$A$ motorcyclist is approaching a large wall with a velocity of $90\,km/hr$. $A$ car is chasing him with a velocity of $108\,km/hr$. If the car sounds a horn at $30\,Hz,$ the beat frequency heard by the motorcyclist is . . . . . . $Hz$. Take the velocity of sound $= 330\,m/s$.

$A$ car $P$ travelling at $20\,ms^{-1}$ sounds its horn at a frequency of $400\,Hz$. Another car $Q$ is travelling behind the first car in the same direction with a velocity $40\,ms^{-1}$. The frequency heard by the passenger of the car $Q$ is approximately $.......\,Hz$. [Take,velocity of sound $= 360\,ms^{-1}$]

When an observer moves towards a stationary source with velocity $V_1$,the apparent frequency of the emitted note is $F_1$. When the observer moves away from the stationary source with velocity $V_1$,the apparent frequency is $F_2$. If $v$ is the velocity of sound in air and $\frac{F_1}{F_2} = 2$,then $\frac{v}{V_1}$ is equal to:

$A$ 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

When the observer moves towards a stationary source with velocity $V_1$,the apparent frequency of the emitted note is $F_1$. When the observer moves away from the source with velocity $V_1$,the apparent frequency is $F_2$. If $V$ is the velocity of sound in air and $F_1 / F_2 = 2$,then $V / V_1$ is equal to:

The engine of a train moving with speed $10\,ms^{-1}$ towards a platform sounds a whistle at frequency $400\,Hz$. The frequency heard by a passenger inside the train is $........\,Hz$ (neglect air speed. Speed of sound in air $330\,ms^{-1}$). (in $,Hz$)