Question

A policeman blows a whistle with a frequency of 500 Hz. A car approaches him with a velocity of 15 m s–1. The change in frequency as heard by the driver of the car as he passes the policeman is (Given, speed of sound in air is 300 m s–1)

• A. 25 Hz
• B. 50 Hz
• C. 100 Hz
• D. 150 Hz

Answer 1

Answer: (B)

50 Hz

Answer 2

If the car approaches the policeman

$\upsilon' = \frac{v - v_{0}}{v - v_{0}} \times \upsilon = \frac{300 - (15)}{300 - 0} \times 500$

$\upsilon' = 525Hz$

If the car moves away from the policeman

$\upsilon'' = \frac{v - v_{0}}{v - v_{5}} \times \upsilon = \frac{300 - \upsilon''}{300 - 0} \times 500$

$\upsilon'' = 475Hz$

Change in frequency $= \upsilon' - \upsilon''$

= 525 – 475

= 50Hz