This is easily demonstrated. Consider the construction ''Physics is the study of ...'' (finish the sentence any way you like). If one would argue that the subject is plural, thus, ''Physics are the study of ...'', then one commits the egregious error of number by not having the verb agree with the predicate nominative. If, to correct the error, one subsequently modifies the sentence to ''Physics are the studies of ...'', one has created a statement which is false-to-fact. The discipline of physics (or mathematics, or ethics, or epistemics, etc.) is a single body of knowledge, not an agglomeration of studies.
Hence, physics is STILL fun, regardless of English lit majors.
Interestingly, when a singular noun ending in 's' has a second denotation, e.g. ''ethics'', which term may refer either to the philosophical body of study or to one's personal moral principles, said noun may take either a singular or a plural verb, according to context. Cf. ''Ethics is a fascinating study.'' vs. ''His ethics are those of a snake.''
(s) The Mad Grammarian (and many fond memories of Professor Sidney Lamb, the ARCH-stickler of grammar in bygone days)
BTW, I agree that physics is/are fun. It was my favorite science subject.
Another one: data v. datum. Strictly speaking, it should be "data are."