A state trooper chases a speeder along a straight road; both vehicles move at 160 km/h. The siren on the trooper's vehicle produces sound at frequency of 500 Hz. What is the Doppler shift in the frequency heard by the speeder?

Answer 1

http://en.wikipedia.org/wiki/Doppler_effect

doppler shift depends on the relative difference between the source and the receiver, in this case, no relative motion, so no doppler shift.