$A$ train whistling at a constant frequency is moving towards a station at a constant speed $V_S$. The train goes past a stationary observer on the station. The frequency $n'$ of the sound as heard by the observer is plotted as a function of time $t$. Identify the expected curve.

$A$ 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 $330 \, m/s$,calculate the speed of the car in $m/s$.

$A$ 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 ... $\text{ Hz}$.

The pitch of a whistle of an engine appears to drop by $30 \%$ 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:

$A$ motorcycle starts from rest from a stationary source of sound and moves away from the source with a uniform acceleration $2 \,m/s^2$. The distance travelled by the motorcycle when the person on it hears the sound of frequency which is $94 \%$ of the true frequency is nearly (speed of sound in air $= 330 \,m/s$): (in $\,m$)

$A$ stationary sound source $S$ of frequency $334 \, Hz$ and a stationary observer $O$ are placed near a reflecting surface moving away from the source with velocity $u = 2 \, m/s$ as shown in the figure. If the velocity of the sound waves in air is $V = 330 \, m/s$,the apparent frequency of the echo heard by the observer is ... $Hz$.