P18 075

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75.

(a) With the detector stationary, we seek a value of source speed v

S

such that the frequency ratio

(heard/emitted) is r = (20 kHz)/(30kHz) = 2/3. From the Doppler effect formula, we find

f



= f



v + 0

v + v

S



=

⇒ v

S

=



1

− r

r



v .

If v = 343 m/s, we get v

S

= 171.5 m/s which converts to 617 km/h.

(b) If r = 20/22, we find v

S

= 34.3 m/s = 123 km/h.


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