Damped oscillations and generalized friction I'm reading damped oscillations from the book Classical Mechanics by Landau and Lifshitz, quoting from the text -

"There exists, however a class of problems where motion in medium can be
  approximately described by including certain additional terms in the
  mechanical equations of motion. Such cases include oscillations
  with frequencies small compared compared with those of dissipative
  processes in the medium. When this is fulfilled we may regard the body
  as being acted by a force of friction which depends (for homogeneous
  medium) only on its velocity."

Can someone explain me how did he argue the portion in italic?
 A: The paragraph may possibly be interpreted along these lines:
1) Compare an object being pushed on the surface of a rough table with an object being pushed while immersed in fluid. 
Both objects experience an opposing frictional force. For the immersed object, frictional force is dependent on velocity. For the table, frictional force does not depend on velocity. The proof of velocity-dependence in fluids is outside the scope of this answer and can be found in many references. 
2) The classification between these two cases can be attributed to dissipation processes within the medium. The table surface experiences microscopic vibrations which quickly convert to heat. The fluid experiences mass displacement, both longitudinal and transverse, which decay slowly. We can say that for fluids, the typical decay-time-constant is long.  
3) An oscillating object imposes an additional complication. An oscillating object on the table (e.g. mass connected to a spring), will experience the same frictional force whatever the oscillating frequency is. This is not true for an oscillating object in fluids. 
If the typical time constant of the oscillation is shorter than the decay-time-constant, than the effect of fluid on the oscillating object is much more complex and does not exhibit a simple velocity dependency (just think of the fluid masses displaced back and forth).   
4) To conclude using the paragraph terms: Only if the oscillating time constant is long (oscillating frequency small) even when compared with the typical long dissipative processes in the medium (long decay constant) -- will then standard fluid physics apply and friction will be strictly velocity dependent.  
