Question

A train is traveling away from you at 120 km/h. it blows its whistle, and you hear a tone of 0.400 kHz. Take the speed of sound to be 340 m/s. What frequency does the whistle actually produce? A) 444 Hz. B) 439 Hz C) 364 Hz D) 361 Hz

Answer 1

Answer:B)439.21 Hz

Explanation:

Given

velocity of train $v=120 km/h\approx 120\times \frac{5}{18}=33.33 m/s$

frequency of train $f_a=0.400 kHz\approx 400 Hz$

speed of sound $c=340 m/s$

frequency from Doppler effect can be calculated by

$f_a=(\frac{c}{c+v})\cdot f$

$400=(\frac{340}{340+33.33})\cdot f$

$400=\frac{340}{373.33}\cdot f$

$f=400\times 1.098$

$f=439.21 Hz$