Question

When a car is approaching the observer, the frequency of horn is 100 Hz. After passing the observer, it is 50 Hz. If the observer moves with the car, the frequency will be $\frac x3$ Hz, where x = _____ .

Answer 1

$100=v_0\frac {v}{v−v_c}$

$50=v_0\frac {v}{v+v_c}$

$2=\frac {v+v_c}{v−v_c}$

$2v–2v_c=v+v_c$

$v_c=\frac v3$

$100=v_0\frac {v×3}{2v}$

$v_0=\frac {200}{3}=\frac x3$

$⇒x=200$

So, the answer is $200$.