Because the period of a pendulum depends only on its length and is insensitive to the amplitude, it is easy to design a pendulum that will have a known period, and easy to guarantee that the period will not change with time or local conditions. For this reason, pendulum clocks were among the first accurate time pieces.

Sound is a vibration in air. It happens too fast for us to realize that the air is moving back and forth, or that the pressure is going up or down. The frequency of audible sounds ranges from 30 cycles per second (for a very low note) to 20,000 cycles per second (for an extremely high pitched squeak). The picture shows a tuning fork that has just been touched to the surface of a shallow container of water. All you would see is that the surface is ruffled (it's still too fast for the eye to follow), but the camera catches the shape at an instant, revealing a pattern of fine waves radiating away from the tuning fork.

The unit on vibration