Question

A whistle revolves in a circle with angular speed $\omega =20$ rad/s using a string of length 50 cm . if the frequency of sound from the whistle is 385 Hz, then what is the maximum frequency heard by an observer who is far away from the centre (velocity of sound = 340 m/s) ?

Answer 1

$A$ where the values in a circle of radius $R=0.5m$ with $w=20rad/s$

Observer heaver the lowest frequency (shriuer sound) when source approaches towards him in nearly (at $A$ ) as he in stationery due to approches effect

At $A$ velocity sources

$v_s=Rw$

$=0.5\infty 20$

$\Rightarrow v_s=10m/s$

Given that, original frequency of sources $v=38542$ and speed of sound in air $340m/s\left(z/s\right)$

Thus, by Dopper in effect. apparent frequency :-

$v^{\prime }=v\left(\frac{v}{v-v_s}\right)$ [observer stationery moving towards him]

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

$v^v=396.7Hz$