Waves - Result Question 72
74. A train moving at a speed of $220 ms^{-1}$ towards a stationary object, emits a sound of frequency $1000 Hz$. Some of the sound reaching the object gets reflected back to the train as echo. The frequency of the echo as detected by the driver of the train is :
[2012M]
(speed of sound in air is $330 ms^{-1}$ )
(a) $3500 Hz$
(b) $4000 Hz$
(c) $5000 Hz$
(d) $3000 Hz$
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Answer:
Correct Answer: 74. (c)
Solution:
- (c) Frequency of the echo detected by the driver of the train is
(According to Doppler effect in sound)
$f^{\prime}=(\frac{v+u}{v-u}) f$
where $f=$ original frequency of source of sound $f^{\prime}=$ Apparent frequency of source because of the relative motion between source and observer.
$f^{\prime}=(\frac{330+220}{330-220}) 1000=5000 Hz$
Velocity of sound direction is always taken from source to observer. All the velocities in the direction of $v$ are positive and in the opposite direction of $v$ are taken negative.
