### 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} $