Designation for the forces of resistance arising between the contact surfaces of adjoining layers of different or the same materials as the result of macroscopic or molecular forces, the latter acting counter to a relative movement of the layers (frictional resistance). The effective macroscopic forces are conditional on the roughness of the contact surfaces, which produces a kind of intermeshing of the surfaces. It is possible to reduce the frictional resistance by carefully smoothing the contact surfaces and/or introducing a thin film of lubricant between the surfaces. The energy required to overcome frictional resistance is transformed almost entirely into heat.