IIT-JEE 2011 · Paper 1 · Physics Q24

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PhysicsSingle correct+3 / -1Question 24 of 69

A police car with a siren of frequency 8 kHz is moving with uniform velocity 36 km/hr towards a tall building which reflects the sound waves. The speed of sound in air is 320 m/s. The frequency of the siren heard by the car driver is

8.50 kHz

8.25 kHz

7.75 kHz

7.50 kHz

Answer and worked solution

Official answer: A

Reflected sound acts as a source moving with the car's image; the driver hears
$f'=f\,\frac{v+u}{v-u}=8\times\frac{320+10}{320-10}=8\times\frac{330}{310}=8.52\ \text{kHz}\approx8.50\ \text{kHz}.$
(36 km/h = 10 m/s.)

(A)

Topics:Doppler effect

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IIT-JEE 2011 · Paper 1, Physics question 24