When a body is vibrating under the influence of an external agency, with the frequency of external source then these are called resonant vibrations.

Two pendulums $C$ and $D$ are suspended from a wire as shown in the given figure. Pendulum $C$ is made to oscillate by displacing it from its mean position. It is seen that $D$ also starts oscillating. What is the name of the phenomenon when the length of $D$ is made equal to $C$?

A vibrating tuning fork is placed over the mouth of a burette filled with water. The tap is opened and the water level gradually falls. It is observed that the sound becomes the loudest for a particular length of air column.

What is the name of the phenomenon taking place when sound is produced for another length of air column and is not the loudest?

Pendulums A, B, C and D are tied to a flexible string PQ and are at rest. Pendulum C is disturbed. Which of the following statements are true. 

In resonance,

A vibrating tuning fork is placed over the mouth of a burette filled with water. The tap is opened and the water level gradually falls. It is observed that the sound becomes the loudest for a particular length of air column.

What is the name of the phenomenon taking place when this happens?

Resonance occurs when

Differentiate between resonance and forced vibrations.