the link http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html has good tips on friction

Friction Assumptions

In the standard model for friction between surfaces, there are certain inherent assumptions:


1. The frictional force is independent of area of contact
2. The frictional force is independent of the velocity of motion
3. The frictional force is proportional to the normal force.


One can readily find circumstances where any or all of these assumptions are invalid, but they provide a straightforward framework for a beginning treatment of friction that satisfactorily describes many phenomena.

the applet can be include real life cases where the rule 1 is valid
and include real life case where rule 1 is not valid due to the increase in microscope contact area during broad tires on snow
[img]http://hyperphysics.phy-astr.gsu.edu/hbase/imgmec/fnor.gif[/img]
http://hyperphysics.phy-astr.gsu.edu/hbase/imgmec/fnor.gif