Thus far we have considered the relations of speed, force, and resistance from a somewhat theoretical standpoint; in actual practice a deduction has to be made from the advantage apparently gained because of the resistance of the machine to free motion. This resistance is due to the rough surfaces of the bearings of the machine, although to the naked eye these bearings may appear perfectly smooth. When polished surfaces are inspected or examined under the microscope (Fig. 34) they are seen to have many inequalities and to be comparatively rough. These inequalities fit into the hollows of the opposite surface, out of which it requires some force to lift or slide them. This is done first by polishing the surfaces until they are as smooth as possible and then by inserting some lubricant, such as oil, grease, or black lead which fills up the little holes and thus reduces friction. Friction is also reduced by having two different substances or metals in contact, as, for example, the brass or sometimes jeweled boxes in which the steel axles of wheels in clocks and watches revolve. The greatest amount of friction arises just before motion takes place, because the inequalities of the upper surface sink into those in the lower more completely at rest than in motion.
Fig. 34. - Metal Under a Magnifying Glass. Imaginative view of a shaft showing microscopic roughness that causes friction.
In going down a hill, drivers of heavy vehicles pass a chain through a spoke of the wheel to increase the friction, and thus prevent the wheel from turning. Friction between the ground and the shoe enables us to walk. Shoes with hob nails are dangerous on a smooth iron plate because the two iron surfaces give little friction.