1) Indeed usually the adjective is used in connection with physical forces and systems.
2) Now I searched with Google the three words in question using the following search strings:
"X of * force" physics
"X of * forces" physics
"X of * system" physics
"X of * systems" physics
where X is either conservativism or conservativity or conservativeness. Similar searches I also performed restricted to the e-print archive http://www.arxiv.org/
(an online collection of professional (mostly physical) papers that at least have review quality).
I guess it would require a deeper study for a new entry in LEO :-)... Anyway, here is what I've found out:
A) on arxiv.org these words are almost not used at all: there are 3 hits for conservativism, 4 hits for conservativity, and 1 hit for conservativeness.
B) As far as the whole www is concerned:
a) "conservativism" seems to be rather used in connection with political (and similar) concepts.
b) conservativity received in connection with the physical concepts 5 hits and conservativeness 16 hits.