Friction, or resistance to motion, is the result of the force acting between two surfaces as they slide against each other. Bearings reduce friction and provide smooth sliding contact between surfaces due to their material, shape or lubrication. Choosing the correct bearing for an application improves efficiency, extends bearing life at high speeds and avoids overheating. Bearings fall into two general categories: radial bearings that support rotating shafts and thrust bearings that support axial loads. Four classifications of bearing tolerances have been established, providing standards of precision for the correct application of ball bearings.