PSA 2014_XI_Q15. A train blew its whistle as it came out of the tunnel. Farzana, who was standing 700 metres from the tunnel beside the straight railway line, heard it. The speed of the sound was 350 m/s. Eighteen seconds after Farzana heard the whistle, the train reached the point at which she was standing beside the track. What was the train’s average speed over that 700 metre railway line ?
(1) 44 m/s
(2) 2 m/s
(3) 35 m/s
(4) 37 m/s
Ans: Had the speed of the sound not been given, the train’s speed could have been straight forward (700/18) m/s. But, here the case is sound travels at 350 m/s and the whistle must have reached Farzana after 700/350 = 2 seconds.
Thus, the train not only has travelled for 18 seconds but also has covered some distance for 2 secs while the whistle sound reached Farzana’s ears.
Average speed = Distance/Total time= 700/(18+2) = 35 m/s. Option (3).