Friction
Friction (Ff) is a force that opposes the relative motion (or tendency for relative
motion) between 2 surfaces or substances in contact. There is always friction
between any two surfaces or substances in contact.
A closer look at the surfaces helps to explain this “retarding” force.
The amount of friction depends on:
i)
ii)
the normal force FN i.e. Ff  FN
characteristics of the 2 surfaces in contact …called the coefficient of friction
().
 pronounced mu, has no unit and is a number that represents how much friction
would exist between 2 surfaces with a given normal force. i.e. Ff  
Ff   FN
Note the lack of the vector symbols here. Although friction and normal force are
vector quantities, here we are concerned with magnitudes only. Often times this
formula will be used as only a part of a greater problem. Whether friction is
negative, depends on your convention and the given problem.
There are two types of friction
i)
Static Friction – that friction which exists between two surfaces that are not
moving relative to one another. In this case the coefficient of friction is
called the coefficient of static friction (s)
ii)
Kinetic Friction – that friction which exists between two surfaces that are
moving relative to one another. In this case the coefficient of friction is
called the coefficient of kinetic friction (k)
s > k (in almost all cases)
Static Friction > Kinetic Friction
(in almost all cases; all things being equal)