# Curvature

## History

## Description

Curvature is a value that measures how curved is the curve at a point on a curve. The most intuitive way to see it is that at any point P on the curve there is a circle of right size that touches P and fits the most. Call this most-fitting circle the tangent circle. The flatter the curve at P, the larger is its tangent circle. The sharper the curve at P, the smaller is its tangent circle. Thus, we can define the value of curvature as 1/r, where r is the radius of the tangent circle. When the tangent circle is large, the curve is flattish, and the curvature 1/r is small.

Ideally, a line would have curvature of 0 everywhere, and its tangent circle doesn't exist. (i.e. have infinite radius) A Circle of radius R would have a constant curvature of 1/R, and its tangent circle at any point on the circle would be the same as the circle itself.

There are several ways to define tangent circle mathematically. One way is by the limit of the intersetion of two normals at points P1 and P2 on the curve as P1 and P2 approach each other and meet at P. The limit of that intersection is the center C of a tangent circle at P. Thus, distance CP would be the radius of the tangent circle, and 1/distance[C,P] would be the curvature.

Another way to define curvature mathematically is the rate of change of the turning of the tangent line. That is, suppose θ is the angle of the tangent at P and the x-axis. As a P moves on the curve, θ changes. The curvature at P is the rate of change of θ at P. This way, see see that a line has curvature of 0 everywhere since it's tangent doesn't change. And a circle would have constant curvature because its tangent turns in a constant way.

## Formula

Given a parametric curve `{xf[t],yf[t]}`

, let `T[t]`

be the unit-length tangent vector at t. That is, `T[t]:={xf'[t]/,yf'[t]}/s[t]`

,
where s[t] is the “speed” defined as `s[t]:=Sqrt[xf'[t]^2 + yf'[t]^2]`

. Let N[t] be the unit-length normal at t, that is, let {t1[t],t2[t]} denoted the coordinates of T[t], then N[t]= {-t2[t],t1[t]}. Now, curvature can be given as:

k[t] := (T'[t] . N[t])/s[t]

Expand it out we have:

k = (x' y'' - x'' y')/(x'^2+y'^2)^(3/2)

(argument suffix [t] dropped for easy reading. xf[t] replaced by x, yf[t] by y.) Note: tangent circle is also called Osculating Circle.

## Interesting Examples

Interesting curves involning the curvature concept, see:

- Evolute curve (the centers of tangent circles)
- Radial curve (locus of tangent circle normals)
- circle (constant curvature everywhere)
- line (curvature of 0 everywhere)
- Clothoid (spiral of linearly increasing curvature)

For a gallery of artifacts that involves fancy curves, see: Curlicues.

## Related Web Sites

Robert Yates: Curves and Their Properties.