Examples
A pendulum swinging in a small arc, a mass bouncing at the end of a stretched spring, and air molecules vibrating back and forth as a sound wave passes are all examples of when Simple Harmonic Motions take place.
A mass suspended by a spring is also known as a simple harmonic oscillator, which represents the most basic oscillating system possible. Once the spring is displaced, or moved, from its resting position, the spring will apply force to try to restore. Due to gravity, however, it will continue to go down a constant force of 9.8m/s. When the spring recoils itself, it creates compressions and rarefactions, which are used to determine the wavelengths and help determine the period.(See experiment page for more details). This harmonic oscillator image was provided by wikipedia at http://en.wikipedia.org/wiki/Image:Simple_harmonic_oscillator.gif
The movement of a pendulum also represents simple harmonic motion. If displaced from rest, it executes simple harmonic motion as it swings back and forth, accelerated by the downward force of gravity. This acceleration keeps it moving from side to side. Unless disturebed, it will continue this motion.
This pendulum clock image was provided by wikipedia at http://en.wikipedia.org/wiki/Image:Old_Pendulum_clock.jpg