The Doppler effect is applicable for

$A$ train approaching a railway platform with a speed of $20 \ ms^{-1}$ starts blowing the whistle. The speed of sound in air is $340 \ ms^{-1}$. If the frequency of the emitted sound from the whistle is $640 \ Hz$, the frequency of sound to a person standing on the platform will appear to be: (in $Hz$)

$A$ train is moving towards a stationary observer (at $t = 0$) with a constant velocity of $20 \ m/s$ and after some time it crosses the observer. Which of the following curves best represents the frequency $f$ received by the observer as a function of time?

$A$ 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 ..... $Hz$.

$A$ sound source is moving towards a stationary listener with $1/10$th of the speed of sound. The ratio of apparent to real frequency is

$A$ source of sound $S$ in the form of a block kept on a smooth horizontal surface is connected to a spring, as shown in the figure. If the spring oscillates with an amplitude of $50 \,cm$ along the horizontal between the wall and the observer $O$, the maximum frequency heard by the observer is $12.5 \%$ more than the minimum frequency heard by him. If the mass of the source of sound is $100 \,g$, the force constant of the spring is (Speed of sound in air is $340 \,ms^{-1}$) (in $\,N m^{-1}$)