Etymonline has this to say:
-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.) it represents a 16c. revival of the classical custom of using the neuter plural of adjectives with -ikos (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their names in English before c.1500, however, tend to remain in singular (e.g. arithmetic, logic).
So yes, at some point in history, there were such things as physic (meaning "natural science"), mathematic (meaning "mathematical science"), etc. that were later turned into plural forms but kept being treated as singular.
Edit: having looked in a few more places, it appears that in contemporary English, it still makes some sense to have both the suffix -ic and its plural form -ics. According to the Collins English Dictionary, the former has kind of specialized in forming adjectives, while the latter is happily forming nouns:
-ic
suffix forming adjectives
- of, relating to, or resembling: allergic, Germanic, periodic. See also -ical.
[...]
[from Latin -icus or Greek -ikos; -ic also occurs in nouns that represent a substantive use of adjectives (magic) and in nouns borrowed directly from Latin or Greek (critic, music)]
[...]
-ics
suffix forming nouns (functioning as singular)
- indicating a science, art, or matters relating to a particular subject: aeronautics, politics
- indicating certain activities or practices: acrobatics
[plural of -ic, representing Latin -ica, from Greek -ika, as in mathēmatika mathematics]
The key here is that they are not just two unrelated suffixes. Much rather, one is etymologically a plural form of the other. As the American Heritage Dictionary succinctly puts it, -ics is "-ic + -s".
In English, every number that is not 1 is considered plural. The correct sentence is the first you wrote.
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Best Answer
US English is is more cut-and-dried on this topic. "An assortment of people was . . ." is correct. "An assortment of people were . . ." is not correct and this very construction is commonly used by test writers to test agreement in number for the very reason that it does sound correct. I tried a couple of grammar check systems and they agree with my analysis. Since the writing samples used for college admission in the US are moving toward a human-less scoring scheme, I expect grammatical standards will become even more concrete. The questioner is from Minnesota and if he starts going around saying stuff like, "The Committee are unable to reach a consensus," he might get deported to a land where HRH Prince William posts, "It.s a Boy. Beautiful Boy." on FB.