You are standing on the sidewalk as a police car recedes from you at a speed of 20 m/s. The car's siren has a fixed frequency of 400 Hz. At the same time a second police car is approaching you. The second car is going at the same speed and has an identical siren.
A) What frequencies do you hear from each police car?
B) What is the frequency difference, if any, between the two observed frequencies?