Sum

A traffic policeman standing on a road sounds a whistle emitting the main frequency of 2.00 kHz. What could be the apparent frequency heard by a scooter-driver approaching the policeman at a speed of 36.0 km h^{−1}? Speed of sound in air = 340 m s^{−1}.

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#### Solution

Velocity of sound in air *v* = 340 ms^{−1}

Velocity of scooter-driver \[v_o\]* _{=}* 36 kmh

^{−1}=

\[36 \times \frac{5}{18} = 10 {\text { ms }}^{- 1}\]

Frequency of sound of whistle \[f_o\]= 2 kHz

Apparent frequency \[\left( f \right)\] heard by the scooter-driver approaching the policeman is given by :

\[f = \left( \frac{v + v_o}{v} \right) \times f_o\]

\[f = \left( \frac{340 + 10}{340} \right) \times 2\]

\[ = \frac{350 \times 2}{340} \text { kHz }\]

\[ = 2 . 06 \text{ kHz }\]

Concept: Wave Motion

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