Question

A bullet passes past a person at a speed of 220 m/s. Find the fractional change in the frequency of the whistling sound heard by the person as the bullet crosses the person. Speed of sound in air =330 m/s.

Answer 1

Let the freqency of the bullet will be f.

Given : u =330 m/s, $v_s=220$ m/s

(a)Apparent frequency before crossing.

$\Rightarrow f^1=\left(\frac{330}{330-220}\right)f=3f$

(b)Apparent frequency after crossing.

$\Rightarrow f^{11}=\left(\frac{330}{330+220}\right)f=0.6f$

So, $\left(\frac{f^{11}}{f^1}\right)=\frac{0.6f}{3f}=0.2$

Therefore,

Fractional change =1-0.2 =0.8