# Orthogonal trajectory

﻿
Orthogonal trajectory

In mathematics, orthogonal trajectories are a family of curves in the plane that intersect a given family of curves at right angles. The problem is classical, but is now understood by means of complex analysis; see for example harmonic conjugate.

For a family of level curves described by g(x,y) = C, where C is a constant, the orthogonal trajectories may be found as the level curves of a new function f(x,y) by solving the partial differential equation

$\nabla f \cdot \nabla g = 0$

for f(x,y). This is literally a statement that the gradients of the functions (which are perpendicular to the curves) are orthogonal. Note that if f and g are functions of three variables instead of two, the equation above will be nonlinear and will specify orthogonal surfaces.

The partial differential equation may be avoided by instead equating the tangent of a parametric curve $\vec r(t)$ with the gradient of g(x,y):

$\frac{d}{d t}\left(\vec r(t)\right) = \nabla g$

which will result in two possibly coupled ordinary differential equations, whose solutions are the orthogonal trajectories. Note that with this formula, if g is a function of three variables its level sets are surfaces, and the family of curves $\vec r(t)$ are orthogonal to the surfaces.

## Example: circle

In polar coordinates, the family of circles centered about the origin is the level curves of

rR = 0

where R is the radius of the circle. Then the orthogonal trajectories are the level curves of f defined by:

$\nabla f \cdot \nabla (r - R) = 0$
$\left(\frac{\partial f}{\partial r}, \frac{1}{r} \frac{\partial f}{\partial \theta}\right) \cdot \left(\frac{\partial}{\partial r}(r - R), \frac{1}{r} \frac{\partial}{\partial \theta}(r - R)\right) = 0$
$\frac{\partial f}{\partial r} = 0 \quad \Rightarrow \quad f = f(\theta)$

The lack of complete boundary data prevents determining f(θ). However, we want our orthogonal trajectories to span every point on every circle, which means that f(θ) must have a range which at least include one period of rotation. Thus, the level curves of f(θ) = 0, with freedom to choose any f, are all of the θ = constant curves that intersect circles, which are (all of the) straight lines passing through the origin. Note that the dot product takes nearly the familiar form since polar coordinates are orthogonal.

The absence of boundary data is a good thing, as it makes solving the PDE simple as one doesn't need to contort the solution to any boundary. In general, though, it must be ensured that all of the trajectories are found.

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• orthogonal trajectory — Math. the locus of a point whose path cuts each curve of a family of curves at right angles. [1810 20] * * * ▪ mathematics  family of curves that intersect another family of curves at right angles (orthogonal; see figure >). Such families of… …   Universalium

• orthogonal trajectory — ortogonalioji trajektorija statusas T sritis fizika atitikmenys: angl. orthogonal trajectory vok. orthogonale Trajektorie, f rus. ортогональная траектория, f pranc. trajectoire orthogonale, f …   Fizikos terminų žodynas

• orthogonal trajectory — noun : a mathematical curve which cuts every curve of a given set at right angles * * * Math. the locus of a point whose path cuts each curve of a family of curves at right angles. [1810 20] …   Useful english dictionary

• Isogonal trajectory — When a family of curves intersects another family at a specific constant angle α, the first family is referred to as an isogonal family of the second one, and in this case it is said that every family is an isogonal trajectory of the other. It is …   Wikipedia

• List of mathematics articles (O) — NOTOC O O minimal theory O Nan group O(n) Obelus Oberwolfach Prize Object of the mind Object theory Oblate spheroid Oblate spheroidal coordinates Oblique projection Oblique reflection Observability Observability Gramian Observable subgroup… …   Wikipedia

• Harmonic conjugate — For geometric conjugate points, see Projective harmonic conjugates. In mathematics, a function defined on some open domain is said to have as a conjugate a function if and only if they are respectively real and imaginary part of a holomorphic… …   Wikipedia

• orthogonale Trajektorie — ortogonalioji trajektorija statusas T sritis fizika atitikmenys: angl. orthogonal trajectory vok. orthogonale Trajektorie, f rus. ортогональная траектория, f pranc. trajectoire orthogonale, f …   Fizikos terminų žodynas

• ortogonalioji trajektorija — statusas T sritis fizika atitikmenys: angl. orthogonal trajectory vok. orthogonale Trajektorie, f rus. ортогональная траектория, f pranc. trajectoire orthogonale, f …   Fizikos terminų žodynas

• trajectoire orthogonale — ortogonalioji trajektorija statusas T sritis fizika atitikmenys: angl. orthogonal trajectory vok. orthogonale Trajektorie, f rus. ортогональная траектория, f pranc. trajectoire orthogonale, f …   Fizikos terminų žodynas

• ортогональная траектория — ortogonalioji trajektorija statusas T sritis fizika atitikmenys: angl. orthogonal trajectory vok. orthogonale Trajektorie, f rus. ортогональная траектория, f pranc. trajectoire orthogonale, f …   Fizikos terminų žodynas