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Question: Answered & Verified by Expert
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
PhysicsWaves and SoundBITSATBITSAT 2020
Options:
  • A $1115 \mathrm{~Hz}$
  • B $115 \mathrm{~Hz}$
  • C $67 \mathrm{~Hz}$
  • D $47.2 \mathrm{~Hz}$
Solution:
2655 Upvotes Verified Answer
The correct answer is: $1115 \mathrm{~Hz}$
By the concept of accoustic, the observer and source are moving towards each other, each with a velocity of $18 \mathrm{~m} \mathrm{~s}^{-1}$ $\therefore \mathrm{v}^{\prime}=\frac{330+18}{330-18} \times 1000 \approx 1115 \mathrm{~Hz}$

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