Which force opposes motion between two surfaces that are in contact?

Friction is the force that opposes motion between two surfaces that are in contact. It arises from the interactions at the microscopic level between the irregularities on the surfaces of the objects. When one surface slides or attempts to slide over another, the frictional force acts in the opposite direction to the motion, effectively resisting it. This force can vary depending on factors like the nature of the surfaces involved (smooth or rough) and the amount of pressure pressing them together.

Understanding friction is crucial in many real-life applications, from determining how easily objects can slide over one another to understanding the physical principles behind vehicle stopping distances. The other forces mentioned, such as gravity, drag, and pressure, play different roles in physical interactions but do not specifically serve to oppose motion between two solid surfaces in contact. Gravity pulls objects toward the earth, drag opposes the motion of objects through fluids, and pressure relates to the force exerted per unit area, but none of these directly counteract motion on a surface like friction does.