A laboratory demonstration uses an electric bell suspended inside a thick-walled glass bell-jar connected to a vacuum pump. As the air is gradually pumped out, students observe changes in the loudness of the ringing.
(i) Define a mechanical wave.
(ii) State why sound is classified as a mechanical wave.
(iii) Describe qualitatively how the loudness of the bell changes as the air is removed from the jar and explain the reason for this observation.
(iv) Using the above explanation, justify why sound cannot travel through outer space.