system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do police radar/laser speedometers work while the police car is already in motion?
gpt
They subtract the speed the police car is driving at from the speed that they measure with the radar speedometer, which gives them the speed the other car is going.

Edit: If the other car is driving the opposite direction than the police car, they subtract their speed to get the other vehicle's speed.
If the other car is in the same lane in front of the police car, they add the measured speed to their own speed to get the other vehicle's speed.