$A$ train is moving towards a stationary observer. Which of the following curves best represents the frequency $f$ received by the observer as a function of time $t$?

$A$ bus moving with a uniform speed of $72 \,km/h$ towards a building blows a horn of frequency $1.7 \,kHz$. If the speed of sound in air is $340 \,m/s$, what will be the frequency of the echo heard by the bus driver (in $\,kHz$)?

$A$ source of sound is moving with a constant velocity of $30 \,m/s$ emitting a note of frequency $256 \,Hz$. The ratio of frequencies observed by a stationary observer while the source is approaching him and after it crosses him is [speed of sound in air $= 330 \,m/s$].

$A$ car travelling at a speed of $54 \ km/h$ towards a wall sounds a horn of frequency $400 \ Hz$. The difference in the frequencies of two sounds,one received directly from the car and the other reflected from the wall,noticed by a person standing between the car and the wall is (speed of sound in air is $335 \ m/s$):

$A$ stationary source of sound $A$ is producing sound of frequency $170 \, Hz$. Another source of sound $B$ producing sound of frequency $240 \, Hz$ is moving towards the source $A$ on a straight path with a uniform speed of $20 \, ms^{-1}$. An observer between $A$ and $B$ is moving towards the source $A$ along the straight path $BA$. If the number of beats heard by the observer is zero, the speed of the observer is . . . . . . $ms^{-1}$. (Speed of sound in air $= 340 \, ms^{-1}$).

What is the Doppler effect? Write its example and explain how its analysis can be performed.