I've read a similar thread, but I believe I have something extra to ask, so please bear with me. I've been thinking about friction(qualitatively) for some time now. I've not got a satisfactory explanation about the nature of the frictional force yet. My physics professor says its due to crests and troughs on the interacting surfaces. Resnick and Halliday(Chapter 6 Page 118 - Principles of Physics- Extended Ninth Edition) doesn't mention peaks and troughs at all. According to them, cold welding is responsible for friction. If the surfaces are very smooth and free of impurities, the contacting surfaces will weld. "If the applied force is great enough to pull one surface across the other, there is first a tearing of welds at breakaway and then a continuous re forming and tearing of welds as movement occurs and chance contacts are made. The kinetic frictional force that oppose the motion is the vector sum of the forces at those many chance contacts. Intuitively, this doesn't seem right. There is considerable friction if you try to drag a tire across a road without rolling it. If this is due to adhesion primarily, wouldn't it be difficult to lift the tire as well? Also, since the contact patch has adhered to the road surface rolling should be much more difficult too. I'm starting to think its a mixture of both. If the surfaces are very smooth or very rough, there is a lot of friction, but if its somewhere in the middle, friction is less. Also, is cold welding only possible between similar kinds of atoms?