Q142 / 160 · AP EAMCET 2006 · Physics

An observer is standing $500 \mathrm{~m}$ away from a vertical hill. Starting between the observer and the hill, a police van sounding a siren of frequency $1000 \mathrm{~Hz}$ moves towards the hill with a uniform speed. If the frequency of the sound heard directly from the siren is $970 \mathrm{~Hz}$, the frequency of the sound heard after reflection from the hill (in Hz) is about, (velocity of sound $=330 \mathrm{~ms}^{-1}$ ) :

  • A
    1042
  • Correct answer
    B
    1032
  • C
    1022
  • D
    1012

Solution

Sound heard directly $f_1=f_o\left(\frac{v}{v+v_s}\right)$ $\therefore \quad 970=1000\left(\frac{330}{330+v_s}\right)$ or $\quad v_s=10.2 \mathrm{~m} / \mathrm{s}$ The frequency of reflected sound is given by $f_2=f_o\left(\frac{v}{v-v_s}\right)$ $=1000\left(\frac{330}{330-10.2}\right)$ $=\frac{1000 \times 330}{319.8}$ $\approx 1032 \mathrm{~Hz}$

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

Relevant Questions