30. A police car horn emits a 250HZ tone when sittingstill. (a) What frequency d

Question

30. A police car horn emits a 250HZ tone when sittingstill.  

(a) What frequency does a stationary observer hear if thepolice car sounds its horn while approaching at a speed of
27.0 m/s (60 mi/h)? _____________________________________________

(b)What frequency is head if the horn is sounded as the car isleaving at 27.0 m/s? _______________________

(c) If the police car were sounding its horn whilestationary, what frequency would be heard by an observer whowas
approaching it at a speed of 27.0 m/s? _____________________________________ (Note that this is notthe same as
in part (a)!).

Explanation / Answer

(a)   f = f0 v / (v-s) = 250 * 343 / (343-27) =   271.4 Hz . (b)  f = f0 v / (v+s) = 250 * 343 / (343+27) =   231.8Hz . (c)   f = f0 (v+s)/ v = 250 * (343+27) / 343 =   269.8Hz

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