Question

A train whistle has a frequency of 100 hertz as heard by the engineer on the train. Assume that the velocity of sound in air is 330 meters per second. If the train is approaching a stationary listener on a windless day at a velocity of 30 meters per second, the whistle frequency that the listener hears is most nearly 

(A) 90 Hz 

(B) 110 Hz 

(C) 120 Hz 

(D) 240 Hz 

(E) 300 Hz

Answer 1

The correct option is: (B) 110 Hz 

Explanation:

Use the Doppler equation

where v = v_snd and we use the top signs of the denominator and numerator equation since we are moving towards the observer. The observer is not moving so V_obs = 0

f’ = 100Hz (330)/(330-30) = 110 Hz