I cannot figure out how to translate 2D formulas of Normal Force (Fn = mg cos a and Ft = mg sin a) to 3D. How do I calculate magnitude of object's normal force Fn on an inclined plane? I calculate normal force direction vector by taking crossproduct of plane normal and gravity, and again of the plane normal and the vector from previous crossproduct. But I can't figure out how to calculate correct magnitude based on gravity and steepness of the plane. For example, on a plane perpendicular to gravity, normal force should equal to zero, and increase as slope gets steeper. [edited by - ZiM on August 18, 2003 1:57:39 PM]

not positive about this, but should it be:

-mg dot plane-normal (assuming normal is up and gravity is down)

?