First: I count 4 fundamental forces in your list, unless you're counting electrical and magnetic separately. If you were to do that, however, you'd have to include the magnetic-like part of each of the fundamental forces separately. So let's say you have 3 or 4 forces: gravity, strong nuclear force, and electroweak. (You might choose to count the electroweak as 1 or 2.)
OK, what about the frictional forces? Generally, these are forces which convert the kinetic energy of large-scale relative motions to various forms of small-scale random thermal energy. Almost all the familiar visible forces are forms of electromagnetism, and that includes common frictional forces.
I guess that on large scales, gravity plays a role in friction. For example, tidal stresses in the earth and moon are part of the path by which large-scale orbital and rotational energy is converted to thermal energy. The last step, however, in which the energy of big tides is converted to slightly warmer oceans, for example, is always electromagnetic. That's because on the scale of molecules gravity is tiny compared to electromagnetic forces.
Mike W.
(published on 03/08/11)
