You are driving along a highway at 28.7 m/s when you hear a siren. You look in the rear-view mirror and see a police car approaching you from behind with a constant speed. The frequency of the siren that you hear is 1790. Hz. Right after the police car passes you, the frequency of the siren that you hear is 1740. Hz.
a) How fast was the police car moving?
b) You are so nervous after the police car passes you that you pull off the road and stop. Then you hear another siren, this time from an ambulance approaching from behind. The frequency of its siren that you hear is 1260. Hz. Once it passes, the frequency is 1030. Hz. What is the actual frequency of the ambulance's siren?