Question

Thinking and Inquiry Two train whistles, A and B each have a frequency of 392Hz. A is at rest while B is moving at 35m/s [right]. A railway worker moving at 15 m/s[left] is between the two trains. Air temp is 293 K. 14. a) What is the frequency heard by the railway worker from Train A? (3)

Answer 1

Part A.

Using Doppler's effect:

When Observer is moving away from Stationary Source

f1 = f0*(V - Vo)/V

Vo = Speed of Observer = 15 m/sec

V = Speed of Sound at 293 K = 343 m/sec

f0 = frequency of Source = 392 Hz

So,

f1 = frequency heard by worker by train A = ?

f1 = 392*(343 - 15)/343

f1 = 374.8 Hz

Part B.

Since train is moving in right and worker is moving in left, So

When Source and observer both moving away from each other, then

f2 = f0*(V - Vo)/(V + Vs)

Vs = Speed of source = 35 m/sec

Using these values:

f2 = 392*(343 - 15)/(343 + 35)

f2 = 340.1 Hz = frequency heard by worker by train B

Part C.

Beat frequency = f1 - f2

Beat frequency = 374.8 - 340.1 = 34.7 Hz

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