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Question
A train whistling at constant frequency is moving towards a station at a constant speed V. The train goes past a stationary observer on the station. The frequency n ′ of the sound as heard by the observer is plotted as a function of time t (figure). Identify the expected curve.
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Solution
Explanation:
Let the original frequency of the source is n0.
Let the speed of sound wave n the medium be v.
As the observer is stationary
Apparent frequency `n_a = (v/(v - v_s)) n_o` ......[When the train is approaching]
= `(v/(v - v_s)) n_o`
= `n_a > n_o`
When the train is going away from the observer
Apparent frequency `n_a = (v/(v +v_s)) n_0`
= `n_a < n_o`
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