Friction is a force of resistance that arises when two surfaces are in contact and moving, or attempting to move, relative to one another. This force acts parallel to the interface, always opposing the direction of motion or the potential for motion. Without friction, simple acts like walking, driving, or holding an object would be impossible, illustrating its fundamental role in daily life. This resisting force is rooted in the physics of surface interactions, and its principles are foundational to mechanical engineering and the design of moving parts. Understanding friction requires examining the historical path that led to the formalization of its governing principles and underlying physical mechanisms.
The Historical Quest for Understanding Friction
The first systematic studies of friction trace back to the Renaissance period. Leonardo da Vinci, in his unpublished notebooks from the late 15th century, conducted the earliest known experiments on sliding friction. He observed that friction was proportional to the load pressing the surfaces together (the normal force) and was independent of the apparent area of contact.
Da Vinci’s pioneering work remained hidden in his private papers for centuries, having no influence on the subsequent development of mechanics. The formal laws of dry friction were independently rediscovered and published approximately 200 years later by the French scientist Guillaume Amontons in 1699. Amontons presented his findings, derived from experiments on various materials, in a memoir to the French Académie Royale des Sciences.
Amontons’s published work established the core empirical relationships governing the force of friction between two solids. He noted that friction was proportional to the normal load and did not depend on the size of the contact patch. This formal presentation secured his place as the figure who officially introduced these principles to the world.
Defining the Fundamental Laws of Dry Friction
The classical understanding of friction is built upon three empirical observations, known collectively as the Amontons-Coulomb Laws of dry friction.
Amontons’s First Law
The force of friction is directly proportional to the normal force exerted between the two surfaces. The normal force is the force pressing the two surfaces together, typically the weight of an object on a flat, horizontal plane. This relationship is quantified by the coefficient of friction, a dimensionless constant represented by the Greek letter mu, which is the ratio of the friction force to the normal force.
Amontons’s Second Law
The total force of friction is independent of the apparent area of contact between the surfaces. This means an object experiences the same resistance whether it is resting on its widest face or its narrowest end. This observation holds true because the actual area where the two surfaces physically touch is only a tiny fraction of the apparent area. This real contact area increases proportionally with the normal force, resulting in the same total frictional force regardless of the block’s apparent size.
Coulomb’s Law
The third law was contributed by the French engineer Charles-Augustin de Coulomb in 1785. His study differentiated between the friction that resists the initiation of motion and the friction that resists ongoing motion. He noted that kinetic friction, the resistance during sliding, is largely independent of the sliding velocity over a moderate range. This distinction formalized the concept of static friction, the force required to start movement, and kinetic friction, the resistance to maintain movement.
The Physics of Friction: Types and Mechanisms
The phenomena described by the Amontons-Coulomb laws are explained by examining the interaction of surfaces at a microscopic level. Even surfaces that appear smooth are covered in microscopic peaks and valleys known as asperities. When two surfaces are pressed together, contact occurs only at the tips of these asperities, making the real area of contact extremely small compared to the overall surface area.
Friction arises primarily from two microscopic mechanisms acting at these contact points: the interlocking of asperities and molecular adhesion. As the surfaces attempt to slide, the asperities must break off, deform, or ride up and over each other, creating mechanical resistance. Simultaneously, the close proximity of atoms at the true contact points leads to strong attractive intermolecular forces, essentially welding the surfaces together in tiny junctions. The force required to slide the object must be sufficient to break these microscopic bonds.
Static friction, the force that prevents an object from starting to move, is generally greater than kinetic friction. This happens because it takes more force to shear the fully formed adhesive bonds and overcome the initial interlocking of asperities than it does to maintain sliding motion. Once motion begins, the surfaces are no longer fully interlocked, and the time for new adhesive bonds to form is reduced, leading to the lower resistance of kinetic friction.
Beyond static and kinetic friction, rolling friction represents a different category of resistance encountered when a round object moves across a surface. Rolling friction is caused mainly by the small deformation of the object and the surface as the wheel compresses the material directly ahead of it. The resistance from rolling friction is significantly less than that of sliding friction, which is why wheels and ball bearings are used to facilitate movement.