When length equals area

The graph of hyperbolic cosine is called a catenary. A catenary has the following curious property: the length of a catenary between two points equals the area under the catenary between those two points.

The proof is surprisingly simple. Start with the following:

\sqrt{1 +\left(\frac{d}{dx} \cosh(x) \right)^2} = \sqrt{1 + \sinh^2(x)} = \cosh(x)

Now integrate the first and last expressions between two points a and b. Note that the former integral gives the arc length of cosh between a and b, and the later integral gives the area under the graph of cosh between a and b.

By the way, the most famous catenary may be the Gateway Arch in St. Louis, Missouri.

Gateway Arch at night

4 thoughts on “When length equals area

  1. Hey John – found this interesting detail about the St. Louis arch.

    “The Gateway Arch in St. Louis, Missouri, United States is sometimes said to be an (inverted) catenary, but this is incorrect.[19] It is close to a more general curve called a flattened catenary, with equation y = A cosh(Bx), which is a catenary if AB = 1. While a catenary is the ideal shape for a freestanding arch of constant thickness, the Gateway Arch is narrower near the top. According to the U.S. National Historic Landmark nomination for the arch, it is a “weighted catenary” instead. Its shape corresponds to the shape that a weighted chain, having lighter links in the middle, would form.”

  2. I never knew that about catenaries. It vaguely feels like some kind of dual to Kepler’s Law that says planets sweep out equal area per time. Any insight on that?

  3. Maybe somewhat related, what I found extremely interesting in school was the Neile-Parabola, in which the arc length is the function itself composed with a polynomial.

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