The coefficients of friction or the coefficients of friction are linked to the opposition to sliding offered by the surfaces "Surface (physical)") of two bodies in contact according to the intensity of the mutual support they experience. It is a dimensionless coefficient "Coefficient (mathematics)"). It is usually represented by the Greek letter μ (mi).

The value of the friction coefficient is characteristic of each pair of materials in contact; It is not an intrinsic property of a material. It also depends on many factors such as temperature, the finish of the surfaces "Surface (physics)"), the relative speed between the surfaces, etc. The nature of this type of force is linked to the interactions of the microscopic particles of the two surfaces involved.

For example, ice on a sheet of polished steel has a low coefficient; while rubber on the pavement has a high coefficient. The coefficient of friction can take values ​​from almost zero and normally does not exceed unity.

Most surfaces, even those considered polished, are extremely rough on a microscopic scale. When two surfaces are brought into contact, the movement of one relative to the other generates tangential forces called friction forces, which have a direction opposite to the movement; the magnitude of this force depends on the dynamic friction coefficient.

There is another form of friction related to the previous one, in which two rigid surfaces at rest do not move with respect to each other as long as the force parallel to the tangent plane is sufficiently small, in this case the relevant coefficient is the static friction coefficient. The condition for there to be no slippage is that:

Where:.

For deformable surfaces it is convenient to consider the previous relationship in terms of normal and tangential stresses at a point; there will be relative slip if at any point:

Where:.

When considering the sliding of a body on an inclined plane, it is observed that by varying the inclination of said plane, the object begins the movement when a critical angle of inclination is reached. This is because as the inclination increases, the perpendicular component of the weight, the force , is gradually reduced, which is proportional to the cosine of the angle of inclination.