In this section, we would like to apply what we have learned in the
previous sections to address the question of what happens when we take
an oscillator and push, or drive, it with a periodic force.
This is somewhat akin to pushing a child on a swing, where the swing
by itself is an oscillatory system with a certain natural
frequency 
, and we can drive the system by pushing with
whatever frequency 
we desire. An interesting questions about
this system is what driving frequency will result in the largest
response? As you may suspect, the answer is to drive at a frequency
which nearly matches the natural frequency of the system, 
. But, as we shall see, the answer isn't quite that
simple.