Always acts in the direction opposite to motion or possible motion.
Friction has a limit which it cannot exceed. This is called limiting friction and is proportional to the normal contact force acting on a object.
The limiting friction depends on the object and the surface it is on and is governed by the equation F=μR, where μ is the coefficient of friction for the specific combination of object and surface.
If object is on the point of moving we say it is in limiting equilibrium.
μ is typically between 0.3 and 0.9, however it can be greater than 1 (it is an experimentally observed approximation and does not represent a mathematical law like Newton’s laws).
Exercise (skip questions 3 and 4 which is about total contact force. Total contact force is the resultant of the friction and the reaction force, which we haven’t talked about)
F ≤ μR
It is only when an object is at the point of moving (in limiting equilibrium), that F = μR.
More Advanced Friction Questions
Exercise (and General Exercises)