Question

A bus moving with an uniform speed of $72\, km / h$ towards a building blows a horn of frequency $1.7\, KHz$. If speed of sound in air is $340\, m / s$, what will be the frequency of echo heard by bus driver?

• A. $1.8\, KHz$
• B. $2.0 \, KHz$
• C. $1.6 \, KHz$
• D. $1.4 \, KHz$

Answer 1

Answer: (A)

According to Doppler's effect,
Observed frequency of sound reflected from building is given by
$f'=f\left(\frac{v+v_{b}}{v-v_{b}}\right) \ldots .$ (i)
Here, $v=$ speed of sound $=340 \,m / s$
$v_{b}=$ speed of bus $=7.2 \,km / h =20 \, ms ^{-1}$
and $f=$ original frequency produced by horn $=1.7 kHz$
Hence, putting these value in Eq. (i),
we get Observed frequency by bus driver,
Hence, putting these value in Eq. (i),
we get Observed frequency by bus driver,
$f'=1.7\left(\frac{340+20}{340-20}\right)$
$=1.9125 \,kHz \approx 1.8 \,kHz$