Waves
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Question : 20
Total: 27
A train, standing at the outer signal of a railway station blows a whistle of frequency 400 Hz in still air.
(i) What is the frequency of the whistle for a platform observer when the train
(a) approaches the platform with a speed of10 m s – 1
(b)recedes from the platform with a speed of10 m s – 1
(ii) What is the speed of sound in each case? The speed of sound in still air can be taken as 340m s – 1
(i) What is the frequency of the whistle for a platform observer when the train
(a) approaches the platform with a speed of
(b)recedes from the platform with a speed of
(ii) What is the speed of sound in each case? The speed of sound in still air can be taken as 340
Solution:
Here, frequency of source of sound,
υ = 400 v = 340 m s – 1
Speed of source= v s = 10 m s – 1
(i) (a) When the train approaches the platform, the apparent frequency as heard by the observer on the platformwill be
υ ′ =
υ =
× 400
=
× 400 = 412.1 H z
(b) When the train recedes from the platform, the apparent frequency as heard by the observer is given by according to the formula :
υ ′ =
υ =
× 400
=
× 400 = 388.6 Hz = 389 Hz
(ii) The speed of sound in each case remains same i.e.340 m s – 1
Speed of source
(i) (a) When the train approaches the platform, the apparent frequency as heard by the observer on the platformwill be
(b) When the train recedes from the platform, the apparent frequency as heard by the observer is given by according to the formula :
(ii) The speed of sound in each case remains same i.e.
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