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Standard XII

Physics

Apparent Frequency:Listener Moving

A major car i...

Question

A major car is approaching a road crossing with a speed of 108 Km/h.A police standing near the crossing hears the frequency of the car's horn as 300 Hz.The real frequency of the horn is [speed of sound in air=332 m/s]

300 Hz

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332 Hz

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273 Hz

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400 Hz

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Solution

The correct option is C 273 Hz
Velocity of source $V_{s o u r c e} = 108 k m / h r = 108 \times \frac{5}{18} = 30 m / s$
Apparent frequency heard by police $f^{'} = 300 H z$
Using Doppler effect when a source approaches a stationary observer :
Apparent frequency heard $f^{'} = f_{o} [\frac{v_{s o u n d}}{v_{s o u n d} - v_{s o u r c e}}]$
$∴$ $300 = f_{o} [\frac{332}{332 - 30}]$
$⟹ f_{o} = 273 H z$

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A major car is approaching a road crossing with a speed of 108 Km/h.A police standing near the crossing hears the frequency of the car's horn as 300 Hz.The real frequency of the horn is [speed of sound in air=332 m/s]

The correct option is C 273 HzVelocity of source Vsource=108 km/hr=108×518=30 m/sApparent frequency heard by police f′=300 HzUsing Doppler effect when a source approaches a stationary observer :Apparent frequency heard f′=fo[vsoundvsound−vsource]∴ 300=fo[332332−30]⟹ fo=273 Hz