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A bus moving with an uniform speed of $72 \mathrm{~km} / \mathrm{h}$ towards a building blows a horn of frequency $1.7 \mathrm{kHz}$. If speed of sound in air is $340 \mathrm{~m} / \mathrm{s}$, what will be the frequency of echo heard by bus driver?
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The correct answer is:
$1.8 \mathrm{kHz}$
According to Doppler's effect, Observed frequency of sound reflected from building is given by
Here, $v=$ speed of sound $=340 \mathrm{~m} / \mathrm{s}$,
$v_b=$ speed of bus $=7.2 \mathrm{~km} / \mathrm{h}=20 \mathrm{~ms}^{-1}$
and $f=$ original frequency produced by horn $=1.7 \mathrm{kHz}$
Hence, putting these value in Eq. (i), we get Observed frequency by bus driver,
$f^{\prime}=1.7\left(\frac{340+20}{340-20}\right)=1.9125 \mathrm{kHz} \approx 1.8 \mathrm{kHz}$
Here, $v=$ speed of sound $=340 \mathrm{~m} / \mathrm{s}$,
$v_b=$ speed of bus $=7.2 \mathrm{~km} / \mathrm{h}=20 \mathrm{~ms}^{-1}$
and $f=$ original frequency produced by horn $=1.7 \mathrm{kHz}$
Hence, putting these value in Eq. (i), we get Observed frequency by bus driver,
$f^{\prime}=1.7\left(\frac{340+20}{340-20}\right)=1.9125 \mathrm{kHz} \approx 1.8 \mathrm{kHz}$
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