Does the siren of a moving police car seem to change pitch to the police officers inside the car? Why or why not?

Yes it does!

Explanation:

As the police car moves towards them, the pitch of the police car's siren goes up because the sound waves get closer together in front of the police car. When the police car moves away from the person, the pitch goes down because the waves spread out behind the police and that makes the frequency go down.

probably not if net force is zero