A police car's siren emits a wave with a frequency of 60 Hz. The speed of sound is 300 m/s and the car is moving 50 m/s. "what is the wavelength of the wave behind the car" So, the answer seems pretty easy. The observer is a stationary point behind the car, and the source is moving away from the observer, which means frequency should decrease: f = 60 (300+0)/(300+50) = 51.4 Hz wavelength = 300/51.4 = 5.8 Is this logic correct? However, the answer in the book states that the final frequency is 70 Hz, by this equation: f = 60 (300+50)/(300+0) = 70 Hz Which one is correct?