First slide

Doppler's effect

Question

A person carrying a whistle emitting continuously a note of 272 Hz is running towards a reflecting surface with a speed of 18 km/hour. The speed of sound in air is $345 {ms}^{- 1}$ . The number of beats heard by him is

Easy

A

4
B

6
C

8
D

3

Solution

According the concept of sound image
$n' = \frac{v + v_{person}}{v - v_{person}} n = \frac{345 + 5}{345 - 5} \times 272 = 280 Hz$
$Δn =$ Number of beats =280 – 272 = 8 Hz

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