$A$ train approaching a railway platform with a speed of $20 \, m \, s^{-1}$ starts blowing the whistle. The speed of sound in air is $340 \, m \, s^{-1}$. If the frequency of the emitted sound from the whistle is $640 \, Hz$,the frequency of sound as heard by a person standing on the platform is .... $Hz$.

If a source emitting waves of frequency $F$ moves towards an observer with a velocity $V/3$ and the observer moves away from the source with a velocity $V/4$,the apparent frequency as heard by the observer will be ($V=$ velocity of sound).

An engine moving away from a vertical cliff blows a horn at a frequency $f$. Its speed is $0.5 \%$ of the speed of sound in air. The frequency of the reflected sound received at the engine is ............ $f$.

$A$ 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:

$A$ car $P$ approaching a crossing at a speed of $10 \, m/s$ sounds a horn of frequency $700 \, Hz$ when $40 \, m$ in front of the crossing. The speed of sound in air is $340 \, m/s$. Another car $Q$ is at rest on a road which is perpendicular to the road on which car $P$ is reaching the crossing. The driver of car $Q$ hears the sound of the horn of car $P$ when he is $30 \, m$ in front of the crossing. The apparent frequency heard by the driver of car $Q$ is ..... $Hz$.

$A$ small source of sound moves on a circle as shown in the figure and an observer is standing at $O.$ Let $n_1, n_2$ and $n_3$ be the frequencies heard when the source is at $A, B$ and $C$ respectively. Then