static friction definition

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

• Dry friction is a force that opposes the relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction (“stiction”) between non-moving surfaces, and kinetic friction between moving surfaces. With the exception of atomic or molecular friction, dry friction generally arises from the interaction of surface features, known as asperities (see Figure 1).
• Fluid friction describes the friction between layers of a viscous fluid that are moving relative to each other.

• Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces.

• Skin friction is a component of drag, the force resisting the motion of a fluid across the surface of a body.
• Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.

Convert kinetic energy into heat energy

When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy (that is, it converts work to heat). This property can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to thermal energy whenever motion with friction occurs, for example when a viscous fluid is stirred. Another important consequence of many types of friction can be wear, which may lead to performance degradation or damage to components. Friction is a component of the science of tribology.

Friction is desirable and important in supplying traction to facilitate motion on land. Most land vehicles rely on friction for acceleration, deceleration and changing direction. Sudden reductions in traction can cause loss of control and accidents.

Fundamental force

Friction is not itself a fundamental force. Dry friction arises from a combination of inter-surface adhesion, surface roughness, surface deformation, and surface contamination. The complexity of these interactions makes the calculation of friction from first principles impractical and necessitates the use of empirical methods for analysis and the development of theory.

Friction is a non-conservative force – work done against friction is path dependent. In the presence of friction, some kinetic energy is always transformed to thermal energy, so mechanical energy is not conserved.

static friction definition

History

The Greeks, including Aristotle, Vitruvius, and Pliny the Elder, were interested in the cause and mitigation of friction. They were aware of differences between static and kinetic friction with Themistius stating in 350 A.D. that “it is easier to further the motion of a moving body than to move a body at rest”.

The classic laws of sliding friction were discovered by Leonardo da Vinci in 1493, a pioneer in tribology, but the laws documented in his notebooks were not published and remained unknown. These laws were rediscovered by Guillaume Amontons in 1699 and became known as Amonton’s three laws of dry friction. Amontons presented the nature of friction in terms of surface irregularities and the force required to raise the weight pressing the surfaces together.

1734-1750

This view was further elaborated by Bernard Forest de Bélidor and Leonhard Euler (1750), who derived the angle of repose of a weight on an inclined plane and first distinguished between static and kinetic friction. John Theophilus Desaguliers (1734) first recognized the role of adhesion in friction. Microscopic forces cause surfaces to stick together; he proposed that friction was the force necessary to tear the adhering surfaces apart.

The understanding of friction was further developed by Charles-Augustin de Coulomb (1785). Coulomb investigated the influence of four main factors on friction: the nature of the materials in contact and their surface coatings; the extent of the surface area; the normal pressure (or load); and the length of time that the surfaces remained in contact (time of repose). Coulomb further considered the influence of sliding velocity, temperature and humidity, in order to decide between the different explanations on the nature of friction that had been proposed.

1758

The distinction between static and dynamic friction is made in Coulomb’s friction law (see below), although this distinction was already drawn by Johann Andreas von Segner in 1758. The effect of the time of repose was explained by Pieter van Musschenbroek (1762) by considering the surfaces of fibrous materials, with fibers meshing together, which takes a finite time in which the friction increases.

John Leslie (1766–1832) noted a weakness in the views of Amontons and Coulomb: If friction arises from a weight being drawn up the inclined plane of successive asperities, why then isn’t it balanced through descending the opposite slope? Leslie was equally skeptical about the role of adhesion proposed by Desaguliers, which should on the whole have the same tendency to accelerate as to retard the motion. In Leslie’s view, friction should be seen as a time-dependent process of flattening, pressing down asperities, which creates new obstacles in what were cavities before.

1877

Arthur Jules Morin (1833) developed the concept of sliding versus rolling friction. Osborne Reynolds (1866) derived the equation of viscous flow. This completed the classic empirical model of friction (static, kinetic, and fluid) commonly used today in engineering. In 1877, Fleeming Jenkin and J. A. Ewing investigated the continuity between static and kinetic friction.

20th century

The focus of research during the 20th century has been to understand the physical mechanisms behind friction. Frank Philip Bowden and David Tabor (1950) showed that, at a microscopic level, the actual area of contact between surfaces is a very small fraction of the apparent area. This actual area of contact, caused by asperities increases with pressure.

The development of the atomic force microscope (ca. 1986) enabled scientists to study friction at the atomic scale, showing that, on that scale, dry friction is the product of the inter-surface shear stress and the contact area. These two discoveries explain Amonton’s first law (below); the macroscopic proportionality between normal force and static frictional force between dry surfaces.

Independence from the contact area

Two simple experimental facts characterize the friction of sliding solids. First, the amount of friction is nearly independent of the area of contact. If a brick is pulled along a table, the frictional force is the same whether the brick is lying flat or standing on end. Second, friction is proportional to the load or weight that presses the surfaces together. If a pile of three bricks is pulled along a table, the friction is three times greater than if one brick is pulled. Thus, the ratio of friction F to load L is constant.

This constant ratio is called the coefficient of friction and is usually symbolized by the Greek letter mu (μ). Mathematically, μ = F/L. Because both friction and load are measured in units of force (such as pounds or newtons), the coefficient of friction is dimensionless. The value of the coefficient of friction for a case of one or more bricks sliding on a clean wooden table is about 0.5, which implies that a force equal to half the weight of the bricks is required just to overcome friction in keeping the bricks moving along at a constant speed.

Between zero and the smallest force

The frictional force itself is directed oppositely to the motion of the object. Because the friction thus far described arises between surfaces in relative motion, it is called kinetic friction.

Static friction, in contrast, acts between surfaces at rest with respect to each other. The value of static friction varies between zero and the smallest force needed to start motion. This smallest force required to start motion, or to overcome static friction, is always greater than the force required to continue the motion, or to overcome kinetic friction.

Rolling friction occurs when a wheel, ball, or cylinder rolls freely over a surface, as in ball and roller bearings. The main source of friction in rolling appears to be dissipation of energy involved in deformation of the objects. If a hard ball is rolling on a level surface, the ball is somewhat flattened and the level surface somewhat indented in the regions in contact.

100 to 1,000 times

The elastic deformation or compression produced at the leading section of the area in contact is a hindrance to motion that is not fully compensated as the substances spring back to normal shape at the trailing section. The internal losses in the two substances are similar to those that keep a ball from bouncing back to the level from which it is dropped. Coefficients of sliding friction are generally 100 to 1,000 times greater than coefficients of rolling friction for corresponding materials. This advantage was realized historically with the transition from sledge to wheel. (See mechanics.)

What Is Static Friction?

Have you ever noticed that it’s harder to get a shopping cart moving than it is to keep it moving. If you try to push your couch across the room, the first push is the hardest part. Maybe people assume that’s nothing more than psychological, but there really is a physics reason for it. The reason is static friction. Friction, in general, is a force that makes it harder for two objects to slide alongside one another. Static friction is the friction that exists between a stationary object and the surface on which it’s resting.

Once the objects have already started moving, kinetic friction takes over. This is the friction that exists between two objects moving relative to each other. Kinetic friction isn’t as strong as static friction, and so it’s easier to keep the shopping cart moving.

What Causes Friction?

If you were to see an extreme close up of an otherwise smooth surface, you’d see that it contains a whole landscape of mountains and valleys, pits and bumps. These imperfections cause two surfaces to grip each other and make it hard for things to slide.

But when an object is stationary, there’s also something called adhesion between two surfaces. Adhesion is where two non-moving surfaces stick together slightly, due to some light chemical bonding between the materials. This is what makes static friction so strong.

Inequality for Static Friction

Force is a push or pull, measured in Newtons (N). Friction is one such force. We can calculate how many Newtons of frictional force there are between two surfaces using this inequality:

The coefficient of friction is just a number that represents how much two surfaces grip each other. The normal force is the force a surface applies to an object to keep it sitting on a surface. Without a normal force, objects would fall through the ground, because there would be nothing to hold them up.

Laws of Static Friction

• The maximum force of static friction is not dependent on the area of contact.

• The maximum force of static friction is comparative to the normal force i.e., if the normal force increases, the maximum external force that the object can endure without moving, also increases.

Limiting Friction:

It is the highest value of static friction which comes into play when an object is just about to slide over the surface of a different object. For an exerted external force greater than the limiting friction, the body begins to move.

Once the motion has started, static friction cannot be considered. A new type of friction termed as kinetic friction comes into play.

Laws of Limiting Friction:

• The direction of limiting frictional force is always contrary to the direction of motion.

• Limiting friction acts tangentially to the two surfaces interacting.

• The magnitude of limiting friction is proportional to the normal reaction amidst the two surfaces.

• The limiting friction hinges upon the material, the nature of the surfaces interacting and their evenness.
• So long as the normal reaction is the same, the magnitude of limiting friction is free of the shape or the area of the surfaces in contact, for any two given surfaces.

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