Which force is the underlying cause of friction?

Answer 1

The force of friction is ultimately the result of forces between atoms of the two constituent objects that are touching.

At the atomic level, the explanation of friction is complex and it is not an resolved and well defined scientific topic, but leaving details aside, one is certain that atomic forces are at the origin of friction.

Two simple aspects are important to mention. First, the atoms in different contacting surfaces experience attractive and repulsive forces when interacting with atoms in another surface when there is a separation of less than a few angstroms.

Second, the irregularity of two surfaces in contact mean that sliding friction and static friction will be different and sliding friction may involve breaking atoms or sections of material from the surfaces.

The types of friction and the complexities of friction are too enormous to begin to characterize in a short answer beyond saying that it must involve atomic and materials processes at the interface of two interacting materials. And, that does not even include internal friction such a a fluid would experience.

Answer 2

The force behind friction is typically caused by the microscopic interactions between the surfaces of two objects that are in contact with each other. This force resists the relative motion or attempts to slide past each other, leading to the resistance we feel when trying to move objects against each other.