Definition:Curvature/Parametric Form/Polar

Definition

Let $C$ be a curve defined by a real function which is twice differentiable.

Let $C$ be embedded in a polar plane and defined by the parametric equations:

$\begin{cases} r = \map r t \\ \theta = \map \theta t \end{cases}$

The curvature $\kappa$ of $C$ at a point $P = \polar {r, \theta}$ is given by:

$\kappa = \dfrac {2 r'^2 \theta' + r r \theta' + r r' \theta + r^2 \theta'^3} {\paren {r'^2 + \paren {r \theta'}^2}^{1/2} }$

where:

$r' = \dfrac {\d r} {\d t}$ is the derivative of $r$ with respect to $t$ at $P$

$\theta' = \dfrac {\d \theta} {\d t}$ is the derivative of $\theta$ with respect to $t$ at $P$

$r$ and $\theta$ are the second derivatives of $r$ and $y$ with respect to $t$ at $P$.

Also see

• Equivalence of Definitions of Curvature

• Results about curvature can be found here.