FRICTION

 

Measured in Newtons (N)

 

Any two surfaces can have friction between them.  The maximum amount of friction is dependent on the nature of the surfaces and the force exerted on them and the ‘stickiness’ of the two surfaces is measured by the Coefficient of Friction (  ). The coefficient of friction has no units, the closer to zero the number the less friction there is ( NB  means there is no friction) and the higher the number the more friction there is.

 

where:

F_max      = maximum amount of friction

          = coefficient of friction

R          = reaction

 

Friction is only ‘present’ when motion is taking place and then the frictional force ALWAYS opposes motion.

so  

 

Friction will always try to match the magnitude of the pushing force.  As the pushing force increases so does the magnitude of the frictional force UNTIL the frictional force reaches its maximum value (given by  ).  This point is called LIMITING EQUILIBRIUM.  At this point the particle can move with the constant velocity (NB: V could equal zero!).  When the pushing force increases beyond this value, as the frictional force can no longer increase,  so the particle accelerates and the magnitude of the acceleration can be found by .

 

1)

 

 

 

There is no motion

 

2)

 

 

 at limiting equilibrium

motion possible at constant velocity

NB: V could = 0

 

3)

 

 beyond limiting equilibrium so

 

 

there is acceleration