Waves - Result Question 73
75. 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
(a) $555.5 Hz$
(b) $720 Hz$
(c) $500 Hz$
(d) $550 Hz$
[2009]
Show Answer
Answer:
Correct Answer: 75. (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} $
