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parabola, plane curve consisting of all points equidistant from a given fixed point (focus) and a given fixed line (directrix). It is the conic section cut by a plane parallel to one of the elements of the cone. The axis of a parabola is the line through the focus perpendicular to the directrix. The vertex is the point at which the axis intersects the curve. The *latus rectum* is the chord through the focus perpendicular to the axis. Examples of this curve are the path of a projectile and the shape of the cross section of a parallel beam reflector.

The Columbia Electronic Encyclopedia Copyright © 2004.

Licensed from Columbia University Press

Licensed from Columbia University Press

Open curve, one of the conic sections. It results when a right circular cone intersects a plane that is parallel to an edge of the cone. It is also the path of a point moving so that its distance from a fixed line (directrix) is always equal to its distance from a fixed point (focus). In analytic geometry its equation is *math.y* = *math.a**math.x*^{2} + *math.b**math.x* + *math.c* (a second-degree, or quadratic, polynomial function). Such a curve has the useful property that any line parallel to its axis of symmetry reflects through its focus, and vice versa. Rotating a parabola about its axis produces a surface (paraboloid) with the same reflection property, making it an ideal shape for satellite dishes and reflectors in headlights. Parabolas occur naturally as the paths of projectiles. The shape is also seen in the design of bridges and arches.

Learn more about parabola with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

In mathematics, the parabola (from the Greek παραβολή) is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface. Given a point (the focus) and a line (the directrix) that lie in a plane, the locus of points in that plane that are equidistant to them is a parabola.

A particular case arises when the plane is tangent to the conical surface of a circle. In this case, the intersection is a degenerate parabola consisting of a straight line.

The parabola is an important concept in abstract mathematics, but it is also seen with considerable frequency in the physical world, and there are many practical applications for the construct in engineering, physics, and other domains.

In Cartesian coordinates, a parabola with an axis parallel to the $y,!$ axis with vertex $(h,\; k),!$, focus $(h,\; k\; +\; p),!$, and directrix $y\; =\; k\; -\; p,!$, with $p,!$ being the distance from the vertex to the focus, has the equation with axis parallel to the y-axis.

- $(x\; -\; h)^2\; =\; 4p(y\; -\; k)\; ,$

or, alternatively with axis parallel to the x-axis

- $(y\; -\; k)^2\; =\; 4p(x\; -\; h)\; ,$

More generally, a parabola is a curve in the Cartesian plane defined by an irreducible equation of the form

- $A\; x^2\; +\; B\; xy\; +\; C\; y^2\; +\; D\; x\; +\; E\; y\; +\; F\; =\; 0\; ,$

such that $B^2\; =\; 4\; AC\; ,$, where all of the coefficients are real, where $A\; not=\; 0\; ,$ or $C\; not=\; 0\; ,$, and where more than one solution, defining a pair of points (x, y) on the parabola, exists. That the equation is irreducible means it does not factor as a product of two not necessarily distinct linear equations.

A parabola may also be characterized as a conic section with an eccentricity of 1. As a consequence of this, all parabolas are similar. A parabola can also be obtained as the limit of a sequence of ellipses where one focus is kept fixed as the other is allowed to move arbitrarily far away in one direction. In this sense, a parabola may be considered an ellipse that has one focus at infinity. The parabola is an inverse transform of a cardioid.

A parabola has a single axis of reflective symmetry, which passes through its focus and is perpendicular to its directrix. The point of intersection of this axis and the parabola is called the vertex. A parabola spun about this axis in three dimensions traces out a shape known as a paraboloid of revolution.

The parabola is found in numerous situations in the physical world (see below).

- $(x\; -\; h)^2\; =\; 4p(y\; -\; k)\; ,$

- $y\; =\; k\; ,$

- $y\; =\; ax^2\; +\; bx\; +\; c\; ,$

- $mbox\{where\; \}a\; =\; frac\{1\}\{4p\};\; b\; =\; frac\{-h\}\{2p\};\; c\; =\; frac\{h^2\}\{4p\}\; +\; k;$

- $h\; =\; frac\{-b\}\{2a\};\; k\; =\; frac\{4ac\; -\; b^2\}\{4a\}$.

- $x(t)\; =\; 2pt\; +\; h;\; y(t)\; =\; pt^2\; +\; k\; ,$

- $(y\; -\; k)^2\; =\; 4p(x\; -\; h)\; ,$

- $x\; =\; a(y\; -\; k)^2\; +\; h\; ,$

- $x\; =\; ay^2\; +\; by\; +\; c\; ,$

- $mbox\{where\; \}a\; =\; frac\{1\}\{4p\};\; b\; =\; frac\{-k\}\{2p\};\; c\; =\; frac\{k^2\}\{4p\}\; +\; h;$

- $h\; =\; frac\{4ac\; -\; b^2\}\{4a\};\; k\; =\; frac\{-b\}\{2a\}$.

- $x(t)\; =\; pt^2\; +\; h;\; y(t)\; =\; 2pt\; +\; k\; ,$'''

- $(Ax+By)^2\; +\; Cx\; +\; Dy\; +\; E\; =\; 0\; ,$

In polar coordinates, a parabola with the focus at the origin and the directrix parallel to the y-axis, is given by the equation

- $r\; (1\; +\; cos\; theta)\; =\; l\; ,$

where l is the semilatus rectum: the distance from the focus to the parabola itself, measured along a line perpendicular to the axis. Note that this is twice the distance from the focus to the apex of the parabola or the perpendicular distance from the focus to the latus rectum.

The latus rectum is the chord that passes through the focus and is perpendicular to the axis. It has a length of 4l.

A Gauss-mapped form: $(tan^2phi,2tanphi)$ has normal $(cosphi,sinphi)$.

Given a parabola whose axis of symmetry is parallel to the y-axis with vertex (0,0) and with equation

- $y\; =\; a\; x^2,\; qquad\; qquad\; qquad\; (1)$

Let F denote the focus, and let Q denote the point at (x,-f). Line FP has the same length as line QP.

- $|\; FP\; |\; =\; sqrt\{\; x^2\; +\; (y\; -\; f)^2\; \},$

- $|\; QP\; |\; =\; y\; +\; f.$

- $|\; FP\; |\; =\; |\; QP\; |$

- $sqrt\{x^2\; +\; (a\; x^2\; -\; f)^2\; \}\; =\; a\; x^2\; +\; f\; qquad$

- $x^2\; +\; a^2\; x^4\; +\; f^2\; -\; 2\; a\; x^2\; f\; =\; a^2\; x^4\; +\; f^2\; +\; 2\; a\; x^2\; f\; quad$

- $x^2\; -\; 2\; a\; x^2\; f\; =\; 2\; a\; x^2\; f,\; quad$

- $x^2\; =\; 4\; a\; x^2\; f.\; quad$

- $1\; =\; 4\; a\; f\; quad$

- $f\; =\; \{1\; over\; 4\; a\; \}$

- $x^2\; =\; 4\; p\; y\; quad$

All this was for a parabola centered at the origin. For any generalized parabola, with its equation given in the standard form

- $y=ax^2+bx+c$,

the focus is located at the point

- $left\; (frac\{-b\}\{2a\},frac\{-b^2\}\{4a\}+c+frac\{1\}\{4a\}\; right)$

which may also be written as

- $left\; (frac\{-b\}\{2a\},c-frac\{b^2-1\}\{4a\}\; right)$

and the directrix is designated by the equation

- $y=frac\{-b^2\}\{4a\}+c-frac\{1\}\{4a\}$

which may also be written as

- $y=c-frac\{b^2+1\}\{4a\}$

The tangent of the parabola described by equation (1) has slope

- $\{dy\; over\; dx\}\; =\; 2\; a\; x\; =\; \{2\; y\; over\; x\}$

- $F\; =\; (0,f),\; quad$

- $Q\; =\; (x,-f),\; quad$

- $\{F\; +\; Q\; over\; 2\}\; =\; \{(0,f)\; +\; (x,-f)\; over\; 2\}\; =\; \{(x,0)\; over\; 2\}\; =\; (\{x\; over\; 2\},\; 0).$

- $|\; FG\; |\; cong\; |\; GQ\; |,$

- $|\; PF\; |\; cong\; |\; PQ\; |,$

- $Delta\; FGP\; cong\; Delta\; QGP$

It follows that $angle\; FPG\; cong\; angle\; GPQ$.

Line QP can be extended beyond P to some point T, and line GP can be extended beyond P to some point R. Then $angle\; RPT$ and $angle\; GPQ$ are vertical, so they are equal (congruent). But $angle\; GPQ$ is equal to $angle\; FPG$. Therefore $angle\; RPT$ is equal to $angle\; FPG$.

The line RG is tangent to the parabola at P, so any light beam bouncing off point P will behave as if line RG were a mirror and it were bouncing off that mirror.

Let a light beam travel down the vertical line TP and bounce off from P. The beam's angle of inclination from the mirror is $angle\; RPT$, so when it bounces off, its angle of inclination must be equal to $angle\; RPT$. But $angle\; FPG$ has been shown to be equal to $angle\; RPT$. Therefore the beam bounces off along the line FP: directly towards the focus.

Conclusion: Any light beam moving vertically downwards in the concavity of the parabola (parallel to the axis of symmetry) will bounce off the parabola moving directly towards the focus. (See parabolic reflector.)

We know the x-coordinate at the vertex is $x=-frac\{b\}\{2a\}$, so substitute it into the equation $y=ax^2+bx+c$

- $y=aleft\; (-frac\{b\}\{2a\}right\; )^2\; +\; b\; left\; (-frac\{b\}\{2a\}\; right\; )\; +\; cqquadtextrm\{Then~simplifyldots\}$

- $=frac\{ab^2\}\{4a^2\}\; -frac\{b^2\}\{2a\}\; +\; c$

- $=frac\{b^2\}\{4a\}\; -frac\{2cdot\; b^2\}\{2cdot\; 2a\}\; +\; ccdotfrac\{4a\}\{4a\}$

- $=frac\{-b^2+4ac\}\{4a\}$

- $=-frac\{b^2-4ac\}\{4a\}=-frac\{D\}\{4a\}$

Thus, the vertex is at point…

- $left\; (-frac\{b\}\{2a\},-frac\{D\}\{4a\}right\; )$

In nature, approximations of parabolas and paraboloids are found in many diverse situations. The most well-known instance of the parabola in the history of physics is the trajectory of a particle or body in motion under the influence of a uniform gravitational field without air resistance (for instance, a baseball flying through the air, neglecting air friction). The parabolic trajectory of projectiles was discovered experimentally by Galileo in the early 17th century, who performed experiments with balls rolling on inclined planes. He also later proved this mathematically in his book 'Dialogue Concerning Two New Sciences'. For objects extended in space, such as a diver jumping from a diving board, the object itself follows a complex motion as it rotates, but the center of mass of the object nevertheless forms a parabola. As in all cases in the physical world, the trajectory is always an approximation of a parabola. The presence of air resistance, for example, always distorts the shape, although at low speeds, the shape is a good approximation of a parabola. At higher speeds, such as in ballistics, the shape is highly distorted and does not resemble a parabola.

Another situation in which parabola may arise in nature is in two-body orbits, for example, of a small planetoid or other object under the influence of the gravitation of the sun. Such parabolic orbits are a special case that are rarely found in nature. Orbits that form a hyperbola or an ellipse are much more common. In fact, the parabolic orbit is the borderline case between those two types of orbit. An object following a parabolic orbit moves at the exact escape velocity of the object it is orbiting, while elliptical orbits are slower and hyperbolic orbits are faster.

Approximations of parabolas are also found in the shape of cables of suspension bridges. Freely hanging cables do not describe parabolas, but rather catenary curves. Under the influence of a uniform load (for example, the deck of bridge), however, the cable is deformed toward a parabola.

Paraboloids arise in several physical situations as well. The most well-known instance is the parabolic reflector, which is a mirror or similar reflective device that concentrates light or other forms of electromagnetic radiation to a common focal point. The principle of the parabolic reflector may have been discovered in the 3rd century BC by the geometer Archimedes, who, according to a legend of debatable veracity, constructed parabolic mirrors to defend Syracuse against the Roman fleet, by concentrating the sun's rays to set fire to the decks of the Roman ships. The principle was applied to telescopes in the 17th century. Today, paraboloid reflectors can be commonly observed throughout much of the world in microwave and satellite dish antennas.

Paraboloids are also observed in the surface of a liquid confined to a container and rotated around the central axis. In this case, the centrifugal force causes the liquid to climb the walls of the container, forming a parabolic surface. This is the principle behind the liquid mirror telescope.

Aircraft used to create a weightless state for purposes of experimentation, such as NASA's “Vomit Comet,” follow a vertically parabolic trajectory for brief periods in order to trace the course of an object in free fall, which produces the same effect as zero gravity for most purposes.

- Catenary
- Conic section
- Ellipse
- Hyperbola
- Parabolic constant
- Parabolic mirror
- Parabolic partial differential equation
- Paraboloid

- Interactive parabola-drag focus, see axis of symmetry, directrix, standard and vertex forms
- Archimedes Triangle and Squaring of Parabola at cut-the-knot
- Two Tangents to Parabola at cut-the-knot
- Parabola As Envelope of Straight Lines at cut-the-knot
- Parabolic Mirror at cut-the-knot
- Three Parabola Tangents at cut-the-knot
- Module for the Tangent Parabola
- Focal Properties of Parabola at cut-the-knot
- Parabola As Envelope II at cut-the-knot
- Parabola Construction - An interactive sketch showing how to trace a parabola. (Requires Java.)
- Quadratic Bezier Construction - An interactive sketch showing how to trace the quadratic Bezier curve (a parabolic segment). (Requires Java.)
- More Interactive Parabola Construction (Java-enabled)

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Last updated on Monday October 06, 2008 at 04:41:56 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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