What frequency does bob hear

Question

Bob’s neighbors are also annoyed by Bob’s flute and file a noise complaint. Two police
officers get into a squad car to investigate. They turn on the siren at 920 Hz and speed
towards Bob’s house at 40 mph. What frequency does Bob hear as the police approach?

Answer 1

This is a Doppler shift situation where the source is moving toward the observer and the observer is at rest.

 

v = speed of sound = ?  [This should be provided to you in the original problem, or you can look it up.]

 

v_s = speed of source (i.e. the police car) = 40 mph

 

f' = frequency detected by the observer (i.e. Bob)

 

f = frequency of the source = 920 Hz

 

f' = [v/(v - v_s)] f

 

You know f and v_s, but you need a value for the speed of sound v.  Look it up.  It should be close to 700 mph.  Plug in the numbers and get f'.