Hi. My problem is: On the surface of half of a rough sphere there is a known path [tex]\theta = \theta(\phi) [/tex]. I would like to known wath is the speed down the curve at any [tex]\theta [/tex] if there are the force of gravity (in the direction of -z) and the force of friction. http://www.shrani.si/f/3G/Hc/41LLD0wT/path.jpg [Broken] I tried but with no success to use the formulas for a 2D curve that are on a picture below and applying them to spherical coordinates in 3D. http://www.shrani.si/f/3v/yA/4EjmK6lQ/formule.jpg [Broken] Thank you for your answers.