eng

### tangent vector

first derivative of a curve parameterized by arc length

Note to entry: If `c(s) = (x(s), y(s), z(s))` is a curve in a 3D Cartesian space (`bbb"E"^3`), and `s` is arc length along `c`, then the tangent is `vec tau(s) = dot c(s) = (dot x(s), dot y(s), dot z(s))`, i.e. the derivative of the coordinate values of `c` with respect to `s`. The curvature vector is `kappa(s) = ddot c(s) = (ddot x(s), ddot y(s), ddot z(s))`.

ORIGIN: ISO/TC 211 Glossary of Terms - English (last updated: 2020-06-02)