What causes friction?
A surface may appear to be smooth, but when examined under a microscope surfaces appear rough with many bumps and hollows.
Friction is caused in part by the tiny bumps and hollows on surfaces as they rub against each other.
The bumps on one surface interlock with the hollows on the other making it hard for the surfaces to slip over each other.
This explains why polishing and lubricating surfaces reduces friction.
Polishing helps to flatten out the bumps and hollows, while oil or grease helps to lift the top surface off the bottom surface by filling the gaps in between.
As a result, the surfaces move over each other more easily.
Friction is also due to attraction between molecules making up the two surfaces, so that there is friction between even extremely smooth surfaces.
When the surfaces move the molecules need to be torn away from each other, and we experience this as friction.
This explains why reducing surface area and separating surfaces reduces friction.
