A cycloid is the curve traced by a point on the rim of a circular wheel as the wheel rolls along a straight line. It is an example of a roulette, a curve generated by a curve rolling on another curve.
The cycloid is the solution to the brachistochrone problem (i.e., it is the curve of fastest descent under gravity) and the related tautochrone problem (i.e., the period of an object in descent without friction inside this curve does not depend on the object's starting position).
In 1658, Christopher Wren showed that the length of one arch of a cycloid is four times the diameter of its generating circle. The cycloid has been called "The Helen of Geometers" as it caused frequent quarrels among 17th-century mathematicians.
The cycloid through the origin, generated by a circle of radius r, consists of the points (x, y), with
Solving for t and replacing, the Cartesian equation would be
The first arch of the cycloid consists of points such that
When y is viewed as a function of x, the cycloid is differentiable everywhere except at the cusps where it hits the x-axis, with the derivative tending toward or as one approaches a cusp. The map from t to (x, y) is a differentiable curve or parametric curve of class C∞ and the singularity where the derivative is 0 is an ordinary cusp.
The cycloid satisfies the differential equation:
One arch of a cycloid generated by a circle of radius r can be parameterized by
the area under the arch is
Arc length 
The arc length S of one arch is given by
Cycloidal pendulum 
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If a simple pendulum is suspended from the cusp of an inverted cycloid, such that the "string" is constrained between the adjacent arcs of the cycloid, and the pendulum's length is equal to that of half the arc length of the cycloid (i.e., twice the diameter of the generating circle), the bob of the pendulum also traces a cycloid path. Such a cycloidal pendulum is isochronous, regardless of amplitude. The equation of motion is given by:
Related curves 
Several curves are related to the cycloid.
- Curtate cycloid: Here the point tracing out the curve is inside the circle, which rolls on a line.
- Prolate cycloid: Here the point tracing out the curve is outside the circle, which rolls on a line.
- Trochoid: refers to any of the cycloid, the curtate cycloid and the prolate cycloid.
- Hypocycloid: The point is on the edge of the circle, which rolls not on a line but on the inside of another circle.
- Epicycloid: The point is on the edge of the circle, which rolls not on a line but on the outside of another circle.
- Hypotrochoid: As hypocycloid but the point need not be on the edge of its circle.
- Epitrochoid: As epicycloid but the point need not be on the edge of its circle.
All these curves are roulettes with a circle rolled along a uniform curvature. The cycloid, epicycloids, and hypocycloids have the property that each is similar to its evolute. If q is the product of that curvature with the circle's radius, signed positive for epi- and negative for hypo-, then the curve:evolute similitude ratio is 1 + 2q.
Use in architecture 
Use in violin plate arching 
Early research indicated that some transverse arching curves of the plates of golden age violins are closely modeled by curtate cycloid curves. Later work indicates that curate cycloids do not serve as general models for these curves, which vary considerably.
See also 
- List of periodic functions
- Tautochrone curve
- Cajori, Florian (1999). A History of Mathematics. New York: Chelsea. p. 177. ISBN 978-0-8218-2102-2.
- C. Huygens, "The Pendulum Clock or Geometrical Demonstrations Concerning the Motion of Pendula (sic) as Applied to Clocks," Translated by R. J. Blackwell, Iowa State University Press (Ames, Iowa, USA, 1986).
- Playfair, Q. "Curtate Cycloid Arching in Golden Age Cremonese Violin Family Instruments". Catgut Acoustical Society Journal. II 4 (7): 48–58.
- Mottola, RM (2011). "Comparison of Arching Profiles of Golden Age Cremonese Violins and Some Mathematically Generated Curves". Savart Journal 1 (1).
Further reading 
- An application from physics: Ghatak, A. & Mahadevan, L. Crack street: the cycloidal wake of a cylinder tearing through a sheet. Physical Review Letters, 91, (2003). link.aps.org
- Edward Kasner & James Newman (1940) Mathematics and the Imagination, pp 196–200, Simon & Schuster.
- Wells D (1991). The Penguin Dictionary of Curious and Interesting Geometry. New York: Penguin Books. pp. 445–47. ISBN 0-14-011813-6.
- O'Connor, John J.; Robertson, Edmund F., "Cycloid", MacTutor History of Mathematics archive, University of St Andrews.
- Weisstein, Eric W., "Cycloid", MathWorld. Retrieved April 27, 2007.
- Cycloids at cut-the-knot
- A Treatise on The Cycloid and all forms of Cycloidal Curves, monograph by Richard A. Proctor, B.A. posted by Cornell University Library.
- Cycloid Curves by Sean Madsen with contributions by David von Seggern, Wolfram Demonstrations Project.
- Cycloid on PlanetPTC (Mathcad