Minimal surface of revolution

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Stretching a soap film between two parallel circular wire loops generates a catenoidal minimal surface of revolution Bulle catenoide.png
Stretching a soap film between two parallel circular wire loops generates a catenoidal minimal surface of revolution

In mathematics, a minimal surface of revolution or minimum surface of revolution is a surface of revolution defined from two points in a half-plane, whose boundary is the axis of revolution of the surface. It is generated by a curve that lies in the half-plane and connects the two points; among all the surfaces that can be generated in this way, it is the one that minimizes the surface area. [1] A basic problem in the calculus of variations is finding the curve between two points that produces this minimal surface of revolution. [1]

Contents

Relation to minimal surfaces

A minimal surface of revolution is a subtype of minimal surface. [1] A minimal surface is defined not as a surface of minimal area, but as a surface with a mean curvature of 0. [2] Since a mean curvature of 0 is a necessary condition of a surface of minimal area, all minimal surfaces of revolution are minimal surfaces, but not all minimal surfaces are minimal surfaces of revolution. As a point forms a circle when rotated about an axis, finding the minimal surface of revolution is equivalent to finding the minimal surface passing through two circular wireframes. [1] A physical realization of a minimal surface of revolution is soap film stretched between two parallel circular wires: the soap film naturally takes on the shape with least surface area. [3] [4]

Catenoid solution

A catenoid Catenoid.svg
A catenoid

If the half-plane containing the two points and the axis of revolution is given Cartesian coordinates, making the axis of revolution into the x-axis of the coordinate system, then the curve connecting the points may be interpreted as the graph of a function. If the Cartesian coordinates of the two given points are , , then the area of the surface generated by a nonnegative differentiable function may be expressed mathematically as

and the problem of finding the minimal surface of revolution becomes one of finding the function that minimizes this integral, subject to the boundary conditions that and . [5] In this case, the optimal curve will necessarily be a catenary. [1] [5] The axis of revolution is the directrix of the catenary, and the minimal surface of revolution will thus be a catenoid. [1] [6] [7]

Goldschmidt solution

Solutions based on discontinuous functions may also be defined. In particular, for some placements of the two points the optimal solution is generated by a discontinuous function that is nonzero at the two points and zero everywhere else. This function leads to a surface of revolution consisting of two circular disks, one for each point, connected by a degenerate line segment along the axis of revolution. This is known as a Goldschmidt solution [5] [8] after German mathematician Carl Wolfgang Benjamin Goldschmidt, [4] who announced his discovery of it in his 1831 paper "Determinatio superficiei minimae rotatione curvae data duo puncta jungentis circa datum axem ortae" ("Determination of the surface-minimal rotation curve given two joined points about a given axis of origin"). [9]

To continue the physical analogy of soap film given above, these Goldschmidt solutions can be visualized as instances in which the soap film breaks as the circular wires are stretched apart. [4] However, in a physical soap film, the connecting line segment would not be present. Additionally, if a soap film is stretched in this way, there is a range of distances within which the catenoid solution is still feasible but has greater area than the Goldschmidt solution, so the soap film may stretch into a configuration in which the area is a local minimum but not a global minimum. For distances greater than this range, the catenary that defines the catenoid crosses the x-axis and leads to a self-intersecting surface, so only the Goldschmidt solution is feasible. [10]

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Catenary Plane curve formed by a hanging cable

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.

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Cubic function Polynomial function of degree 3

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Catenoid

A catenoid is a type of surface, arising by rotating a catenary curve about an axis. It is a minimal surface, meaning that it occupies the least area when bounded by a closed space. It was formally described in 1744 by the mathematician Leonhard Euler.

Minimal surface Surface that locally minimizes its area

In mathematics, a minimal surface is a surface that locally minimizes its area. This is equivalent to having zero mean curvature.

Maxima and minima Largest and smallest value taken by a function takes at a given point

In mathematical analysis, the maxima and minima of a function, known collectively as extrema, are the largest and smallest value of the function, either within a given range, or on the entire domain. Pierre de Fermat was one of the first mathematicians to propose a general technique, adequality, for finding the maxima and minima of functions.

Surface of revolution Mathematical term

A surface of revolution is a surface in Euclidean space created by rotating a curve around an axis of rotation.

Helicoid Mathematical shape

The helicoid, after the plane and the catenoid, is the third minimal surface to be known.

Tractrix Curve traced by a point pulled through a segment of fixed length by a point moving on a line

A tractrix is the curve along which an object moves, under the influence of friction, when pulled on a horizontal plane by a line segment attached to a tractor (pulling) point that moves at a right angle to the initial line between the object and the puller at an infinitesimal speed. It is therefore a curve of pursuit. It was first introduced by Claude Perrault in 1670, and later studied by Isaac Newton (1676) and Christiaan Huygens (1692).

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Scherk surface

In mathematics, a Scherk surface is an example of a minimal surface. Scherk described two complete embedded minimal surfaces in 1834; his first surface is a doubly periodic surface, his second surface is singly periodic. They were the third non-trivial examples of minimal surfaces. The two surfaces are conjugates of each other.

Unduloid

In geometry, an unduloid, or onduloid, is a surface with constant nonzero mean curvature obtained as a surface of revolution of an elliptic catenary: that is, by rolling an ellipse along a fixed line, tracing the focus, and revolving the resulting curve around the line. In 1841 Delaunay proved that the only surfaces of revolution with constant mean curvature were the surfaces obtained by rotating the roulettes of the conics. These are the plane, cylinder, sphere, the catenoid, the unduloid and nodoid.

Differential geometry of surfaces

In mathematics, the differential geometry of surfaces deals with the differential geometry of smooth surfaces with various additional structures, most often, a Riemannian metric. Surfaces have been extensively studied from various perspectives: extrinsically, relating to their embedding in Euclidean space and intrinsically, reflecting their properties determined solely by the distance within the surface as measured along curves on the surface. One of the fundamental concepts investigated is the Gaussian curvature, first studied in depth by Carl Friedrich Gauss, who showed that curvature was an intrinsic property of a surface, independent of its isometric embedding in Euclidean space.

Unit circle

In mathematics, a unit circle is a circle of unit radius—that is, a radius of 1. Frequently, especially in trigonometry, the unit circle is the circle of radius 1 centered at the origin in the Cartesian coordinate system in the Euclidean plane. In topology, it is often denoted as S1 because it is a one-dimensional unit n-sphere.

Carl Wolfgang Benjamin Goldschmidt was a German astronomer, mathematician, and physicist of Jewish descent who was a professor of astronomy at the University of Göttingen. He is also known as Benjamin Goldschmidt, C. W. B. Goldschmidt, Carl Goldschmidt, and Karl Goldschmidt.

Bours minimal surface

In mathematics, Bour's minimal surface is a two-dimensional minimal surface, embedded with self-crossings into three-dimensional Euclidean space. It is named after Edmond Bour, whose work on minimal surfaces won him the 1861 mathematics prize of the French Academy of Sciences.

References

  1. 1 2 3 4 5 6 Weisstein, Eric W. "Minimal Surface of Revolution". Mathworld . Wolfram Research . Retrieved 2012-08-29.CS1 maint: discouraged parameter (link)
  2. Weisstein, Eric W. "Minimal Surface". Mathworld . Wolfram Research . Retrieved 2012-08-29.CS1 maint: discouraged parameter (link)
  3. Olver, Peter J. (2012). "Chapter 21: The Calculus of Variations". Applied Mathematics Lecture Notes (PDF). Retrieved 2012-08-29.CS1 maint: discouraged parameter (link)
  4. 1 2 3 Nahin, Paul J. (2011). When Least Is Best: How Mathematicians Discovered Many Clever Ways to Make Things as Small (or as Large) as Possible. Princeton University Press. pp. 265–6. So what happens to the soap film after it breaks [...]? This discontinuous behavior is called the Goldschmidt solution, after the German mathematician C. W. B. Goldschmidt (1807-51) who discovered it (on paper) in 1831.
  5. 1 2 3 Sagan, Hans (1992), "2.6 The problem of minimal surfaces of revolution", Introduction to the Calculus of Variations, Courier Dover Publications, pp. 62–66, ISBN   9780486673660
  6. Colding, Tobias Holck; Minicozzi II, William P. (2011). "Chapter 1: The Beginning of the Theory". A Course in Minimal Surfaces (PDF). Graduate Studies in Mathematics. American Mathematical Society . Retrieved 2012-08-29.CS1 maint: discouraged parameter (link)
  7. Meeks III, William H.; Pérez, Joaquín (2012). "Chapter 2.5: Some interesting examples of complete minimal surfaces.". A Survey on Classical Minimal Surface Theory (PDF). University Lectures Series. 60. American Mathematical Society . Retrieved 2012-08-29.CS1 maint: discouraged parameter (link)
  8. Weisstein, Eric W. "Goldschmidt Solution". Mathworld . Wolfram Research . Retrieved 2012-08-29.CS1 maint: discouraged parameter (link)
  9. "Bibliographic Information: Determinatio superficiei minimae rotatione curvae data duo puncta jungentis circa datum axem ortae". Google Books . Retrieved 2012-08-27.CS1 maint: discouraged parameter (link)
  10. Isenberg, Cyril (1992), The Science of Soap Films and Soap Bubbles, Courier Dover Publications, p. 165, ISBN   9780486269603 CS1 maint: discouraged parameter (link).