Clarification on Frictional Forces?

[mneox]

Hi, I am requiring some assistance in getting some clarification on frictional forces.

I know that Ff = u(Fn).

How do you generally find the frictional force though? I noticed how sometimes Ff is the same as the applied force, and sometimes you find it by using Fnet = Fa - Ff.

Why is this? And how do I know when to use which? Thank you!

 

[neeraj93]

Frictional force acts on a body due it's interactions with the surface on which it rests. So, when you apply a force on the body, there results an opposing force due to these interactions; the body being static all the while, the force is due to static friction. It is experimentally found that this force is proportional to the normal reaction acting on the body due to the the surface. Now when you go on increasing the applied force, then for a particular value of it, the frictional force reaches a limitting value,i.e, on applying further force the body will move. Thus,
f_s=\mu_sN
where f_s is the force of static friction, N is the normal reaction, & \mu_s is the the coefficient of static friciton. \mu_s is defined only for the limitting value of the static frictional force, acting on the body at rest.
On applying a force greater than the limitting value of the frictional force, the body starts translating, and now the value of the frictional force reduces to smaller value. For this condition, \mu_k, the co-efficient of kinetic friction is defined, and the force is called the force of kinetic friction. We have,
f_k=\mu_kN
So, when you're given the condition that the body is just about to move, force applied f_a is,
f_a=f_s
But when the body is translating, the net force is given by,
F_net=f_a-f_k