Question

An engine's whistle frequency is experienced as 5/6th part when it crosses a stationary observer. If velocity of sound in air is 330 m/s then calculate the velocity of engine.

Answer 1

Given; $n^{\prime }=\frac{5}{6}n;v=330m{s}^{-1};v_s=?$

Ref. image

$\because$ Apparent frequency,

$n^{\prime }=\frac{v}{v+v_s}n$

$\therefore \frac{5}{6}n=\frac{v}{v+v_s}n$

or $\frac{5}{6}=\frac{v}{v+v_s}$

or $5v+5v_s=6v$

or $5v_s=6v-5v=v$

$\therefore v_s=\frac{v}{5}=\frac{330}{5}$

or $v_s=66m{s}^{-1}$