# Point kinetics in a fixed coordinate system

Definitions
The position the velocity and the acceleration are defined by:

The following holds:

When the acceleration is constant this gives:
For the unit vectors in a direction perpendicular to the orbit and parallel to it holds:

For the curvature k and the radius of curvature holds:

### Polar coordinates

Polar coordinates are defined by: . So, for the unit coordinate vectors holds:

The velocity and the acceleration are derived from: