Question

A train approaches a crossing at a speed of 20.0 m/s, sounding a horn with a frequency

of 1000 Hz. A driver stopped at the crossing blows his horn (f = 800 Hz) in frustration.

(a) What frequency does the driver hear coming from the approaching train horn?

(b) The driver continues blowing his horn after the train passes. What frequency does the

train engineer hear coming from the car horn as the train moves away from the crossing?

Answer 1

a)According to Doppler's effect, if observer is stationary and source is moving towards the observer at speed u, then:

f=fo*v/(v-u), where f is apparent frequency , fo is original frequency, v is velocity of sound in air = 340 m/s, u is velocity of source.

Here,fo=1000 Hz, u=20 m/s.

So,f = 1000*340/(340-20)1062.5 Hz.

b)According to Doppler's effect, if source is stationary and observer is moving away from the source, then:

f=fo*(v-u)/v , where f is apparent frequency , fo is original frequency, v is velocity of sound in air = 340 m/s, u is velocity of source.

Here,fo=800 Hz, u=20 m/s.

So, f = 800*(340-20)/340 = 752.94 Hz.