As you may know instruments can be set into vibration from other instruments that vibrate at the same frequency. These instruments can be put together and they will find the same frequency and pitch because they naturally vibrate at the same frequency. This is and easily demonstrated in a classroom with tuning forks. If you find two forks that are the same note, if start one and hold it very close to the other, the second tuning fork will pick up the same vibration. This creates resonance as shown in the picture.


A forced vibration is the tendency of one object to force another adjoining or interconnected object into vibrational motion. A forced vibration can even cause bridges to collapse. An example would be the Tacoma Bridge collapse in 1940.
This was caused because of the wind blowing at a certain frequency causing the bridge to vibrate. As the bridge was vibrating, the vibrations kept getting larger and larger until the bridge was moving like a roller coaster. Finally the bridge collapsed from the large vibrations.

A link to a video: