Natural and forced vibrations
When we strike a tuning fork with a rubber hammer it vibrates with certain frequency.
The frequency does not change even if we hit it harder. Thus every body has its own frequency called ‘natural frequency'.
When a body is set into vibrations and then left to itself, the vibrations are called natural vibrations.
If we strike a tuning form and then press its stem against the top of a table, the sound becomes louder because the table is forced to vibrate with the frequency of the tuning fork.
Hence, when a body executes vibrations under the action of an external periodic force, then the vibrations of the body are called forced vibrations.