Friction is a force that resists the sliding or rolling of one solid object over another.
About 20 percent of the engine power of automobiles is consumed in overcoming frictional forces in the moving parts. The major cause of friction between metals appears to be the forces of attraction, known as adhesion, between the contact regions of the surfaces, which are always microscopically irregular. Two simple experimental facts characterize the friction of sliding solids. First, the amount of friction is nearly independent of the area of contact. If a brick is pulled along a table, the frictional force is the same whether the brick is lying flat or standing on end.