The causes of mechanical friction have been investigated for many centuries.

Several different types of frictional effects have now been extensively described.

Some are purely mechanical - for example the friction caused by surface-details 'interlocking'. Imagine the friction between two sheets of sandpaper, or on a more coarse scale, dragging car tyres over a rocky terrain.

Other types come from 'adhesive' forces of various kinds between materials. Again, there are many types of adhesive forces.

• Chemical (closely touching materials forming chemical bonds)
• Dispersive (held together by van der Waals forces etc)
• Electrostatic (due to attractions between electrons + 'holes')
• Difussive (where substances effectively 'leak' into each other)
• Atmospheric 'suction' (in the presence of liquids on smooth surfaces)

In real world scenarios, it's often extremely difficult to identify and quantify which types of adhesion or mechanical interactions are causing (or will cause) friction in any particular situation.

Surface smoothness, temperature, atmospheric pressure, and the presence or absence of extra materials such as lubricants (wet or dry) can also very dramatically affect the frictional forces. As can the velocity of the realative movements - some material interactions 'stick' dramatically when stationary. (See Stiction at Wikipedia)

The picture is further complicated by the fact that the pressure applied between materials has a large effect on the frictional forces - and not always as an increase, one famous example being the blades of ice-skating boots which glide more easily under pressure.

In assessing newly developed materials, experimental measurements are often the only way of assessing frictional forces. (ref.)

Some extremely simple commonplace examples are still not fully understood even after several decades of research. An example being the friction between rubber wiper blades and glass windscreens (windshields US). (ref.)