$A$ car sounding a horn of frequency $1000 \ Hz$ passes a stationary observer. The ratio of frequencies of the horn noted by the observer before and after passing the car is $11:9$. If the speed of sound is $v$,the speed of the car is:

An obstacle is moving towards the source with velocity $v$. The sound is reflected from the obstacle. If $c$ is the speed of sound and $\lambda$ is the wavelength,then the wavelength of the reflected wave $(\lambda_{r})$ is

$A$ $SONAR$ system fixed in a submarine operates at a frequency of $40.0 \; kHz$. An enemy submarine moves towards the $SONAR$ with a speed of $360 \; km/h$. What is the frequency (in $kHz$) of sound reflected by the submarine (in $; kHz$)? Take the speed of sound in water to be $1450 \; m/s$.

$A$ submarine $(A)$ travelling at $18\, km/hr$ is being chased along the line of its velocity by another submarine $(B)$ travelling at $27\, km/hr$. $B$ sends a sonar signal of $500\, Hz$ to detect $A$ and receives a reflected sound of frequency $v$. The value of $v$ is close to ... $Hz$ (Speed of sound in water $= 1500\, ms^{-1}$)

$A$ man is watching two trains,one leaving and the other coming with an equal speed of $4\,m/s$. If they sound their whistles each of frequency $240\,Hz$,the number of beats per second heard by the man will be equal to: (velocity of sound in air $= 320\,m/s$)

When a train is approaching a stationary observer,the apparent frequency of the whistle of the train is $n_1$,and when the train is moving away from the observer,the apparent frequency is $n_2$. The frequency of the whistle noticed by the observer when he moves with the train is . . . . . . .