English: Schematic representation of the forces acting on an object situated on a sloping surface. The red arrow represents the force of gravity. The green arrow represents the normal force. Since those two forces are not aligned in this situation, they do not entirely cancel each other. The resultant force is directed down the slope. In this image the shape is a cross section of a parabolic dish. When the dish is rotating then there is one rotation rate that exactly matches the parabolic shape. If the rotation rate matches the shape, then an object that is co-rotating with the disk will neither climb up the slope, nor slide down the slope.