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

Two waves represented by

$y_1 = 10\,sin\,(2000\,\pi t + 2x)$

and ${y_2} = 10{\mkern 1mu} \,sin\,{\mkern 1mu} \left( {2000{\mkern 1mu} \pi t + 2x + \frac{\pi }{2}} \right)$ are superposed at any point at a particular instant. The resultant amplitude is ..... $unit$

A man is watching two trains, one leaving and the other coming with equal speed of $4\,m/s$ . If they sound their whistles each of frequency $240\, Hz$ , the number of beats per sec heard by man will be equal to: (velocity of sound in air $= 320\, m/s$ )

A small source of sound moves on a circle as shown in the figure and an observer is standing on $O.$ Let $n_1,\, n_2$ and $n_3$ be the frequencies heard when the source is at $A, B$ and $C$ respectively. Then

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. 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 (see figure). 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   .... $Hz$

When two sound sources of the same amplitude but of slightly different frequencies $v_1$ and $v_2$ are sounded simultaneously, the sound one hears has a frequency equal to