Here is a brief treatment of damped oscillations with a friction force that is proportional to the velocity:
Without a proof, here is the general solution:
(for w0 > wd ) Here are: wd = b/2m, w' = (w02 - wd2)1/2, and w0 = the frequency of the oscillator without the friction force present = (k/m)1/2.
Instead of fighting with a derivation, we can experimentally explore the damped oscillator in this applet:
© W. Bauer, 1999