MadSci Network: Physics

Re: could you explain what exactly causes friction?

Date: Thu Nov 30 12:24:40 2000
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 974330972.Ph

Exciting research is happening in tribology--the science of rubbing 
surfaces.  However, there still is much to learn about what exactly 
produces it.  The simple answer to your question is no--I cannot explain 
what 'exactly' causes friction.  

We presently know that there are a number of contributing causes to what 
we call friction.  Some of these are reasonably well understood.  For 
example, real surfaces have some texture.  They are not perfectly smooth 
and flat.  For this reason, there can be some geometric components to the 
frictional force between two surfaces.  Particles can get between these 
surfaces and affect friction.  
Whether the surfaces are smooth or flat, with or without particles, when 
one object is pressed against another, there are deformations.  These 
produce energy losses within the deformed materials.  All rubbing contacts 
produce some mechanical deformation friction loss.  

One source of friction results from these deformations.  But these are 
only part of friction.

Adhesion is the second basic source of friction loss.  When we get into 
the details of adhesion, there are many theories, but much is yet to be 
learned.  Electrons from the one surface may interact with those from 
another.  Some think that friction results from vibrations induced by the 
rubbing.  Others believe that quantum effects are involved as atoms of one 
surface pass close to those of another.  We do know that there are 
electrostatic forces involved in friction.  The electron clouds that 
surround the nucleus of an atom fluctuate randomly.  For example, a dense-
more negative-part of the cloud will repel the electron cloud on another 
atom, inducing a positive charge.  These will then attract each other.  
Such forces are known as van der Waals forces. 

What makes friction here on earth very complex is the fact that all 
surfaces are contaminated by films, oxide layers, and adsorbed gasses.  
These alter friction, generally lowering it.  For this reason, friction in 
the vacuum of space tends to be much higher than here on earth.

You may find it worth your time to look up the following references on the 
Internet.  The first two are basic, but should be easily understood.

If you wish more information, a good reference on science that includes a 
section on electrostatic forces is:

Finally, you wish to tackle an article from New Scientist magazine, vol. 
160 no. 2156, p.30(1998)

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