Two trains move towards each other with the same speed. The speed of sound is $340 \; m/s$. If the pitch of the whistle of one train heard by an observer on the other train changes by a factor of $9/8$,then the speed of each train is ........... $m/s$.

$A$ source is moving towards an observer with a speed of $20 \ m/s$ and having a frequency of $240 \ Hz$. The observer is now moving towards the source with a speed of $20 \ m/s$. If the velocity of sound is $340 \ m/s$,the apparent frequency heard by the observer is ... $Hz$.

$A$ police car horn emits a sound at a frequency $240 \text{ Hz}$ when the car is at rest. If the speed of sound is $330 \text{ m/s}$,the frequency heard by an observer who is approaching the car at a speed of $11 \text{ m/s}$ is ... $\text{ Hz}$.

$A$ source and an observer are moving towards each other with a speed equal to $\frac{v}{2}$,where $v$ is the speed of sound. The source is emitting sound of frequency $n$. The frequency heard by the observer will be

$A$ sound source is tied to one end of a string of length $50 \ cm$ and is rotated with an angular speed of $40 \ rad \ s^{-1}$ in a horizontal plane. The ratio of the maximum and minimum frequencies of the sound heard by an observer standing at a distance of $10 \ m$ from the fixed end of the string is (speed of sound in air $= 340 \ m \ s^{-1}$).

$A$ train standing at the outer signal of a railway station blows a whistle of frequency $400\, Hz$ in still air. What is the frequency of the whistle for a platform observer when the train recedes from the platform with a speed of $10\, m/s$? (Speed of sound $= 340\, m/s$)