Friktionstal

Friktionstal, or coefficient of friction, is a dimensionless quantity that characterizes the resistance to sliding between two contacting surfaces. It is usually denoted by the Greek letter mu (μ) and is defined as the ratio between the frictional force F and the normal force N acting perpendicular to the contact surface. Two main forms are distinguished: the static (or impending) friction coefficient μs, which limits the frictional force when the surfaces are at rest, and the kinetic (or dynamic) friction coefficient μk, which describes the friction when the surfaces are sliding past each other. In sliding motion, the frictional force is typically F ≈ μk N, while when at rest the friction can vary up to F ≤ μs N.

Historically, Amontons’ laws described friction as proportional to the normal force and largely independent of contact area, with μs and μk representing fixed properties of the material pair. However, modern understanding recognizes that friktionstal depends on many factors beyond material identity, including surface roughness, temperature, contamination, lubrication, contact pressure, and sliding velocity. Real surfaces may exhibit adhesion, plowing, and changes in lubrication regime that cause μ to vary with conditions.

Friktionstal is widely used in engineering and physics to predict and analyze forces, energy loss, wear, and stability. Values vary greatly across material pairs and conditions; for example, dry metal-to-metal contacts generally have higher μ than lubricated or ice-on-metal contacts. Accurate determination often requires measurements with tribological testing or material-specific data under the relevant operating conditions.