Question 143: Waves

Question

A person observes two moving trains, 'A' reaching the station and 'B' leaving the station with equal speed of $$30 \mathrm{~m} / \mathrm{s}$$. If both trains emit sounds with frequency $$300 \mathrm{~Hz}$$, (Speed of sound: $$330 \mathrm{~m} / \mathrm{s}$$) approximate difference of frequencies heard by the person will be:

Accepted Answer

By doppler effect : $f^{\prime}=f_{0}\left[\frac{v-v_{0}}{v-v_{s}}ight]$
$\Rightarrow \quad f_{A}^{\prime}=300\left[\frac{330}{330-30}ight] \mathrm{Hz}$
$$
=330 \mathrm{~Hz}
$$
And $f_{B}^{\prime}=300\left[\frac{330}{330+30}ight] \mathrm{Hz}$
$$
=\frac{5}{6} 	imes 330 \mathrm{~Hz}=275 \mathrm{~Hz}
$$
$	herefore \Delta \mathrm{f}=330-275=55 \mathrm{~Hz}$.

Answer

10 Hz

Answer

55 Hz

Answer

80 Hz

Answer

33 Hz

Waves

Choose the correct answer: