Question

Two trains travelling with equal speeds along parallel tracks in opposite directions blow whistle of frequency. The frequency of whistle of one train as heard by the passenger in another train is 9/8 of actual frequency of whistle. If the speed of sound in air is 340 m/s then speed of either train will be

Answer 1

Answer:

Let the speed of the trains be 'u'

Speed of sound c = 340m/s

fo - observed frequency

ft - actual train frequency

Given

fo = (9/8)ft

Using Dopplers formula:

Numerator u - observers train velocity

Denominator u - source train velocity

fo = [(c+u)/(c-u)] *ft

fo/ft = (340+u)/(340-u)

9/8 = (340+u)/(340-u)

9*340 - 9u = 8*340 + 8u

9*340 - 8*340 = 9u+8u

340 = 17u

u = 340/17 = 20m/s

The trains are travelling with u = 20m/s speed