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Halliday 2 Cap 18 - P18 075

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    December 2018

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75. (a) With the detector stationary, we seek a value of source speed vS such that the frequency ratio (heard/emitted) is r = (20 kHz)/(30 kHz) = 2/3. From the Doppler effect formula, we find     v+0 1−r  f =f =⇒ vS = v. v + vS r If v = 343 m/s, we get vS = 171.5 m/s which converts to 617 km/h. (b) If r = 20/22, we find vS = 34.3 m/s = 123 km/h.