Archimedean Spiral

archimedes spiral
Archimedes's Spiral
archimedean spirals
Archemedean spirals.

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History

Studied by Archimedes (~287 BCE – ~212 BCE).

The reason parabolic spiral and hyperbolic spiral are so named is because their equation in polar system r*θ == 1 and r^2 == θ resembles the equation for hyperbola x*y == 1 and parabola x^2 == y in rectangular coordinates system. What's Fermat's involvement with parabolic spiral?

Hyperbolic spiral is also called reciprocal spiral, because it is the inverse curve of Archemedes' spiral, with inversion center at the center. The inversion curve of all Archemedean spirals with respect to a circle on center is another Archemedean spiral. (see below)

Description

Archimedean spiral is defined by the polar equation r == θ^n. Special names are given for some value of n.

archimedesSpiral archimedean spiral
Archimedean spirals
archimedeanSpiral2
n from 0 to 2
archimedean spiral archimedean spiral
Archimedean spirals
archimedeanSpiral1
r == θ^n, n from -1.6 to 0
archimedean spiral
Archimedean spiral

Formula

Properties

Inversion

The inverse curve of a Archimedean spiral with respect to the center is another Archimedean spiral scaled. Archimedean spiral in parametric form is {t^n*Cos[t], t^n*Sin[t]}. The inversion at origin with radius b of a point {x,y} is {(b^2*x)/(x^2 + y^2), (b^2*y)/(x^2 + y^2)}. Apply this to the parametric form and simply we get b^2*{Cos[t]*t^-n, Sin[t]*t^-n}, which is in polar form r==b^2*θ^(-n). When b==1, there's no scaling.

The inverse curve of Archimedes' spiral with inversion circle of radius 1 at center is the reciprocal spiral.

inversion of Archimedes's spiral inversion of Archimedes's spiral
Archimedes' spirals as inverse to each other. The yellow curve is the inversion circle. Inversion of Archimedes's spiral

The inverse curve of Fermat's spiral with inversion circle of radius 1 at center is the lituus.

inversion of lituus
Lituus and Fermat's spiral. The red curve is the Fermat's spiral. The blue curve is its inversion, which is a lituus scaled by 5^2. The yellow circle is the inversion circle with radius 5. Note that points inside the circle gets mapped to outside of the circle. The closer the point is to the origin, the farther is its corresponding point outside the circle.

Nested Tangent Circles

The tangent circles of Archimedes's spiral are all nested. need to proof that archimedes spiral's tangent circles are nested inside each other.

archimedean spiral
Tangent circles of a Archimedes's spiral.
archimedean spiral
A plot of the curvature function (2 + t^2)/(1 + t^2)^(3/2) of the polar function r==θ.

See also

equiangular spiral, Lituus, clothoid, Mathematics of Seashell Shapes, Spirals In Nature

galleries.nb.zip.

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Related Web Sites

See: Websites on Plane Curves, Printed References On Plane Curves.

Robert Yates: Curves and Their Properties.

The MacTutor History of Mathematics archive

Archimedes spiral.