Question

Two engines pass each other moving in opposite directions with uniform speed of $30m/s$. One of them is blowing a whistle of frequency $540Hz$. Calculate the frequency heard by driver of second engine before they pass each other. Speed of sound is $330m/sec$.

• A. $648Hz$
• B. $540Hz$
• C. $270Hz$
• D. $450Hz$

Answer 1

The correct option is A $648Hz$

$f=\left(\frac{c+v_r}{c-v_s}\right)f_o$

where

$f:$ Observed frequency, $f_o:$ source frequency

$c:$ speed of sound

$v_r:$ velocity of the receiver relative to the medium (positive if the receiver is moving towards the source)

$v_s:$ velocity of the source relative to the medium (positive if the source is moving away from the receiver)

$f=\left(\frac{330+30}{330-30}\right)\times 540=648Hz$