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)
![](https://s3-us-west-2.amazonaws.com/infinitestudent-images/ckeditor_assets/pictures/529362/original_18-1.jpg)