1. The document discusses periodic motion and harmonic motion, which refers to any motion that repeats on a regular time basis such as a pendulum or mass on a spring. 2. Harmonic motion is characterized by sinusoidal oscillations where the applied force depends on position and reverses direction. This results in periodic motion that can be described by amplitude, period, frequency, and angular displacement equations. 3. Pendulums and mass-spring systems are examples of harmonic motion systems, with the period of a pendulum depending on length and gravity, while the angular frequency of a mass-spring system depends on the mass and spring constant.