Grammatical-Number – Is ‘Hair’ Singular or Plural?

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

1. 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)

1. indicating a science, art, or matters relating to a particular subject: aeronautics, politics
2. 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".

It has less to do with the actual number, and more to do with how the number is said or written.

Singular nouns:

Any time the number is "one", or a fraction with "one" in the numerator, the result is singular. This also applies to negatives. See Is -1 followed by a singular or plural noun?

• One apple
• 1 apple ("one apple")
• Half an apple
• One half of an apple
• 1/2 apple ("one half apple" or "half of an apple")
• 1/4 apple ("one quarter apple" or "one quarter of an apple" or "one fourth of an apple")
• -1 volt ("minus one volt", "negative one volt")

Plural nouns:

Any decimal number, including 1.0, is plural. See Should we use plural or singular for a fraction of a mile?

• 1.0 apples ("one point zero apples", "one point oh apples")
• 0.5 apples ("zero point five apples", "oh point five apples")

Complicated cases:

Fractions with numerators larger than one can be handled both ways. This also applies to percentages. The plural form is used for countable objects, and the singular form is used for non-countable objects. See Is two-thirds plural?

• 2/3 of the people are here. (We are counting people.)
• 2/3 of the soda was left over. (We are not counting soda.)
• 75% of the computers are broken. (countable)
• 75% of the rice was eaten. (not countable)

Complex and imaginary numbers:

Complex and imaginary numbers only appear in technical contexts. I can only think of examples with units, for example:

• 5.7+3.1j kΩ at 500 Hz
• -1.0+0.9j mV at 10 kHz

Note that engineers usually use "j" instead of "i" to avoid confusion with I, the symbol for current. Mathematicians use "i".

In technical contexts, quantities for should be written with numerals and units should be written with abbreviations, which do not take plural. So "5 V" is okay, but "five volts" is only okay in non-technical contexts.