1. The problem statement, all variables and given/known data A state trooper chases a speeder along a straight stretch of road; both vehicles move at 160 km/h. The siren on the trooper's vehicle produces sound at a frequency of 500 Hz. What is the Doppler shift in the frequency heard by the speeder? 2. Relevant equations This is what I need the answer to. 3. The attempt at a solution I understand why the answer is 500 Hz. They are both traveling at the velocity with respect to one another, so there is no doppler effect taking place---it is cancelled out by the equal velocity and direction of the vehicles. What I do not understand is, how do I prove it? Thanks!