$A$ source of sound is travelling towards a stationary observer. The frequency of sound heard by the observer is three times the original frequency. If the velocity of sound is $v \ m/s$,the speed of the source will be:

The pitch of the whistle of an engine appears to drop by $20 \%$ of its original value when it passes a stationary observer. If the speed of sound in air is $350 \ m/s$,then the speed of the engine in $m/s$ is:

$A$ locomotive approaching a crossing at a speed of $20 \, m/s$ sounds a whistle of frequency $640 \, Hz$ when it is $1 \, km$ from the crossing. There is no wind and the speed of sound in air is $330 \, m/s$. What frequency is heard by an observer located $\sqrt{3} \, km$ away from the crossing on a straight road at a right angle to the locomotive's path (in $, Hz$)?

$A$ policeman on duty detects a drop of $10\%$ in the pitch of the horn of a moving car as it crosses him. If the velocity of sound is $330 \, m/s$,the speed of the car will be ..... $m/s$.

$A$ car moving towards a wall at a velocity of $30 \, m/s$ sounds a horn of frequency $600 \, Hz$. What frequency $(Hz)$ will the driver hear? (Speed of sound in air = $330 \, m/s$)

The driver of a car travelling with speed $30 \, m/s$ towards a hill sounds a horn of frequency $600 \, Hz$. If the velocity of sound in air is $330 \, m/s$,the frequency of reflected sound as heard by the driver is .... $Hz$.