45. A train station bell gives off a fundamental tone of 500 Hz as the train app

Question

45. A train station bell gives off a fundamental tone of 500 Hz as the train approaches the station at a speed of 20 m/s. If the speed of sound in air is 335 m/s, what will be the apparent frequency of the bell to an observer riding the train? a. 532 Hz b. 530 Hz c. 470 Hz d. 472 Hz 46. You stand by the railroad tracks as a train passes by. You hear a 1 000-Hz frequency when the train approaches, which changes to 800 Hz as it goes away. How fast is the train moving? The speed of sound in air is 340 m/s a. 15.7 m/s b. 21.2 m/s c. 28.0 m/s d. 37.8 m/s 47. A sound source of frequency 1 000 Hz moves at 50.0 m/s toward a listener who is at rest. What is the apparent frequency heard by the listener? (speed of sound-340 m/s) a.853 Hz b. 872 Hz c. 150 Hz d. 170 Hz

Explanation / Answer

45) a) 532 Hz

f' = f*(v_sound)/(v_sound - v_source)

= 500*335/(335 - 20)

= 532 Hz

46) d) 37.8 m/s

when train approches,

f' = f*(v_sound)/(v_sound - v_source)

1000 = f*340/(340 - v_source) ----(1)

when train goes away

f'' = f*(v_sound)/(v_sound - v_source)

800 = f*340/(340 + v_source) ---(2)

from equation 1 and 2

1000/800 = (340 + v_source)/(340 - v_source)

v_source = 37.8 m/s

47) d) 1170 Hz

f' = f*(v_sound)/(v_sound - v_source)

= 1000*340/(340 - 50)

= 1170 Hz

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