police car vs motorcar

police car and motorcar both are nouns.

Word	Noun	Adjective	Verb	Adverb
police car	Yes	No	No	No
motorcar	Yes	No	No	No

As nouns, motorcar is a hypernym of police car; that is, motorcar is a word with a broader meaning than police car:

police car: a car in which policemen cruise the streets; equipped with radiotelephonic communications to headquarters
motorcar: a motor vehicle with four wheels; usually propelled by an internal combustion engine

Other hypernyms of police car include auto, automobile, car, machine.

police car (noun)	motorcar (noun)
a car in which policemen cruise the streets; equipped with radiotelephonic communications to headquarters	a motor vehicle with four wheels; usually propelled by an internal combustion engine