Question

A person $P$ is $600\text{m}$ away from the station when train is approaching station with $72\text{km/h}$, it blows a whistle of frequency $800\text{Hz}$ when $800\text{m}$ away from the station. Find the frequency heard by the person. Speed of sound $=340{\text{ms}}^{-1}$.

• A. $800\text{Hz}$
• B. $839.5\text{Hz}$
• C. $829.5\text{Hz}$
• D. $843.5\text{Hz}$

Answer 1

The correct option is B $839.5\text{Hz}$


$f_{\text{app}}=\frac{v}{v-v_s\cos \theta }f$
$=\frac{340}{340-20\times 0.8}\times 800=839.5\text{Hz}$