Friction is a fundamental force that opposes motion between two surfaces in contact, and the observation that it is more difficult to initiate movement than to sustain it is a common experience. Pushing a heavy piece of furniture across a floor, for example, requires a greater initial shove than the continuous force needed to keep it sliding. This difference in required force is quantified by two separate coefficients: the coefficient of static friction (\(mu_s\)) and the coefficient of kinetic friction (\(mu_k\)). The physical reason the static coefficient is always greater lies in the microscopic interactions at the interface, which fundamentally change when relative motion begins.
Defining Static and Kinetic Friction
Static friction is the resistance force that must be overcome to initiate relative motion between two objects at rest against each other. This force increases in magnitude to match any applied force up to a maximum threshold. Once the applied force exceeds this maximum static friction, the object begins to slide, and the opposing force immediately drops to a lower, relatively constant value known as kinetic friction. Kinetic friction resists the motion of objects already sliding against one another. The relationship defining these forces is \(F = mu N\), where \(F\) is the frictional force, \(N\) is the normal force pressing the surfaces together, and \(mu\) is the coefficient of friction.
The Role of Microscopic Surface Roughness
To understand why the coefficients differ, the view must shift from the macroscopic level to the microscopic. Every surface possesses microscopic irregularities, consisting of peaks and valleys. These tiny features are known as asperities, giving the surface a texture resembling a mountain range at a minuscule scale. When two surfaces are pressed together, actual contact occurs only at the tips of these asperities, meaning the true area of contact is substantially smaller than the apparent area. This concentration of force onto a tiny fraction of the total surface area sets the stage for the physics of static friction.
The Force Required to Break Adhesive Bonds
When the surfaces are static and in contact, the high pressure exerted at the points of asperity contact causes localized deformation. This pressure allows the atoms of the two surfaces to come into extremely close proximity. At this atomic distance, powerful short-range forces, such as van der Waals or electrostatic forces, act to form adhesive bonds across the interface. These established atomic connections are often referred to as “cold welding.” The force required to initiate motion must be sufficient to shear or fracture this network of fully formed, static adhesive bonds, in addition to overcoming the mechanical interlocking of the asperities. This necessity of breaking the atomic welds is the physical reason that the maximum force of static friction, and thus its coefficient (\(mu_s\)), is higher.
Why Movement Maintains a Lower Coefficient
Once the external force has fractured the static adhesive bonds and overcome the initial mechanical resistance, the surfaces begin to slide. In this state of kinetic friction, the surfaces are in continuous relative motion, and the true contact points are constantly changing. The asperities do not remain in contact long enough for strong, extensive atomic bonds to fully re-form. Instead, the friction force primarily involves the continuous formation and immediate shearing of much weaker, fleeting bonds, along with the mechanical resistance of asperities sliding over the opposing surface’s peaks. Because the energy required to continuously disrupt these temporary bonds is less than the energy needed to break the initial static bonds, the resulting kinetic friction force is lower.