The Feynman Lectures on Physics offers a simple dimensional argument for why friction acts like it does, and then says that the details of the coefficient of fr

The Feynman Lectures on Physics offers a simple dimensional argument for why friction acts like it does, and then says that the details of the coefficient of friction are beyond our ability to estimate from first principles. It then offers a simple experiment to illustrate that the properties have to do with surface impurities. If you slide glass over dry glass, it slides easily. Do the same with wet glass and you can feel a "gritty" motion. This is due to the water lifting the impurities off of the glass and allowing a direct glass on glass contact, at which point they fuse and have to be broken. You can see where this happened because it leaves visible scratches. I found this unbelievable and had to test it. My mother was not thrilled to see that her good crystal got visible scratches on it...