Question

A policeman on duty detects a drop of 10 % in the frequency of motor car as it is crosses him If the velocity of sound is 330 m/s, the speed of the car is

• A. $20m/sec$
• B. $36.3m/sec$
• C. $25m/sec$
• D. none of the above

Answer 1

The correct option is B $36.3m/sec$

According to the Doppler effect

$f_{new}=f_0\left(\frac{V\pm V_{obs}}{V\pm V_{source}}\right)$

$\frac{90}{100}{f}_o=f_o\left(\frac{330}{330+V_{source}}\right)$ [observer is stationary car is moving away]

$330+V_{source}=33\times \frac{100}{9}$

$V_{source}=36.67$m/sec

Thus, the car moves with speed $36.67$ m/s.