A motor car is approaching towards a crossing with a velocity of 72 k m h - 1 . The frequency of the sound of its horn as heard by a policeman standing on the crossing is 260 Hz . The frequency of hor

Question

A motor car is approaching towards a crossing with a velocity of 72 k m h - 1 . The frequency of the sound of its horn as heard by a policeman standing on the crossing is 260 Hz . The frequency of horn is

TestHub · Accepted Answer

From Doppler's effect, perceived frequency is

$ν^{'} = ν (\frac{v - v_{o}}{v - v_{s}})$

$v_{s} = 72 km h^{- 1} = \frac{72 \times 1000}{60 \times 60} = 20 m s^{- 1}$

$v_{o} = 0, v = 332 m s^{- 1}, ν^{'} = 260 Hz$

$260 = ν (\frac{332}{332 - 20})$

$⟹ ν = \frac{260 \times 312}{332} = 244 Hz$

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