So far, we have considered primarily steady-state force balances. For the rest of the course we will consider a particular type of time-dependent force balance, where the motion of fluid parcels repeats with a characteristic period of time. Repeating motion will be created if a displacement from some equilibrium position results in a restoring force, i.e., a force proportional to the displacement that opposes the motion. These motions are called oscillations (if stationary) or waves (if moving through space as well as oscillating in time).
The first two kinds of waves that we will consider (ocean surface waves and acoustic waves) are waves in a fluid medium, and the force balance will be between the cyclical acceleration and a restoring force due to pressure gradient forces (along with gravity, in the case of ocean surface waves). The third kind of wave we will consider, electromagnetic waves, are not a fluid mechanics phenomenon. However, this form of oscillation is so important in oceanography that we will look at its applications without first understanding its theoretical basis. If you have interest in learning more about electromagnetism, consider taking a dedicated course on that topic.