Before revision: If inertial forces and terrain roughness are set side, a ball rolling down a slope, or water flowing down, will accelerate in the direction of greatest slope.
After revision: In mountain biking and skiing, the line of greatest slope is sometimes called the fall URL Farlow Gap - Mountain Biking: fall line (2nd paragraph) — "The trail becomes loose and rocky, and becomes fall line into a wide open rock garden..."If inertial forces and terrain roughness are set side, a ball rolling down a slope, or water flowing down, will accelerate in the direction of greatest slope.