The resistance to motion which is called into play when it is attempted to slide one surface over another, with which it is in contact. The frictional force opposing the motion is equal to the moving force up to a value known as the limiting friction. Any increase in the moving force will then cause slipping. Static friction is the value of the limiting friction just before slipping occurs. Kinetic friction is the value of the limiting friction after slipping has occurred. This is slightly less than the static friction.

The coefficient of friction is the ratio of the limiting friction to the normal reaction between the sliding surfaces. It is constant for a given pair of surfaces.

MaterialStatic frictionSliding friction
Brakes~0.30.3 - 0.7
Clutches0.3 - 0.40.3 - 0.7
Earth - Earth0.25 - 1.0 
Lignum vitae - Steel 0.1
Metal - Metal (dry)0.15 - 0.60.1 - 0.5
Nylon - Steel 0.3 - 0.5
Perspex - Steel~0.5 
PTFE - Steel 0.05 - 0.2
Rubber - Steel0.6 - 0.90.3 - 0.6
Rubber - Asphalt0.7 - 0.90.5 - 0.8
Wood - Wood0.25 - 0.50.25 - 0.5

Power Lost to Engine Friction

This can be calculated from the difference between Indicated Power and Brake Power.

Roughly 10% of the energy input into an internal combustion engine is lost in friction in the engine.


George Fenske, Robert Erck, Layo Ajayi, Ali Erdemir and Osman Eryilmaz, "Parasitic Energy Loss Mechanisms Impact on Vehicle System Efficiency", Argonne National Laboratory

See also: Coefficient of Friction, Fluid Friction, Friction Clutch, Friction Gear, Friction Horse Power, Friction Torque, Galling, Lubricant, Power Lost to Engine Friction, Viscosity.

Previous PageView links to and from this pageNext Page

Subjects: Mechanical Engineering Physics