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单词 CircularHelix
释义

circular helix


The space curve traced out by the parameterization

𝜸(t)=[acos(t)asin(t)bt],t,a,b

is called a circular helix (plur. helices).

Its Frenet frame is:

𝐓=1a2+b2[-asintacostb],
𝐍=[-cost-sint0],
𝐁=1a2+b2[bsint-bcosta].

Its curvaturePlanetmathPlanetmath and torsion are the following constants:

κ=aa2+b2,τ=ba2+b2.

A circular helix can be conceived of as a space curve with constant,non-zero curvature, and constant, non-zero torsion. Indeed, one canshow that if a space curve satisfies the above constraints, then thereexists a system of Cartesian coordinatesMathworldPlanetmath in which the curve has aparameterization of the form shown above.

Figure 1: A plot of a circular helix with a=b=1, and κ=τ=1/2.

An important property of the circular helix is that for any point of it, the angle φ between its tangent and the helix axis is constant. Indeed, if we consider the position vector of that arbitrary point, we have (where 𝐤 is the unit vector parallelMathworldPlanetmathPlanetmath to helix axis)

d𝜸dt𝐤=[-asintacostb][0 0 1]=bd𝜸dtcosφ=a2+b2cosφ.

Therefore,

cosφ=ba2+b2constant,

as was to be shown.

There is also another parameter, the so-called pitch of the helix P which is the separationMathworldPlanetmathPlanetmath between two consecutive turns.(It is mostly used in the manufacture of screws.)Thus,

P=γ3(t+2π)-γ3(t)=b(t+2π)-bt=2πb,

and P is also a constant.

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更新时间:2025/5/4 20:44:36