divideby0 you will not encounter these problems in physics, (the worst will be "this object experiences 30N of friction....)

but if you need to know:

friction is the retardation force when two objects rubs together. It represents the imperfection in the surface of the material. The friction force is directly proportional to the normal force, that is, the harder it is pulled down towards the surface, the harder it is to move along it. the constant "k" in this relationship is your friction coefficients

The static coefficient represents the friction when the object is not moving, and allowing even some of the imperfections on the atomic level to lock together. The kinetic coefficient is lower, as movement does not allow the smaller imperfections to "lock" together, however, larger imperfections still do.

These coefficients are averages, as imperfection is not spread evenly, but rather randomly.

and there can be frictionless surfaces, these are either impossibly hard to produce (nanomachines with perfect surfaces), or are found under extreme gravitational forces (the surface of a neutron-star)

the general equation for friction is F

_{friction}=k F

_{normal}, where k is the appropriate coefficient for the given case (where velocity>0 use kinetic otherwise use static, unless of course you are dealing with rolling friction which is different yet again)

hope that made sense