ABSTRACT
Friction-Complex and Simplified Friction occurs between all matter in contact. Any combination of two materials, whether gaseous, liquid, or solid, will have unique friction characteristics dependent on many factors: the materials themselves, their surface finish, their speed relative to each other; the presence of any lubricant or impurities (dust and dirt); the direction of movement relative to any grain-like structure in a material; the pressure between the materials; temperature, and on and on. Because of all these factors, accurate mathematical models of frictional effects are extremely complex, and this is especially so for fluids. Fortunately, for the rough predictions of the loads imposed by friction on scenery movement, the complexity can be reduced considerably. Scenery movement speeds and “sail” area are usually low enough that air resistance can be considered nil. The flow of liquids, and of solids through liquids, is likewise beyond 99.9% of scenic situations (friction does affect oil flow in any hydraulic systems that we may use, but that topic is beyond the scope of this book). So for our needs here, friction will be considered only between solid materials, that can either slide against or roll over each other, in two states of motion-static, or not moving relative to each other, and dynamic, when movement does occur. The effect of friction on a move fits into the general formula for the maximum power needed to move an object as the friction force, :
Friction always opposes motion, and it never creates it. The term will never be zero (though it is sometimes small enough to be negligible), and so even in the absence of acceleration and lifting, power will be required to keep something moving because friction is always present. Friction during motion wastes power, and is usually minimized as much as possible, by using wheels rather than sliding, by using ball bearings instead of plain bearings, and through the use of lubricating oils for instance. The friction that exists in the absence of motion between materials is on the other hand primarily beneficial. The pull on a rope from a hands grip, or the traction between your shoes and the floor are instances of the usefulness of this aspect of friction.
