Q141 / 160 · TS EAMCET 2018 (04 May Shift 2) · Physics

A source of sound whose frequency is $1000 \mathrm{~Hz}$ is moving with a speed $33 \mathrm{~m} / \mathrm{s}$. The waves reflected by a fixed obstacle are registered by a receiver that moves together with the source. The speed of the sound waves is $330 \mathrm{~m} / \mathrm{s}$, then the frequency registered by the receiver is

  • A
    $0.9 \mathrm{kHz}$
  • B
    $1.1 \mathrm{kHz}$
  • Correct answer
    C
    $1.2 \mathrm{kHz}$
  • D
    $2.2 \mathrm{kHz}$

Solution

Frequency of reflected sound, $f_1=f\left(\frac{v}{v-v_s}\right)$ $$ =1000\left(\frac{330}{330-33}\right)=\frac{1000 \times 330}{297} $$ Frequency observed by observer, $$ \begin{aligned} & f_2=f_1\left(\frac{v+v_D}{v}\right)=\frac{1000 \times 330}{297}\left(\frac{330+33}{330}\right) \\ = & \frac{1000 \times 363}{297}=1.2 \times 1000=1.2 \mathrm{kHz} \end{aligned} $$

Hence, the correct answer is option: 'C'.

Relevant Questions