both inelastic and elastic hanging cables using nonlinear equations and Newton iteration.

Findings

Nonlinear equations are essential for calculating cable tension.

Two-dimensional Newton iteration effectively solves coupled equations for elastic threads.

Explicit procedures for scaling and shifting hyperbolic cosine shapes are provided.

Abstract

It has been known for more than 300 years that the shape of an inelastic hanging cable, chain, or rope of uniform linear mass density is the graph of the hyperbolic cosine, up to scaling and shifting coordinates. But given two points at which the ends of the cable are attached, how exactly should we scale and shift the coordinates? Many otherwise excellent expositions of the problem are a little vague about that. They might for instance give the answer in terms of the tension at the lowest point, but without explaining how to compute that tension. Here we discuss how to obtain all necessary parameters. To obtain the tension at the lowest point, one has to solve a nonlinear equation numerically. When the two ends of the cable are attached at different heights, a second nonlinear equation must be solved to determine the location of the lowest point. When the cable is elastic, think of a thread in a spider's web for instance, the two equations can no longer be decoupled, but they can be solved using two-dimensional Newton iteration.