Q109 / 125 · BITSAT 2020 · Physics

A whistle of frequency $1000 \mathrm{~Hz}$ is sounded on a car travelling towards a cliff with velocity of 18 $\mathrm{~m} \mathrm{~s}^{-1}$ normal to the cliff. If $\mathrm{c}=330 \mathrm{~m} \mathrm{~s}^{-1}$, then the apparent frequency of the echo as heard by the car driver is nearly

  • Correct answer
    A
    $1115 \mathrm{~Hz}$
  • B
    $115 \mathrm{~Hz}$
  • C
    $67 \mathrm{~Hz}$
  • D
    $47.2 \mathrm{~Hz}$

Solution

By the concept of accoustic, the observer and source are moving towards each other, each with a velocity of $18 \mathrm{~m} \mathrm{~s}^{-1}$ $\therefore \mathrm{v}^{\prime}=\frac{330+18}{330-18} \times 1000 \approx 1115 \mathrm{~Hz}$

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

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