Question

An observer moves towards a stationary source of sound with a speed$1/5^{th}$ of speed of sound . the wavelength and frequency of the source emitted are $\lambda$ and f respectively. The apparent frequency recorded by the observer

• A. 396.7
• B. 400
• C. 500
• D. 600

Answer 1

The correct option is A 396.7

A whistle revolves in a circle of radius $R=0.5m$ with $W=20$ rad/s

observe hears the lowest frequency (shriller sound) when source approaches towards him linearly (at A) as he is stationery, due to Doppler's effect.

At A, velocity of source

$V_s=RW$

$=0.5\times 20$

$V_s=10m/s$

Given that, original frequency of source $V=385Hz$ and speed of sound in air $=340m/s$

Thus, by Doppler's effect, apparent frequency :-

$V^{\prime }=V\left(\frac{V}{V-V_s}\right)$

$=385\times \left(\frac{340}{340-10}\right)$

$V^{\prime }=396.7Hz$