Friction is a natural phenomenon which resists the motion of a solid material body upon another solid surface or liquid mass or gaseous atmosphere. Different theories are available which tells about the causes of friction.
Friction comes to play in action in the following cases, when –
- A solid body moves or tend to move upon another solid surface.
- Solid body moves or tend to move in a liquid mass.
- One liquid layer slides or tends to slide over another layer.
- A solid object moves or tends to move in a gaseous atmosphere.
- A solid body undergoes deformation, then friction develops between its molecules.
Therefore, if there is a relative motion between two surface, a frictional force is exerted on the body in a direction opposite to the direction of movement of body.
Friction also founds in moving fluid layers. When liquid surfaces or layers in contact move or tend to move relative to each other, friction between the two layer develops.
To be noted –
Different theories are available for causes of friction. The branch of study of friction is called tribology. Greek word tribos means rubbing.
Tribology is the branch of science which deals with a frictional force and its causes and effects.
The science of tribology tells about contact mechanics that generally involves energy dissipation. It deals with scientific approach of tribological properties like Adhesion, Friction, Lubrication and Wear.Leonardo da Vinci (1452-1519) is known as “father of modern tribology“. He studied about the topics such as friction, wear, bearing materials, plain bearings, lubrication systems, gears, screw-jacks, and rolling-element bearings.Guillaume Amontons (1663-1705) and Charles-Augustin Coulomb (1736-1806) are other famous tribologist. Their findings about friction are summarized in following three hypothesis –
- Amonton’s 1st Law – Force of friction is directly proportional to applied load.
- Amonton’s 2nd Law – Force of friction is independent of apparent area of contact.
- Coulomb’s Law – Kinetic friction is independent of sliding velocity.
As per tribology study, the main causes of friction are –
- Molecular adhesion.
- Interlocking due to surface roughness & deformation.
- Electromagnetic attraction.
Adhesion is the force required to separate two different bodies which are in contact with each other. A famous tribologist, Desanguliers (1734) proposed adhesion as a cause of friction process.
Another tribologist, Amonton, suggested that, friction doesn’t depend upon the contact area. By experiments he derive a hypothesis in contradiction to the concept of adhesion.
- He developed the interlocking theory of mechanical asperities.
- According to this, the real area of contact is made up of a large number of small regions of contact called asperities.
- These are junctions of contact where atom-to-atom contact takes place.
This theory is known as concept of real area of contact.Contradiction between the Desangulier’s adhesion theory and Amonton’s hypothesis cleared up by the introduction of the concept of the real area of contact.
- Other scientists, Bowden and Tabor (1954) showed that, the force of static friction between two sliding surfaces depend on the real area of contact causing abrasive wear.
- When a hard rough surface slides against a softer surface, asperity junctions plastically deform above a critical shear strength.
- This process depends on the adhesive forces of the two surfaces in contact. This is a fully plastic flow situation of all asperities and called plastic flow of asperities.
- In this situation, friction is found to change linearly with the applied load as demanded by Amonton’s hypothesis.
Some scientists believe that friction is the result of electromagnetic force of attraction between the charged particles in two contacting surfaces.
- The frictional force always act in a direction opposite to the movement of body or its tendency to move.
- This frictional force converts kinetic energy into heat and light energy.
Frictional force has effects in both forms of “usable property” or a “waste property“.
|FRICTION – As a usable property
|FRICTION – As a waste property
|In various applications, friction is an usable property or a desirable quality.
|In other applications, friction is a waste property or undesirable quality.