$A$ person observes two moving trains,$A$ reaching the station and $B$ leaving the station with equal speed of $30\,m/s$. If both trains emit sounds with frequency $300\,Hz$,(Speed of sound : $330\,m/s$) the approximate difference of frequencies heard by the person will be $..........Hz$.

When a source of sound crosses a stationary observer,the change in apparent frequency of sound observed by the observer,when $V_s << V$,will be -

$A$ train standing at the outer signal of a railway station blows a whistle of frequency $400 \ Hz$ in still air. The train begins to move with a speed of $10 \ ms^{-1}$ towards the platform. What is the frequency of the sound for an observer standing on the platform (in $Hz$)? (Sound velocity in air $= 330 \ ms^{-1}$).

The Doppler effect is applicable for

$A$ musician using an open flute of length $50\,cm$ produces second harmonic sound waves. $A$ person runs towards the musician from another end of a hall at a speed of $10\,km/h.$ If the wave speed is $330\,m/s,$ the frequency heard by the running person shall be close to...... $Hz$

$A$ 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 of the car is $11:9$. The speed of the car is (Speed of sound $v = 340 \,ms^{-1}$) (in $\,ms^{-1}$)