Friction is a fundamental force present in nearly every physical interaction. It manifests in two primary forms: static friction and kinetic friction. Static friction opposes the initiation of movement between two surfaces at rest, preventing objects from starting to move. In contrast, kinetic friction opposes an object’s motion once it is already sliding. This difference highlights a central question: why is the coefficient of static friction typically greater than the coefficient of kinetic friction?
How Surfaces Interact
Even surfaces that appear perfectly smooth possess microscopic irregularities, often described as peaks and valleys, or asperities. When two surfaces come into contact, these tiny irregularities interlock. True contact occurs only at a very small fraction of their apparent area, specifically at the tips of these asperities.
Intermolecular forces also contribute to the interaction between surfaces. These attractive or repulsive forces exist between atoms and molecules at the points of contact, including Van der Waals forces that contribute to friction.
The Grip of Static Friction
When an object rests on a surface, microscopic irregularities have time to settle into each other, creating a strong mechanical interlock. This allows asperities to mesh deeply, maximizing contact points. At these true contact points, intermolecular bonds form between the surfaces.
These bonds create robust resistance to initial motion. To overcome static friction, an applied force must be sufficient to break these stronger intermolecular bonds and deform or break the deeply interlocked asperities. The coefficient of static friction represents the maximum resistance that must be overcome before relative motion begins.
The Slide of Kinetic Friction
Once an object begins to slide, the nature of the interaction between surfaces changes significantly. The surfaces are no longer at rest, so asperities have less time to interlock deeply. Continuous relative motion also limits the time for strong intermolecular bonds to fully form at new contact points.
While some bonds do form, they are constantly being broken and reformed as surfaces slide past one another. This continuous “breaking and making” of weaker bonds results in less overall resistance compared to static friction. The coefficient of kinetic friction represents the resistance to this ongoing motion and is lower because the surfaces are in a state of continuous disruption.
Why the Difference Matters
Understanding the distinction between static and kinetic friction is important for many everyday applications and engineering designs. For instance, when a person walks, they rely on static friction between their shoes and the ground to push off and propel themselves forward. If there were only kinetic friction, walking would be more akin to slipping on ice.
Automobile braking systems, particularly Anti-lock Braking Systems (ABS), are another prime example. ABS works by rapidly pulsing the brakes, preventing the wheels from fully locking up and skidding. This action ensures that the tires maintain static friction with the road, which provides greater stopping power than the weaker kinetic friction that occurs during a skid.