Friction

Friction is a retarding force that resists relative motion between surfaces. This resistance occurs between solid surfaces, fluid layers, or material elements. It can happen when objects slide or grind against one another. Friction is a massive factor in global energy use. It accounts for about 20% of the total energy expenditure of the world. Scientists who study these processes call the field tribology.

To understand friction, we must look at the microscopic level. Most surfaces are not perfectly smooth. They are covered in tiny surface features called asperities.

When two bodies touch, these asperities interact. This interaction causes mechanical energy to transform into heat. It can also create changes in local structure or electrical charges. The total energy lost per unit of distance is the frictional force. This complex process makes calculating friction from first principles very difficult. Scientists often use empirical methods to study it instead.

There are several distinct types of friction. Dry friction occurs between two solid surfaces in contact. This is subdivided into static friction and kinetic friction. Static friction, or "stiction," acts between surfaces that are not moving. Kinetic friction acts between surfaces that are already in motion.

Fluid friction describes the resistance between layers of a viscous fluid. Lubricated friction is a specific type of fluid friction. In this case, a lubricant fluid separates two solid surfaces. Skin friction is a component of drag, which resists fluid moving across a body. Finally, internal friction resists motion between elements within a solid during deformation.

Humans have studied friction for over 2,000 years. Ancient authors like Aristotle and Pliny the Elder investigated its causes. In 1493, Leonardo da Vinci documented laws of sliding friction in his notebooks. However, these notes remained unknown for a long time. Guillaume Amontons rediscovered these laws in 1699. He described friction using surface irregularities and the force pressing surfaces together. Later, Leonhard Euler helped distinguish between static and kinetic friction. In 1785, Charles-Augustin de Coulomb expanded this by studying material types and pressure.

Modern science provides specific mathematical models for these forces. Amontons' first law states friction is proportional to the applied load. His second law says friction is independent of the apparent contact area. Coulomb's law states that kinetic friction is independent of sliding velocity.

We use the coefficient of friction to describe these interactions. This is an empirical property of the specific materials in contact. In the static case, friction acts as a threshold called traction. Once the force exceeds this threshold, motion begins.

History shows how our understanding of energy changed with friction. In 1798, Benjamin Thompson reported on heat created by friction. In 1845, James Joule published work on the mechanical equivalent of heat. He used a rotating paddle wheel in water to measure this. This helped prove that friction converts mechanical work into heat. In 1950, Bowden and Tabor showed that actual contact area is tiny. They proved that true solid-to-solid contact only happens at a few asperity points. This explains why friction increases when you increase the pressure.

Friction connects to many different fields of science. It is essential to the study of thermodynamics. For example, Max Planck described heat from rubbing as an irreversible process. It also relates to fluid dynamics through the study of viscous flow. Engineers must account for friction to prevent wear and damage. Wear can lead to the degradation of machine components. Understanding these forces allows us to control everything from car tires to microscopic machines.