Which of the four fundamental forces causes friction?

Answer 1

The Electromagnetic force is the only microscopic force causing friction, but the explanation of that force is not simply based on electrostatics or classical electromagnetic theory.

The forces involved with friction are ultimately the forces between atoms at the interface between two materials.

Atoms may attract, repel and become chemically bonded and those processes must be described using quantum theory.

In some cases, atoms or molecules at the interface are charges or have dipoles or other complex charge distributions which interact with other charges or charge distributions through simple laws of electrostatics.

But, neutral atoms interact even when the isolated atoms have no charge and a simple spherical charge distribution. The interaction between neutral atoms and molecules which are far apart (further apart than a few angstroms) are termed "van der Walls forces" and such forces are weakly attractive. When atoms or molecules are closer, then they may develop a chemical bond. If they do not develop a chemical bond that moving even closer results in a rapidly increasing repulsion when the electron clouds of two atoms begin to overlap. This is a result of an increase in the kinetic energy of electrons and also includes some coulomb (electrostatic) effects.

The forces between atoms at close range (the repulsive forces and the van der Waals forces) are not simply electrostatic in origin. The full explanation resides in the quantum mechanical laws governing the electronic structure of the atoms and molecules.