First slide

Doppler's effect

Question

A car sounding a horn of frequency 1000 Hz passes an observer. The ratio of frequencies of the horn noted by the observer before and after passing of the car is 11 : 9. If the speed of sound is v, the speed of the car is

Moderate

A

$\frac{1}{10} v$
B

$\frac{1}{2} v$
C

$\frac{1}{5} v$
D

v

Solution

$n_{Before} = \frac{v}{v - v_{c}} n and n_{After} = \frac{v}{v + v_{c}} . n$
$\frac{n_{Before}}{n_{After}} = \frac{11}{9} = (\frac{v + v_{c}}{v - v_{c}}) \Rightarrow v_{c} \Rightarrow \frac{v}{10}$

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