Co-efficient of Friction (COF) defines the sliding interaction between two surfaces. Static Friction and Dynamic Friction (or Kinetic Friction) are defined in many standards including ASTM D1894, ASTM D4918, ASTM G219, ISO8295, TAPPI T549, TAPPI T815, TAPPI T503, TAPPI T548, and NFQ 03083.
The co-efficient defines the sliding resistance of two surfaces, such as a ski on snow (A), or feet on grass (B). In the absence of friction, the effort needed to slide an object would be significantly reduced, as would the effort needed to stop or change direction.
Newton’s first law of motion states that a body remains at rest or in uniform motion unless acted upon by a force. Gravity causes a mass to press down with a force equal to its mass multiplied by acceleration. It follows that friction is the force needed to start sliding an object (Static Friction), or the force needed to keep it sliding (Kinetic Friction), expressed as a ratio by dividing into the downward force of the mass.