Question

A train S1, moving with a uniform velocity of 108 km/h, approaches another train S2 standing on a platform. An observer O moves with a uniform velocity of 36 km/h towards S2, as shown in figure.Both the trains are blowing whistles of same
frequency 120 Hz. When O is 600 m away from S2 and distance between S1 and S2 is 800 m, the number of beats heard by O is _____.
[Speed of the sound = 330 m/s]

Answer 1

Thus the number of beats heard by O is f  b  = 8.128 Hz

Explanation:

Speed of sound =330 m/s

Calculate beat

f  b  = 120 [ ( 330 + 10 cos 53° / 330 + 30 cos 37° ) - ( 330 + 10 / 330 )]

f  b  = 120 [ 336 / 306 - 34 / 33 ]

f  b  = 8.128 Hz

Thus the number of beats heard by O is f  b  = 8.128 Hz