Friction

Friction is the force that resists the sliding of one surface over another. The force of friction is responsible for the wear experienced by mechanical devices using surfaces in contact and results in the heating of sliding surfaces.

Overview

Friction is the "contact force," a force between two surfaces that are in contact that resists the sliding motion of one surface over the other. The friction force exerted by one object sliding over another object is always parallel to the contact surface between the two objects and always occurs in a direction opposite that of the motion.

The friction force is important in the common activities of everyday life. Friction produces the force between our feet and the floor, allowing us to walk; provides the force between automobile tires and the road, allowing cars to accelerate and to stop; and provides the force between an object and our hands, allowing us to pick things up, for example. A person walking normally along a sidewalk will generally slip when unexpectedly encountering a spot where friction is significantly reduced, such as an unseen icy patch.

When two objects are sliding, however, friction is a "dissipative force," a force that turns energy into heat. Under these circumstances, friction is frequently regarded as a nuisance, requiring the operator of mechanical devices to supply more energy to these devices than is used to accomplish the desired objective. Most of the excess energy is transformed into heat by the friction force, though a small part of the excess energy actually abrades, or wears away, the surface of the weaker of the two materials. For example, some of the energy generated by burning gasoline in an automobile engine is dissipated by friction within the engine, transmission, and drive mechanism, rather than being used to accelerate the vehicle. Friction causes most mechanical objects to wear out as a result of abrasion (the loss of material as the surfaces in contact interact with each other) and because of the heating produced as one surface slides over another.

A complete theoretical description of friction has, thus far, eluded scientists, because the phenomenon involves the interaction forces between all the individual atoms or molecules on or near the two surfaces that are in contact. The modern theoretical understanding of friction attributes the friction force to the atomic forces of adhesion between atoms or molecules on the surfaces of the two materials. These forces are similar to the electronic bonding forces that hold the individual atoms together in liquids and solids. The more strongly the atoms on one surface attract those on the other surface, the greater the friction force between the two surfaces. However, detailed modeling of this adhesion force is possible only if a few atoms are interacting; such modeling cannot be done for the vast number of atoms that interact over the surface areas of real objects.

Because of the complexity of a complete theoretical description of friction, the friction force is usually approximated using "laws of friction" that are derived from experiment. These laws of friction, which were originally developed by the Italian scientist, engineer, and artist Leonardo da Vinci in the fifteenth century, are as follows: the friction force is proportional to the contact force that presses the two surfaces together; the friction force depends on the two types of material that are in contact; for objects in motion, the friction force is independent of the speed at which one surface is moving over the other surface; and the friction force is independent of the area of contact between the two surfaces.

There are two types of friction, "static friction" and "kinetic friction." Static friction occurs between two surfaces that are in contact but are not moving with respect to each other, for example, a book resting on top of a flat, horizontal table. If you push gently on one of the vertical sides of the book, the book will not move. Static friction provides the force that resists the motion of the book. However, there is a maximum force that the two surfaces in contact can exert. If you push hard enough to overcome this force, the book will begin to move.

The maximum force of static friction is modeled by physicists as the product of the "coefficient of static friction," a quantity that must be measured for each type of material sliding on another type of material, and the "normal force," which is a measure of how hard one surface is pushing on the other. In the case of the book on the table, the normal force is the weight of the book, which pushes it down onto the table. If the weight is increased by stacking a second book on top of the first one, the normal force increases, and the force that must be exerted on the side of the book to make it move also increases.

The coefficient of static friction between the book and the table can be measured by tilting the table, increasing the angle that the surface of the table makes with the horizontal until the book just starts to slide down the incline. The coefficient of static friction is equal to the numerical value of the tangent of the angle at which the table's surface is inclined relative to the horizontal when the book just begins to slide. This technique is used to measure the coefficient of static friction for many surface materials.

Kinetic friction occurs when one of the surfaces is moving with respect to the other one. Kinetic friction is modeled in the same way as static friction except that the coefficient of static friction is replaced by a coefficient of kinetic friction. For most materials, the coefficient of kinetic friction is less than the coefficient of static friction, so it requires a stronger push to start the book in sliding motion than it does to keep it sliding across the table at a constant speed.

The coefficient of kinetic friction can also be measured using an incline. If the angle of the incline is adjusted so that the object, once moving, slides down the incline at a constant speed, then the coefficient of kinetic friction is equal to the tangent of the angle of the incline relative to the horizontal.

Friction is one of a group of "drag forces," which resist the motion of one object in contact with another. Objects moving through gases or liquids experience similar dissipative forces, called "aerodynamic" drag or "fluid" drag.

Applications

Most likely, the first application of friction was by prehistoric humans who used it to start fires. The friction force between two objects, such as sticks or stones that are rubbed together, can generate enough heat to ignite kindling material. This application of friction is still used today, as matches rubbed along a rough surface ignite the heat-sensitive chemical material on their surfaces.

In everyday life, friction is the force that allows us to lift objects without putting our hands underneath them. A cylindrical water glass can be lifted by gripping the hand around the glass. The fingers and palm pushing in on the sides of the glass produce a contact force, and static friction, which is parallel to the contact surface and opposite the direction of intended motion, exerts a force that is directed upward, keeping the glass from sliding downward as the glass is raised. However, if a lubricant, such as oil, is applied to the outer surface of the glass or to the hand, the coefficient of static friction is reduced, and the glass may slip downward when the fingers and palm apply the same contact force.

Friction is the critical force involved in stopping most moving objects, whether by rubbing a foot along the pavement to stop a wagon or skateboard or by applying the brakes in a car. Two friction devices are used in stopping an automobile: the brakes and the tires. Automobile brakes work on the principle that the kinetic energy (the energy associated with motion) of the car can be transformed into heat energy by forcing one surface to slide over another. When the brakes in an automobile are activated, the stationary brake pads or brake shoes are brought into contact with a disk or a drum that rotates with each wheel. Kinetic friction between the brake pads or shoes and the rotating disks or drums transforms the kinetic energy of the vehicle into heat. Static friction between the tires and the road then slows the car as the wheels rotate more slowly. If the brakes are applied too strongly, the wheels will stop rotating, and the kinetic friction of the nonrotating tires sliding along the road surface will bring the vehicle to a stop. In this case, the abrasion of the soft surface of the tires by the stronger surface of the road results in a skid mark, as material wearing from the tires is deposited along the road.

The efficiency of braking depends on the magnitude of the coefficient of friction. A normal automobile on a dry road surface can come to a stop, from an initial speed of 60 miles per hour, in about 150 feet. However, if the road surface is wet, thus reducing the coefficient of friction between the tires and the road, the stopping distance increases to about 250 feet. Similarly, if the surfaces of the brakes get wet, for example by driving through a deep puddle, the braking effectiveness will be poor until the contact surfaces have dried out. For a road surface covered with ice, which further decreases the coefficient of friction between the tires and the road, the stopping distance increases even farther. A train, which has steel wheels on a steel track, has a much lower coefficient of friction between its wheels and the track than that of automobile tires on the road. As a result, a train usually requires 400 to 500 feet to come to a stop from an initial speed of 60 miles per hour.

Much of the study of friction concentrates on methods to minimize or overcome friction, since the minimization of friction improves the "efficiency," a measure of the amount of useful work a device can do for a given amount of input energy. Lubrication is frequently used to reduce friction between two contact surfaces. Ice skates, for example, are designed to produce and trap a thin layer of water between the metal blade of the skate and the ice surface, allowing the skater to move on a thin layer of lubricant, reducing the frictional resistance from that of metal on ice.

One metal surface sliding on another metal surface can have a friction coefficient as high as 4 in a vacuum. In air, this friction coefficient is reduced to a value near 1. However, the introduction of a liquid lubricant can further reduce the friction coefficient. Values near 0.3 are measured using water as the lubricant; high-quality lubricating oils can reduce the friction coefficient to values near 0.1. Because lubricants can improve the efficiency of mechanical devices and minimize their wear, thus prolonging their useful lifetime, the development of better lubricants is an important area of engineering research.

In the adhesion model of friction, the action of lubricants is understood to reduce the atomic forces between the two surfaces. In this model, friction is attributed to the atomic forces between the atoms at the surfaces in contact. In solids, these atomic forces are generally quite high. The ability of liquids to flow is attributed to weaker atomic forces. Thus liquids, with their lower atomic forces, are believed to bond to both of the surfaces, and the atomic forces between these liquids is lower than between the two solid surfaces.

Friction can also be reduced by coating one of the surfaces with a solid material having a low coefficient of friction. Coatings of teflon or graphite reduce the coefficient of friction between two surfaces. The coefficient of kinetic friction between teflon and steel is 0.04, while that between steel and steel is 0.57. Thus, the application of a teflon coating to a steel surface will reduce the friction between the two surfaces by more than a factor of 10, increasing efficiency and reducing wear. Teflon coatings are frequently applied at points where two metal surfaces would normally come into contact with each other.

The laws of friction are only approximately true, and small deviations from the third law--that the friction force is independent of the speed of motion of the two surfaces--give rise to "frictional oscillations," which have a variety of useful applications. Generally, for surfaces that are either not lubricated or are poorly lubricated, the coefficient of sliding friction decreases as the speed increases, resulting in an oscillatory motion, or vibration, by which the moving object speeds up, then slows down in a repeating fashion. This frictional oscillation produces the screech that is sometimes heard when chalk is pushed across a blackboard. Frictional oscillation is also the mechanism by which bowed instruments, such as a violin, produce their harmonic tones. Frictional oscillations can also provide a warning that the lubricant has been lost from mechanical systems, and observant operators frequently have enough warning to shut down the system before it overheats or seizes.

Context

Prior to the recognition of the friction force, the understanding of mechanics—the science of how objects move—was impeded. Early humans moved heavy objects on sleds or skids that they pulled across the ground. Because these sleds or skids have a high coefficient of friction with the ground, this method of moving objects dissipates, or wastes, a lot of energy. It is suspected that early humans developed lubricants, possibly wetting the ground surface with water, to reduce friction. A revolution occurred when it was recognized that the sled could be replaced with the wagon, a device that rolled, probably first along a runway of logs and, later, on its own wheels mounted on axles. The wagon greatly reduced the friction force that must be overcome to move objects and made it practical to transport heavy objects over large distances.

Natural scientists in ancient Rome and Greece observed that when they ceased pushing on an object, it would quickly come to a stop. Based on this observation, they believed that an object in motion required the continual application of a force to remain in motion. By the fifteenth century, Leonardo da Vinci had engaged in a study of friction in an effort to reduce its effects on mechanical systems. Experiments by Galileo Galilei and Isaac Newton recognized how friction had impeded earlier ideas about the motion of objects, and this resulted in a new formulation of the laws of motion. Newton's first law of motion states that an object in motion will remain in motion in a straight line at a constant speed until it is acted upon by an external force. The major reason why most moving objects eventually slow down is friction.

Friction was originally explained as a force that resulted from the imperfections of the surfaces of the two objects. It was imagined that one piece of rough wood had difficulty sliding over another piece of rough wood because the high points on one surface would get stuck in the low points of the second surface. However, in the twentieth century, as it became possible to prepare highly polished surfaces, it became clear that the friction force still persisted as the surfaces became flatter. In fact, two highly polished metal surfaces placed in contact in a vacuum will weld together, and the resistance to sliding will exceed that of rougher metal surfaces that do not experience welding. A detailed understanding of how polished metal surfaces weld in a vacuum has become important in the exploration of space, where metal is a common material used in construction of moving parts on spacecraft that operate in the high vacuum of space.

Developments in modeling the chemical bonding and atomic forces between atoms led to the modern understanding of friction as adhesion, or the atomic-scale attraction between the atoms or molecules in one surface and those in the other surface. Efforts to understand and control friction now focus on the detailed understanding of these atomic-scale forces.

The ultimate objective for certain mechanical devices is to eliminate friction entirely, thus eliminating wear and removing the energy penalty resulting from the production of heat. Very low-friction conditions can be achieved by keeping one surface suspended above the second, so that motion occurs without any contact between the surfaces. In the laboratory, this can be accomplished using air tracks or air tables, devices in which a continuous stream of high-pressure air is forced from small holes in the surface. The result is that an object placed on the surface is suspended on a thin cushion of air, never actually coming in contact with the track or table. This technology is used in the construction of "air-hockey" games, for example; however, it is generally not practical to use air tracks for large or heavy objects. Magnetic levitation, in which an object is suspended by magnet fields, provides another means of greatly reducing friction. This has been perhaps most famously deployed in maglev trains, but these have not been widely adopted due to high costs and other drawbacks that counterbalance the advantages of improved speed and performance.

Principal terms

ABRASION: The wearing away of surface material by friction

ADHESION: The molecular attraction exerted between surfaces that are in contact

ATOMIC FORCES: Electronic forces that arise because of the interactions between electrons of nearby atoms

COEFFICIENT OF FRICTION: A dimensionless quantity, which depends on the types of materials involved, that characterizes the magnitude of the friction force between two surfaces of unit area

CONTACT FORCE: A force that occurs only when two objects are in direct contact

DISSIPATIVE FORCE: A force that, by its action, turns energy into heat

KINETIC FRICTION: The force that opposes one surface sliding over another when one surface is moving with respect to the other

NORMAL FORCE: A force in the direction normal, or perpendicular, to two surfaces in contact that pushes the two surfaces together

STATIC FRICTION: The force that resists one surface sliding over another when the two surfaces are not moving

Bibliography

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