Question

A car passes a person standing on road, when car was approaching it, frequency received by person was $n$. When it recedes, the frequency received by the person is $3n/5$, find speed of car. (Speed of sound = $330m/s$)

• A. 60.4 m/s
• B. 82.5 m/s
• C. 32.8 m/s
• D. 4.34 m/s

Answer 1

The correct option is B 82.5 m/s

By using doppler's effect we can say:

When car is approaching the person:
$n=n_0\left\{\frac{330-0}{330-x}\right\}$

When car is receding the person:
$\frac{3n}{5}=n_0\left\{\frac{330-0}{330+x}\right\}\frac{\mathrm{n/}}{\frac{3\mathrm{n/}}{5}}=\frac{n_0\left(\frac{\text{/}330}{(330-x)}\right)}{n_0\frac{\text{/}330}{(330+x)}}=\frac{330+x}{330-x}\Rightarrow \frac{5}{3}=\frac{330+x}{330-x}$

$\Rightarrow x=82.5m/s$