natural frequency

Imagine you are pushing a child on a swing. If you pull the swing back towards you and let go, it will swing freely to-and-fro until the motion dies out. The repeated to-and-fro movement is called oscillation.

Each to-and-fro movement is called one complete cycle of oscillation . Have you ever noticed that the time for each complete cycle is the same, even though the motion is dying out.

The number of complete cycles the swing makes every second is called its natural frequency. A different swing with longer or shorter chains will have a different natural frequency.

This may be hard to imagine, but buildings can also sway to-and-fro ,and sometimes this happens during an earthquake. For buildings, the repeated swaying motion is called vibration. Buildings also have natural frequency, but each building has a whole set of natural frequencies.

Other structures, like bridges and dams, vibrate with their own set of natural frequencies too
You can find out more about this on the IDEERS Resistant Buildings pages.