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

Apparent frequency due to train which is coming in is m1=vv−vsn Apparent frequency due to train which is leaving is m2=vv−vsn So the number of beats is n1−n2=(1316−1324)320×240⇒n1−n2=6

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