heaviness vs sadness

heaviness and sadness both are nouns.

Word	Noun	Adjective	Verb	Adverb
heaviness	Yes	No	No	No
sadness	Yes	No	No	No

As nouns, sadness is a hypernym of heaviness; that is, sadness is a word with a broader meaning than heaviness:

Other hypernyms of heaviness include unhappiness.

heaviness (noun)	sadness (noun)
the property of being comparatively great in weight	the quality of excessive mournfulness and uncheerfulness
used of a line or mark	emotions experienced when not in a state of well-being
unwelcome burdensome difficulty	the state of being sad
an oppressive quality that is laborious and solemn and lacks grace or fluency
persisting sadness