Friction is a resistive external force that affects objects in motion relative to another surface, being essential for movement and stopping. It allows everyday activities such as walking and driving by opposing relative motion, while its complexity in behavior continues to be a subject of study.
Friction is a force that opposes an object's motion relative to another surface or medium it is in contact with.
Friction is a critical force in everyday physics, acting as a resistive force that opposes the relative motion between two systems in contact. It manifests when objects slide past each other and is a universal experience, like when trying to walk on a smooth surface versus ice. The complexity of friction is a subject of study, but we recognize its general behavior to oppose motion or the attempt to move. Standing on the ground, writing with a pencil, driving a car—all these actions rely on friction. The magnitude of kinetic friction can be expressed mathematically as
with µk being the coefficient of kinetic friction and N being the normal force.
Ultimately, friction is an external force that plays a crucial role in allowing systems in contact to both move and cease moving as needed, acting contrary to an object's attempts at relative motion.