Re: Is friction a reaction force?

In a sense all forces are reaction forces because all forces are partners 
in action reaction pairs. But I think that you mean something different. 
Friction is a response to motion or to incipient motion, as it were. I 
would not use the term "reaction force" because that has a specific and 
technical meaning in Newtonian physics. Consider first static friction and 
imagine a wooden block on a rough wooden surface. If you push on the block 
parallel to the surface, no motion occurs until a certain threshold applied 
force is reached. Until that force is applied, the static frictional force 
adjusts itself to be  equal and opposite the applied force, like the force 
which a spring exerts on your hand when you pull on it. Note however, that 
the static friction force and the applied force are ON THE SAME BODY , 
unlike the action reaction pair of Newton's Third Law.
  The static frictional force thus matches the applied force until the 
applied force is made large enough finally to overcome the attractive 
forces between the surfaces. The bonds are finally broken. These forces are 
intermolecular in nature, and are applied only where the surfaces are in 
very close contact,  which is  only over an area  much smaller than the 
actual physical area of the block. The close- contact area , where the 
surface separation is  of molecular dimensions, is proportional to the 
normal force - that is, the weight of the block, or more generally, the 
downward force which the block applies to the surface. It is this force 
which effectively presses the surfaces together and creates close contact.
  Now suppose that you are pushing the block along the surface. Static 
frictional forces have been overcome, but the surfaces still are constantly 
binding to each other and unbinding as relative motion breaks the bonds. 
The result is a force which is opposite to the direction of velocity of the 
block. This is the force of kinetic friction.  However kinetic friction does 
not depend on the magnitude of velocity . Both kinetic and static friction 
depend on the nature of the surfaces and on the normal force , and , for 
most materials, on nothing else. For a certain choice of surfaces friction 
is characterized by two coefficients of friction,  mu static and mu 
kinetic. The static mu is alsways greater than the kinetic and the force of 
friction in either case is mu times the normal force. Sometimes ( steel on 
steel ) the coefficients differ little ( 0.7, 0.6 ). But for glass on glass, 
we have 0.9 compared to 0.4 .