doppler effect?(20點)

2007-04-16 6:08 am

1.a truck moving at 25 m/s emits a 400-Hz signal.find the wavelength between the source and a stationary observer given that the truck is (A)approaching;(B)moving away.
2.a source emit sound with a characteristic frequency of 200 Hz.calculate the observed frequency and the wave-length between source and observer in each of the follow-ing situations:(A)the source approaches a stationary ob-server at 40 m/s

更新1:

2.;(B)the observer approaches the stationary source at 40 m/s;(C)both the source and the observer move toward each other at 20 m/s relative to the ground.

回答 (1)

魏王將張遼

2007-04-16 6:49 am

✔ 最佳答案

(1) With reference to the equation of Doppler effect:

圖片參考：http://i117.photobucket.com/albums/o61/billy_hywung/April07/Doppler.jpg

where
f' = Apparent frequency
f = Original source frequency
v = Sound velocity
uo = Observer's velocity
us = Source's velocity
Taking the sound velocity in air = 340 m/s, we have:
Part (a) Apparent frequency f' = [(340 + 25)/(340)] × 400
= 429.1 Hz
Therefore the apparent wavelength λ' = v/f' = 0.792 m
Part (b) Apparent frequency f' = [(340 - 25)/(340)] × 400
= 370.6 Hz
Therefore the apparent wavelength λ' = v/f' = 0.917 m
(2) Again, applying the Doppler's effect equation again, we have:
Part (a) Apparent frequency f' = [(340 + 40)/(340)] × 200
= 223.5 Hz
Therefore the apparent wavelength λ' = v/f' = 1.521 m
Part (b) Apparent frequency f' = [(340)/(340 - 40)] × 200
= 226.7 Hz
Therefore the apparent wavelength λ' = v/f' = 1.5 m
Part (c) Apparent frequency f' = [(340 + 20)/(340 - 20)] × 200
= 225 Hz
Therefore the apparent wavelength λ' = v/f' = 1.511 m

參考: My physics knowledge

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