A toy-car, blowing its horn, is moving with a steady speed of $5\, m/s$ , away from a wall. An observer, towards whom the toy car is moving, is able to hear $5\, beats$ per second. If the velocity of sound in air is $340\, m/s$, the frequency of the horn of the toy car is close to ... $Hz$
JEE MAIN 2016, Diffcult
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From Doppler's effect
$f(\text { direct })=f\left(\frac{340}{340-5}\right)=f_{1}$
$f\left(b y \text { wall) } f\left(\frac{340}{340+5}\right)=f_{2}\right.$
Beats $=\left(f_{1}-f_{2}\right)$
$5=f\left(\frac{340}{340-5}-\frac{340}{340+5}\right)$
$\Rightarrow \mathrm{f}=170 \mathrm{Hz}$
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