If I'm understanding it correctly, my book says that frictional forces are the result of cold welds - that when two surfaces are in contact with one another, the highest points on the surfaces on an atomic scale weld together to form a single object, and these welds are what resists movement when some object tries to slide over some other object. This seems intuitive, but it raises a question in my mind: Usually, I would think of a rougher surface as one with greater friction, and a smoother surface as one that is easier to slide across. By my book's explanation, a smoother surface has the opportunity for more atom-to-atom contact, and therefore more points of cold-welding, leading to greater friction. Why the discrepancy? Is it because I'm thinking of "rough" and "smooth" on entirely the wrong scale, or is it something else? And, on a mildly related note, does this mean that any time friction opposes the movement of two surfaces across one another, some atoms of each surface are ripped off and stuck to the other surface? Thanks for taking the time to answer my neophyte questions.