The force that resists the motion of one surface relative to another with which it is in contact. For a body resting on a horizontal surface there is a normal contact force, R, between the body and surface, acting perpendicularly to the surface. If a horizontal force B is applied to the body with the intention of moving it to the right, there will be an equal horizontal friction force, F, to the left, resisting the motion. If B is increased until the body just moves, the value of F will also increase until it reaches the limiting frictional force (FL), which is the maximum value of F. FL is then equal to μsR, where μs is the coefficient of static friction, the value of which depends on the nature of the surfaces. Once the body is moving with constant velocity, the value of F falls to a value Fk, which is equal to μkR, where μk is the coefficient of kinetic friction. Both μs and μk are independent of the surface area of the body unless this is very small and μk is almost independent of the relative velocity of the body and surface.
The cause of friction is that surfaces, however smooth they may look to the eye, on the microscopic scale have many humps and crests. Therefore the actual area of contact is very small indeed, and the consequent very high pressure leads to local pressure welding of the surfaces. In motion the welds are broken and remade continually. See also rolling friction.
Friction.
For two bodies in contact, say the frictional force and the normal reaction have magnitudes F and N respectively (see contact force). The coefficient of static friction μs is the ratio F/N in the limiting case when the bodies are just about to move relative to each other. Thus if the bodies are at rest relative to each other, F ≤ μsN. The coefficient of kinetic friction μk is the ratio F/N when the bodies are sliding, in contact with each other. These coefficients of friction depend on the materials of which the bodies are made. Normally, μk is somewhat less than μs. See also angle of friction.
A force which resists the motion of solid or fluid bodies that are in contact and slide against each other, thereby converting kinetic energy into thermal energy. It is described by the two laws of Amontons (1. friction is directly proportional to the applied load; 2. friction is independent of the apparent area of contact) and by Coulomb’s law (friction is independent of the sliding velocity).
A force that resists the movement of one body in relation to another body with which it is in contact. Where one body rests upon another there is a contact force (R) between them acting at right angles to the contact surface. If a force B acting parallel to the contact surface is applied to one of the bodies, a force F will resist it. If B increases until the bodies move against each other, F will also increase until it reaches a maximum (FL). FL = μgR, where μg is the coefficient of static friction. Once the two bodies are moving against each other at a constant velocity, F will fall to a value Fk = μkR, where μk is the coefficient of kinetic friction.
The force which resists the movement of one surface over another. Friction between the surfaces of two mineral grains is related to the hardness of the mineral, the roughness of the surface, and the number and area of the points of contact between the grains. It is of major significance in any study of the movement of sediment since the forces moving the sediment must be greater than the resistance provided by friction. In meteorology, the frictional force is the roughness and irregularity of the Earth’s surface that reduces wind speeds. The friction layer, where this effect is strongest, roughly comprises the lowest 100 m of the atmosphere. See Arya (1985) J. Appl. Met. 24, 9.
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