Question

What is the difference between free and forced vibration?

Answer 1

When no energy transfer between the vibrating object and the surrounding hence it is called the free vibrations.
When the driving force acts on a vibrating object then it is called the forced vibration.
In a forced vibration the energy transfer between the vibrating object and surrounding occurs.

Complete answer:
If the vibration occurs in the absence of friction then it is called the free vibration. When repeated force occurs on a system then it is called the forced vibration.
The difference between the free and forced vibration is tabulated below,

Free vibration	Forced vibration
When the body gets disturbed during its equilibrium position it produced the free vibrations	When external periodic force act on a body it produced the forced vibration
The force is required to initiate the free vibration	Continuous periodic force is required to initiate the forced vibration.
It is a self-sustained vibration	It is an externally sustained vibration
The free vibrations always depend on the body hence it is also called the natural frequency	The forced vibration frequency is equal to the external periodic force.
The free vibrations are stopped fastly	The forced vibrations are stopped based on the external periodic force
Example: simple pendulum oscillations	Example: music instruments that have soundboards.

Note: The audible frequency range is $20Hz$ to $20KHz$
The frequency below $20Hz$ is called the infrasonic.
The frequency above $20KHz$ is called the ultrasonic.