center of mass vs midpoint

center of mass and midpoint both are nouns.

Word	Noun	Adjective	Verb	Adverb
center of mass	Yes	No	No	No
midpoint	Yes	No	No	No

As nouns, midpoint is a hypernym of center of mass; that is, midpoint is a word with a broader meaning than center of mass:

center of mass: point representing the mean position of the matter in a body
midpoint: a point equidistant from the ends of a line or the extremities of a figure

Other hypernyms of center of mass include center, centre.

center of mass (noun)	midpoint (noun)
point representing the mean position of the matter in a body	a point equidistant from the ends of a line or the extremities of a figure