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单词 SurfaceNormal
释义

surface normal


Let S be a smooth surface in 3. The surface normal of S at a point P of S is the line passing through P and perpendicularMathworldPlanetmathPlanetmathPlanetmathPlanetmath to the tangent planeMathworldPlanetmath τ of S at the point P, i.e. perpendicular to all lines in τ.

If the surface S is given in a parametric form

x=x(u,v),y=y(u,v),z=z(u,v),

it is useful to interpret the parameters u and v as the rectangular coordinates of a point in a plane, the so-called parameter plane. We can consider on S the so-called parameter curves, namely the u-curves which correspond the lines parallelMathworldPlanetmathPlanetmath to the u-axis and the v-curves which correspond the lines parallel to the v-axis in the parameter plane. One u-curve and one v-curve passes through every point on the surface (the values of u and v in a point of S are the Gaussian coordinates of this point). The surface normal at any point of S is perpendicular to both parameter curves, and thus its direction cosinesMathworldPlanetmath a, b, c satisfy the equations

{axu+byu+czu=0,axv+byv+czv=0.

This homogeneousPlanetmathPlanetmath pair of linear equations determines the ratio of the direction cosines

a:b:c=(y,z)(u,v):(z,x)(u,v):(x,y)(u,v)

via the JacobiansMathworldPlanetmathPlanetmath.

Example. Determine the direction cosines of the normal of the helicoid

x=ucosv,y=usinv,z=cv.

We have the Jacobians

|yuzuyvzv|=|sinv0ucosvc|=csinv,|zuxuzvxv|=|0cosvc-usinv|=-ccosv,|xuyuxvyv|=|cosvsinv-usinvucosv|=u.

These are the componentsPlanetmathPlanetmathPlanetmath of the normal vector of the helicoid surface in the point with the Gaussian coordinates u and v.  The length of the vector is  (csinv)2+(-ccosv)2+u2=u2+c2.  If we divide (http://planetmath.org/Division) the vector by its length, we obtain a unit vectorMathworldPlanetmath, the components of which are the direction cosines of the surface normal:

csinvu2+c2,-ccosvu2+c2,uu2+c2.
Titlesurface normal
Canonical nameSurfaceNormal
Date of creation2013-03-22 17:23:10
Last modified on2013-03-22 17:23:10
Ownerpahio (2872)
Last modified bypahio (2872)
Numerical id16
Authorpahio (2872)
Entry typeDefinition
Classificationmsc 26B05
Classificationmsc 26A24
Classificationmsc 53A04
Classificationmsc 53A05
Synonymsurface normal line
Synonymnormal of surface
Related topicNormalLine
Related topicEquationOfPlane
Related topicParameter
Definesparametre plane
Definesparameter plane
Definesparametre curve
Definesparameter curve
DefinesGaussian coordinates
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