Which interaction most directly causes friction?

The interaction that most directly causes friction is described by the electromagnetic forces between the surfaces of two objects. When two surfaces are in contact, the microscopic roughness of each surface leads to specific interactions at a molecular level. These interactions primarily involve electromagnetic forces, as the attraction and repulsion between electrons of the atoms in the surfaces create resistance to motion – which we observe as friction.

While the normal force is also an important factor in understanding friction, it does not directly cause it. Instead, the normal force impacts the amount of friction experienced – the greater the normal force, the greater the potential friction that can occur. Gravitational force contributes to the normal force but does not create friction on its own. Additionally, the idea that the frictional force is always equal to an object's weight is misleading; friction varies based on the conditions and materials in contact rather than being a fixed proportion of weight. Thus, the electromagnetic interactions within the surfaces are fundamental in generating the force we recognize as friction.