Waves Ques 73
73. The driver of a car travelling with speed $30$ $ m / sec$ towards a hill sounds a horn of frequency $600 $ $ Hz$. If the velocity of sound in air is $330$ $ m / s$, the frequency of reflected sound as heard by driver is
[2009]
(a) $555.5 $ $Hz$
(b) $720 $ $Hz$
(c) $500 $ $Hz$
(d) $550 $ $Hz$
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Answer:
Correct Answer: 73.(b)
Solution:
- (b) From the given condition
$f^{\prime}$ is the apparent frequency received by an observer at the hill. $f^{\prime \prime}$ is the frequency of the reflected sound as heard by driver.
$ \begin{aligned} f^{\prime} & =\frac{v}{v-30} f^{\text{and }} \\ f^{\prime \prime} & =\frac{v+30}{v} f^{\prime} \Rightarrow \frac{v+30}{v-30} f=\frac{360}{300} \times 600 \\ & =720 Hz \end{aligned} $
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