Friction is a key concept when you are attempting to understand car accidents. The force of friction is a force that resists motion when two objects are in contact. If you look at the surfaces of all objects, there are tiny bumps and ridges. Those microscopic peaks and valleys catch on one another when two objects are moving past each other.
Graphing frictional forces verses force applied.
As a force is applied to the 100 N box, static friction, which is a reactionary force, responses with the strength equal to the force applied keeping the object fixed. Force applied continues to increase, and the force of static friction increases equally until it reaches what is called "the threshold value". In the diagram below the threshold value is 50 N, at that point the box breaks free and slides across the surface. From this point on, as long as the block continues to move the force of friction stays at a constant 40 N even as the block accelerates.
Difference between static and kinetic friction
