As it happens, both
inertia and
inertness have scientific meanings.
Inertia, in physics, is the tendency of an object to remain at rest or continue in steady motion, and more generally, any tendency to resist change - e.g. the
thermal inertia of a body of water.
Inertness is the lack of chemical reactivity of a substance.
This reminds me of a
Yes Minister episode where an issue came up about the safety or otherwise of a chemical said to be inert:
Joan Littler : What does "inert" mean?
Sir Humphrey Appleby : Eh, it means it's not... ert.
Bernard Woolley : Wouldn't 'urt a fly.
Both words can be used in a non-scientific context, meaning a tendency to do nothing or remain unchanged, but
inertia seems to be used much more often. A very recent example, from
Business Insider on the 15th of this month:
By taking a proactive role on climate change - particularly against a backdrop of government inertia - tech giants have an opportunity to repair their image with the public.
Nevertheless, I think most players would know that
inertness is a word. In the end more than half of the players found it in the puzzle that sparked the initial query. The
only reason for tagging words as common or rare is so that players can aim to get as many as possible of the common words, with little risk of being caught out by words they've never heard of.
I'm not persuaded to change the common status of
inertness or to drop it as a puzzle seed word - nothing has been put forward that would overcome my inertia.