There are two reasons for the friction force: unevenness of surfaces and adhesion. The adhesion comes due to interaction between neutral molecules aka the Van Der Waals force. Because of adhesion the friction force can exceed object’s weight, which is so for rubber for instance. The friction force at rest can appreciably exceed that in motion: about two times for a rubber tire over dry asphalt, four times for steel over steel and ten times for glass over glass. Guillaume Amontons considered (1699) the force of friction to be directly proportional to the applied load and independent of the apparent area of contact. What actually happens between two surfaces is that due to the roughness of surfaces their actual contact area grows proportionally to the load applied. Using an atomic force microscope of subnanometer resolution they found the actual area being much less then the apparent one. As only that actual contact area is responsible for the friction there is the proportionality law between a body’s weight and a force of friction. It holds until saturation comes though I guess the adhesion would dominate well before. When an thermoplastic is used in a bearing, for a greater load the friction can even fall. There is also the rolling resistance called the rolling friction which comes from the deformation of a wheel or rollers/balls in bearings. Now as a consequence of the newly discovered light pressure Einstein has predicted (1909) for an object moving in space the radiation friction due to more radiation is reflected per second off its fore than that its rear.