To the extent that wasn't just movies and TV, I think "Stop in the name of the law" goes back several centuries to the notion of "hue and cry." By loudly calling out for the wanted man to stop in the name of the law, the un-uniformed, badgeless, officer was informing everyone that he was an officer of the law, and he was attempting to catch a suspected criminal. Every able bodied man had a duty to drop what he was doing, join in the "hue and cry" and help the officer pursue and catch the suspect.
Nowdays, with a uniformed and badged cop in a marked patrol vehicle, his stauts is clear, and anyone rushing up to "help" is liable to be arrested for interefring with the duties of a peace officer, if not just shot on the spot, so yelling "Stop in the name of the law" is passe.
That being said, I have to recycle the old joke about he difference between American cops and English cops.
American cops yell: Stop!, or I'll shoot!
English cops yell: Stop!, or I'll yell Stop! again.
Informative and funny, thanks.