Possible webpage title: Calculating Frequency Shift for Moving Sound Source

[buffgilville]

A man standing by a rail track, hears the whistle of a train at a frequency of 10419.7 Hertz. The train is moving towards the man at a speed of 39.9 meters/sec. The frequency of the whistle (in Hertz) is

Student response 9207.612
Correct answer 9164.147

frequency heard by the man = [1/(1-[u/v])} * frequency of whistle
u=39.9 m/s
v=343 m/s
frequency heard by the man=10419.7
so, the frequency of the whistle is 9207.612 but the correct answer is 9164.147
What did I do wrong? please help, thanks.

 

[Andrew Mason]

A man standing by a rail track, hears the whistle of a train at a frequency of 10419.7 Hertz. The train is moving towards the man at a speed of 39.9 meters/sec. The frequency of the whistle (in Hertz) is

Student response 9207.612
Correct answer 9164.147

frequency heard by the man = [1/(1-[u/v])} * frequency of whistle
u=39.9 m/s
v=343 m/s
frequency heard by the man=10419.7
so, the frequency of the whistle is 9207.612 but the correct answer is 9164.147
What did I do wrong? please help, thanks.

You are right if the speed of sound is 343 m/sec. That assumes temperature of 20 degrees C. If the temperature is 0 degrees C, the speed is 331.4 m/sec which results in f_actual = 9165 m/sec. What does the problem give for the speed of sound or temperature?

AM

 

[buffgilville]

It does not give the temperature nor speed of sound.