You stand by the railroad tracks as a train passes by.You hear a 950-Hz frequency when the train approaches,which changes to 820 Hz as it goes away.How fast is the train moving? The speed of sound in air is 340 m/s.
A) 14.0 m/s
B) 20.0 m/s
C) 25.0 m/s
D) 37.8 m/s

Question

Quizplus · Accepted Answer

The Doppler effect formula for frequency change is used: $ f' = f \frac{v + v_0}{v - v_s} $ for approaching and $ f' = f \frac{v - v_0}{v + v_s} $ for receding. Solving these equations with given frequencies and speed of sound gives the train speed as 25.0 m/s.

You Stand by the Railroad Tracks as a Train Passes