Question

A railroad crossing warning signal sounds at 120 Hz.A) If you are approaching the crossing at 3% of the speed of sound, find the frequency you would hear.B) If you were going away from the crossing at 3% of sound speed, find the observed frequency.

Answer 1

Answer

given,

frequency of sound (f)= 120 Hz

speed of sound (v)= 343 m/s

a) speed of approaching = 3 % of speed of sound

                          = 0.03 x 343 = 10.3 m/s

     frequency you would hear

     $f' = f\dfrac{v+v_0}{v}$

     $f' = 120 \times \dfrac{343+10.3}{343}$

            f' = 123.60 Hz

b)  speed of going away = 3 % of speed of sound

                          = 0.03 x 343 = 10.3 m/s

     frequency you would hear

     $f' = f\dfrac{v-v_0}{v}$

     $f' = 120 \times \dfrac{343-10.3}{343}$

            f' = 116.39 Hz