A person standing on a train platform listening to the whistle frequency of 450Hz of the train as it approaches her at a constant speed. As the train approaches the person on the platform, she will hear the frequency (pitch) of the whistle as:
A person standing on a train platform listening to the whistle frequency of 450Hz of the train as it approaches her at a constant speed. As the train approaches the person on the platform, she will hear the frequency (pitch) of the whistle as:
The correct option is A Greater than 450Hz but constant
It is a common perception we feel while standing on a
platform at a train station that if the train is approaching one feels as
though the sound level of its horn is increasing gradually and on the other
hand if the train is receding we hear a fainter sound.
This is because of the Doppler shift in its frequency, given
by (for stationary observer):
(for approaching source) f′=fo(1+vsv)⟶(1)
(for receding source) f′=fo(1−vsv)⟶(2)
f′ apparent frequency (this is what we hear), fo real frequency, vs
velocity of source (gives as constant), v velocity of sound wave in the medium (air) which is also a constant.
Notice that from equation (1) f′>fo
Therefore option A is correct.
It is a common perception we feel while standing on a platform at a train station that if the train is approaching one feels as though the sound level of its horn is increasing gradually and on the other hand if the train is receding we hear a fainter sound.
This is because of the Doppler shift in its frequency, given by (for stationary observer):
(for approaching source) f′=fo(1+vsv)⟶(1)
(for receding source) f′=fo(1−vsv)⟶(2)
f′ apparent frequency (this is what we hear), fo real frequency, vs velocity of source (gives as constant), v velocity of sound wave in the medium (air) which is also a constant.
Notice that from equation (1) f′>fo
Therefore option A is correct.
