Chloe Zhang
Friction is the force exerted by a surface when an object moves across it or tries to move across it. The degree of friction is expressed as the coefficient of friction (COF), which indicates the friction between two surfaces and how easily one material can slide over the other. The COF is not based on a material's property alone but is a system's property. The COF of engineering plastics compared to steel can be determined by running a practical test under real service conditions. The static friction coefficient of some plastics against steel and aluminium under different contact conditions has been studied, revealing that the static friction coefficient decreases when normal load or roughness increases.
| Characteristics | Values |
|---|---|
| Friction force | The force exerted by a surface when an object moves across it or makes an effort to move across it |
| Kinetic or sliding friction coefficient | Used when there is relative motion between surfaces |
| Static friction coefficient | Used when there is no relative motion between surfaces |
| Coefficient of Friction (COF) | A system's property, not just a material's property; the degree of friction between two surfaces, expressed as a number (a larger number indicates higher friction) |
| Steel on steel dry static friction coefficient | 0.8 |
| Steel on steel lubricated static friction coefficient | 0.16 |
| Static friction coefficient of plastics against steel | A study of three thermoplastics (UHMWPE, PA 66, and POM) revealed that the static friction coefficient decreases when normal load or roughness increases |
| Dynamic friction coefficient | The ratio between the friction force and the normal force when the surfaces are moving relative to each other |
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What You'll Learn
- Coefficient of Friction (COF) is a system's property, not a material property
- The dynamic coefficient of friction is the ratio between friction force and normal force when surfaces move
- Static friction is friction between solid objects not moving relative to each other
- Rougher materials have higher friction
- The static coefficient of friction is the ratio between the maximum frictional force and normal force before movement
Coefficient of Friction (COF) is a system's property, not a material property
Friction is the force exerted by a surface when an object moves across it or makes an effort to move across it. The degree of friction between two surfaces is expressed as the coefficient of friction (COF). The COF is a value that indicates how easily one material can slide over another.
The COF is not based on a material's property alone but is a system's property. This means that the COF of a material cannot be determined by looking at just one part or piece of a system; the total solution must be evaluated. For example, the COF of engineering plastics compared to steel cannot be determined by looking at steel or plastics in isolation. Instead, the COF of the two materials together must be assessed.
The COF of a material can be determined by running a practical test under real-world service conditions. For instance, the static friction coefficient of plastics against steel can be tested under different contact conditions. The normal force, stick time, apparent area of contact, and lubrication of the interface can be assessed. The COF can also be determined by using a tribological device under standard laboratory conditions, although this data should not be used to predict the frictional behaviour of materials under real service conditions.
The COF is influenced by various factors, including the normal force, speed, surface roughness, and temperature. For instance, the COF of steel on steel dry static friction is 0.8, but this drops to 0.4 when sliding is initiated. Similarly, the COF of steel on steel lubricated static friction is 0.16, dropping to 0.04 when sliding is initiated.
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The dynamic coefficient of friction is the ratio between friction force and normal force when surfaces move
Friction is a force that resists motion when the surface of one object comes into contact with another. The coefficient of friction is a measure of the amount of friction between two surfaces. The dynamic coefficient of friction is the ratio between friction force and normal force when surfaces move.
The normal force is the support force exerted upon an object that is in contact with another stable object. The normal force can be described by the formula N = mg, where m is the object's mass, and g is the acceleration due to gravity. For example, the normal force of a 2-kg block of wood sitting on a surface can be calculated as N = 2 kg × 9.8 N/kg = 19.6 N.
The coefficient of friction is a dimensionless scalar value, meaning it has no units. It is calculated as the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of static friction is the ratio of the maximum static friction force between surfaces before movement to the normal force. The coefficient of kinetic friction is the ratio of the kinetic friction force between surfaces during movement to the normal force.
The dynamic coefficient of friction is the force required to maintain movement at a constant rate. It is commonly thought that the static coefficient of friction is higher than the dynamic coefficient. However, this is a simplistic statement that can be misleading, especially for brake materials. If we consider the static situation at ambient temperature, the static coefficient of friction is often significantly lower than the quoted average dynamic value.
The dynamic coefficient of friction is influenced by various factors, including the speed and pressure of sliding, surface pressure, and operating temperature. For example, at higher pressures, the minimum friction occurs at a velocity of 2 ft/s (0.7 m/s), and friction increases with the square root of velocity. The coefficient of friction can also be adjusted by using fillers, such as graphite or PTFE, which can reduce it.
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Static friction is friction between solid objects not moving relative to each other
Friction is the force exerted by a surface when an object moves across it or makes an effort to move across it. It varies with speed at low pressure. The friction force can be calculated using the equation: Fmax = Frictional force (N, lb) x μ = static (μ s) or kinetic (μ k) frictional coefficient x Fn = Applied Normal force (N, lb).
Static friction is a type of friction that occurs between solid objects that are not moving relative to each other. It can be understood as the friction that prevents an object from sliding down a sloped surface or the force that keeps an object stationary when pushed with a force that is not strong enough to move it. For example, when you gently push a book on a table, static friction keeps the book in place. The moment the book starts to move, static friction becomes kinetic friction.
The coefficient of static friction (μs) is typically higher than the coefficient of kinetic friction. For instance, the steel-on-steel dry static friction coefficient is 0.8, but it drops to 0.4 when sliding is initiated. Similarly, the steel-on-steel lubricated static friction coefficient is 0.16, which decreases to 0.04 when sliding begins.
Static friction arises due to the microscopic interlocking of bumps and valleys on the surfaces in contact. Even surfaces that appear smooth have microscopic roughness. When two such surfaces come into contact, their bumps and valleys interlock, creating resistance to movement. Overcoming this interlocking requires sufficient energy to initiate the motion of an object at rest.
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Rougher materials have higher friction
The relationship between friction and surface texture is a complex one, and it depends on a variety of factors, including the type of texture and the materials involved. While it is commonly thought that rougher surfaces produce more friction, this is not always the case.
The notion of the "Real Area of Contact" is key to understanding how friction and surface texture are related. This idea was first introduced by Frank Phillip Bowden, a pioneer in the field of tribology, and further developed by Greenwood and Williamson (GW66). Bowden's work established that dry sliding friction arises from the shearing of junctions formed at the points of contact between two surfaces. The "Real Area of Contact" refers to the amount of surface area shared by two objects in contact. When two surfaces are in contact, the real area of contact depends on the nature of the surface texture.
In some cases, rougher surfaces can indeed lead to increased friction. This is because a rougher surface can have a larger real area of contact with another surface, resulting in higher friction. This is especially true for elastically contacting surfaces, where lower surface roughness can lead to a larger real area of contact and, consequently, higher friction. Additionally, the roughness of a surface can affect the effectiveness of lubricants used to reduce friction. Lubrication works by introducing a substance, such as oil or grease, between two surfaces, reducing the direct interaction between them and decreasing the frictional forces. However, if a surface is rough, the lubricant may not be able to fill in all the gaps and valleys, reducing its effectiveness.
On the other hand, it is important to note that making a surface rougher does not always increase friction. In certain cases, a rougher surface can actually reduce friction. The relationship between surface roughness and friction is highly dependent on the specific materials involved and the conditions under which they interact. For example, in some situations, a rough surface texture coated with a hard material can act like a "saw" against a smooth surface, reducing friction. Additionally, the elasticity, hardness, and presence of lubricants can all influence the frictional behaviour, and these factors may outweigh the impact of surface roughness.
While the relationship between friction and surface texture is intricate, understanding it is of utmost importance in materials science, especially in the realm of surface treatment. By comprehending how friction varies with surface texture, scientists can develop more efficient machinery, reduce energy losses, and improve surface treatments.
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The static coefficient of friction is the ratio between the maximum frictional force and normal force before movement
Friction is a force that resists relative motion between objects in contact. The frictional force depends on the materials that are pressing against one another. The frictional force is calculated as the product of the coefficient of friction and the normal force. The normal force is the support force exerted on an object in contact with another stable object. It can be calculated as the product of the object's mass and acceleration due to gravity.
The coefficient of friction is a measure of the amount of friction between two surfaces. It is a dimensionless number that depends only on the two surfaces in contact. The coefficient of friction is typically higher for static objects than for kinetic objects. This is because static objects require more energy to push into motion than to keep them moving.
The static coefficient of friction for dry, hard steels is 0.43. For steel on steel, the dry static friction coefficient is 0.8, dropping to 0.4 when sliding is initiated. The static coefficient of friction for wood is 0.5.
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Frequently asked questions
Friction is the force exerted by a surface when an object moves across it or makes an effort to move across it. The degree of friction is expressed as the coefficient of friction (COF).
The coefficient of friction indicates the friction between two surfaces or how easily one material can slide over the other. This is expressed in a number—a larger number indicates higher friction.
The coefficient of friction is not based on a material's property alone but is a system's property. The COF of engineering plastics can be used for comparative purposes to select the appropriate material for an application. The COF of plastic against steel can be determined by running a practical test under real service conditions.
Static friction is friction between two or more solid objects that are not moving relative to each other. The static coefficient of friction is the ratio between the maximum frictional force before the surfaces start moving relative to each other and the normal force.
A study of the static friction coefficient of three thermoplastics (UHMWPE, PA 66, and POM) revealed that the static friction coefficient decreases when normal load or roughness increases. The same conclusion was reached in a study on the effect of load and surface roughness on the static friction of steel.
