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)

If the car approaches the policeman
$\upsilon'=\frac{v+v_{0}}{v-v_{s}}\times\upsilon $
$=\frac{300+15}{300-0}\times500$
$\Rightarrow \upsilon' =525\,Hz$
If the car moves away from the policeman
$\upsilon''=\frac{v-v_{0}}{v-v_{s}}\times\upsilon$
$=\frac{300-15}{300-0}\times500$
$\upsilon''=475\,Hz$
Change in frequency $= \upsilon'-\upsilon''=525-475$
$=50\,Hz$