Segment V4.6: Pathlines

(Related to Textbook Section 4.1.4 - Streamlines, Streaklines, and Pathlines)

A pathline is the line traced out by a given particle as it flows from one point to another. The lawn sprinkler rotates because the nozzle at the end of each arm points "backwards".

This rotation give the water a "forward" component. As a result, water particles leave the end of each spray arm in a nearly radial direction. Once the water exits the spray arm there is no external force to change its velocity (neglecting aerodynamic drag and gravity). Thus, the pathlines are essentially straight radial lines, whereas the shape of the water streams (which are not pathlines) is a complex spiral about the axis of rotation.