where 
is the magnitude of the reaction force on the body concerned, and 
is a constant known as the coefficient of sliding friction. [P2.3, P5.2]
is: a force that arises from friction. [P5.2]
acts in a direction: that opposes the actual or potential relative motion that gives rise to it. [P5.2]
is, when there is actual relative motion: sliding friction, the magnitude of which is given by where is the magnitude of the reaction force on the body concerned, and is a constant known as the coefficient of sliding friction. [P2.3, P5.2]
is, when there is only potential relative motion: static friction, the maximum magnitude of which is given by 
where is the magnitude of the reaction force on the body concerned, and 
is a constant known as the coefficient of static friction. [P2.3, P5.2]
Copyright 1997, The Open University