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(1) |
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(2) |
x | = | ![]() |
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= | ![]() |
(3) | |
y | = | ![]() |
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= | ![]() |
(4) |
If x(0)=a instead so the first point is at maximum radius (on the Circle), then the equations of the hypocycloid are
x | = | ![]() |
(5) |
y | = | ![]() |
(6) |
An n-cusped non-self-intersecting hypocycloid has a/b=n. A 2-cusped hypocycloid is a Line Segment (Steinhaus 1983, p. 145), as can be seen by setting a=b in equations (3) and (4) and noting that the equations simplify to
x | = | ![]() |
(7) |
y | = | 0. | (8) |
A 3-cusped hypocycloid is called a Deltoid
or Tricuspoid,
and a 4-cusped hypocycloid is called anAstroid.
If a/b is rational, the curve closes
on itself and has b cusps. If a/b
is Irrational,
the curve never closes and fills the entire interior of the Circle.
Let r be the radial distance from
a fixed point. For Radius
of Torsion
and Arc
Lengths, a hypocycloid can given by
the equation
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(9) |
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(10) |
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(11) |
The Arc Length of the hypocycloid can be computed as follows
x' | = | ![]() |
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= | ![]() |
(12) | |
y' | = | ![]() |
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= | ![]() |
(13) |
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(14) |
so
ds | = | ![]() |
(15) |
for .
Integrating,
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(16) |
The length of a single cusp is then
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(17) |
x | = | ![]() |
(18) |
y | = | ![]() |
(19) |
and
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(20) |
xy'-yx' | = | ![]() |
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= | ![]() |
(21) |
The Area of one cusp is then
A | = | ![]() |
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(22) |
If n=a/b is rational,
then after n cusps,
An | = | ![]() |
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= | ![]() |
(23) |
The equation of the hypocycloid can be put in a form which is useful
in the solution of Calculus
of Variations problems with radial symmetry. Consider the case ,
then
r2 | = | x2+y2 | |
= | ![]() |
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(24) |
But ,
so
,
which gives
(a-b)2+b2 | = | ![]() |
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(25) | |
2(a-b)b | = | ![]() |
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= | ![]() |
(26) |
Now let
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(27) |
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(28) |
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(29) |
r2 | = | ![]() |
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= | ![]() |
(30) |
The Polar
Angle is
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(31) |
b | = | ![]() |
(32) |
a-b | = | ![]() |
(33) |
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(34) |
so
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(35) |
Computing
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(36) |
then gives
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(37) |
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(38) |
This form is useful in the solution of the Sphere
with Tunnel problem, which is the generalization of theBrachistochrone
Problem, to find the shape of a tunnel drilled through a Sphere
(with
gravity
varying according to Gauss's
law for gravitation
) such
that the travel time between two points on the surface of the Sphere
under the force of
gravity
is minimized.
See also Astroid, Cycloid, Deltoid, Epicycloid
References
Bogomolny, A. ``Cycloids.'' http://www.cut-the-knot.com/pythagoras/cycloids.html.
Kreyszig, E. Differential Geometry. New York: Dover, 1991.
Lawrence, J. D. A Catalog of Special Plane Curves. New York: Dover, pp. 171-173, 1972.
Lee, X. ``Epicycloid and Hypocycloid.'' http://www.best.com/~xah/SpecialPlaneCurves_dir/EpiHypocycloid_dir/epiHypocycloid.html.
MacTutor History of Mathematics Archive. ``Hypocycloid.'' http://www-groups.dcs.st-and.ac.uk/~history/Curves/Hypocycloid.html.
Madachy, J. S. Madachy's Mathematical Recreations. New York: Dover, pp. 225-231, 1979.
Steinhaus, H. Mathematical Snapshots, 3rd American ed. New York: Oxford University Press, 1983.
Wagon, S. Mathematica in Action. New York: W. H. Freeman, pp. 50-52, 1991.
Yates, R. C. ``Epi- and Hypo-Cycloids.'' A
Handbook on Curves and Their Properties. Ann Arbor, MI:
J. W. Edwards, pp. 81-85, 1952.
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© 1996-9 Eric W. Weisstein