$A$ car is moving towards a high cliff. The car driver sounds a horn of frequency $f$. The reflected sound heard by the driver has a frequency $2f$. If $v$ is the velocity of sound,then the velocity of the car,in the same velocity units,will be:

$A$ train blowing a whistle of frequency $320\,Hz$ approaches an observer standing on the platform at a speed of $66\,m/s$. The frequency observed by the observer will be (given speed of sound $= 330\,m/s$) $.............Hz$.

$A$ source of sound $S$ emitting waves of frequency $100 \, Hz$ and an observer $O$ are located at some distance from each other. The source is moving with a speed of $19.4 \, m s^{-1}$ at an angle of $60^{\circ}$ with the source-observer line as shown in the figure. The observer is at rest. The apparent frequency observed by the observer is .... $Hz$ (velocity of sound in air $330 \, m s^{-1}$).

Two cars $X$ and $Y$ are approaching each other with velocities $36 \; km/h$ and $72 \; km/h$ respectively. The frequency of a whistle sound emitted by a passenger in car $X$,heard by the passenger in car $Y$,is $1320 \; Hz$. If the velocity of sound in air is $340 \; m/s$,the actual frequency of the whistle sound produced is ........ $Hz$.

$A$ source of sound of frequency $90 \text{ vibrations/sec}$ is approaching a stationary observer with a speed equal to $1/10$ the speed of sound. What will be the frequency heard by the observer in $\text{vibrations/sec}$?

$A$ stationary tuning fork is in resonance with an air column in a pipe. If the tuning fork is moved with a speed of $2 \ m/s$ in front of the open end of the pipe and parallel to it,the length of the pipe should be changed for the resonance to occur with the moving tuning fork. If the speed of sound in air is $320 \ m/s$,the smallest value of the percentage change required in the length of the pipe is. . . . . .