PeroK said:
You've already been told that friction has nothing to do with Newton's third law.
You are, of course, correct that friction has little to do with Newton's
third law. However, there is room for confusion.
In the case of
static friction the frictional force will be whatever it has to be to prevent relative motion between the mating surfaces. In the usual case of a mobile object on a stationary surface, this means that the force of static friction will be equal and opposite to the sum of all other applied forces.
It is tempting to take this statement and say that static friction is a "reaction" to the other applied forces. It is then also tempting to think "equal and opposite" and "reaction". Well then, we must be talking about the third law!
Not so fast. That is actually what I like to call a "
second law" action/reaction force pair. You have forces on an object whose momentum does not change (because it has negligible acceleration or negligible mass or both). The forces must sum to zero (##\sum F = ma##). So the one force must be equal and opposite to the sum of all the others.
As rules of thumb:
For static friction, you determine friction by summing the other forces on the object and negating the sum to determine both magnitude and direction of the frictional force. If the magnitude is within the bounds imposed by the coefficient of static friction, you are done. If the magnitude is too high, then slipping will be occurring. If not already, then after only an instant more.
For kinetic friction, you determine the magnitude based on the coefficient of kinetic friction. You determine direction by the opposite of the direction of relative motion. If the objects are at relative rest and about to begin slipping, you can determine the initial direction based on the opposite of the direction of the sum of the other forces.