# Two cars A and B are moving away from each other in opposite directions. Both the cars are moving with speed of  $20\:m^{-1}$ with respect to the ground . If an observer in car A detects a frequency $2000\:\:Hz$  of the sound coming from car B , what is the natural frequency of the sound source in car B ? Option 1) $2250\:\:Hz$ Option 2) $2060\:\:Hz$ Option 3) $2300\:\:Hz$ Option 4) $2150\:\:Hz$

Frequency detected by A = 2000 Hz

$\Rightarrow2000=f\left [ \frac{340-v}{340+v} \right ]$

$\Rightarrow2000=f\left [ \frac{320}{360} \right ]$

$\Rightarrow2000=f \times \frac{8}{9}$

$\Rightarrow f=2250\:Hz$

Option 1)

$2250\:\:Hz$

Option 2)

$2060\:\:Hz$

Option 3)

$2300\:\:Hz$

Option 4)

$2150\:\:Hz$

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