depends on the materials of the roller and supporting surface, and is proportional to the normal pressure exercised by the roller on the rolling surface. It depends also on the diameter of the roller, being less for large rollers than for small ones. On highways with soft compressible surfaces, the resistance is also affected by the width of the wheel tires, being greater for narrow tires than for wide ones. ROLLING FRICTION FOR ROADWAY DIFFERENT The accompanying table gives the maximum, minimum, and mean values of the coefficient of rolling friction for different roadway surfaces. They are expressed in pounds required to overcome the resistance on a level road of a gross ton (2,240 lb.). The mean value is also expressed as a ratio between the frictional resistance and the load. STRENGTH OF MATERIALS DEFINITIONS OF TERMS Stress is the cohesive force by which the particles of a body resist the external load that tends to produce an alteration in the form of the body. It is always equal to the effective external force acting upon the body; thus, a bar subjected to a direct pulling force of 1,000 lb. endures a stress of 1,000 lb. Unit stress is the stress or load per unit of area, usually taken per square inch of section. For instance, if the bar mentioned above is 1 in. X2 in. in section, the unit stress of the bar will be 1,000÷2 (sectional area) = 500 lb. Tensile stress is produced when the external forces tend to stretch a body, or pull the particles away from one another. A rope by which a weight is suspended is an example of a body subjected to tensile stress. Compressive stress is produced when the forces tend to compress the body, or push the particles closer together. A post or column of a building is subjected to compressive stress. Shearing stress is produced when the forces tend to cause the particles in one section of a body to slide over those of the adjacent section. A steel plate acted on by the knives of a shear, and a beam carrying a load, are subjected to shearing stress. Tension, compression, and shear are called simple or direct stresses, to distinguish them from bending and torsion. The amount of alteration in form of a body produced by a stress is called deformation, or strain. It may be tensile deformation, compressive deformation, or shearing deformation, according as the stress producing it is tensile, compressive, or shearing. The rate of deformation, also called unit deformation, is the deformation of a body, subjected to tension or compression, per unit of length. If an iron bar 6 ft. long is subjected to a force that elongates it 1 in., the rate of deformation will be 1 in.÷72 (length of the bar in inches) = .0139 in. The modulus or coefficient of elasticity is the ratio between the stresses and corresponding deformations for a given material, which may have a somewhat different modulus of |