Question

Two trains are moving towards each other with speeds of 20m/s and 15 m/s relative to the ground. The first train sounds a whistle of frequency 600 Hz. The frequency of the whistle heard by a passenger in the second train before the train meets, is ______. (the speed of sound in air is 340 m/s) 

पर्याय

• 600 Hz

• 585 Hz

• 645 Hz

• 666 Hz

Answer 1

Two trains are moving towards each other with speeds of 20m/s and 15 m/s relative to the ground. The first train sounds a whistle of frequency 600 Hz. The frequency of the whistle heard by a passenger in the second train before the train meets, is 666 Hz. (the speed of sound in air is 340 m/s) 

Explanation:

v' = v = `(("v"+"v"_"D")/("v"-"v"_"S"))`

Here, v = 600 Hz, v_D = 15 m/s 

v_S = 20 m/s, v = 340 m/s

∴ v' = 600`(355/320)` ≈ 666 Hz