JEE Advanced 2017 · Paper 1 · Physics Q11

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PhysicsInteger+3Question 11 of 54

A stationary source emits sound of frequency $f_0 = 492$ Hz. The sound is reflected by a large car approaching the source with a speed of $2\ \mathrm{ms^{-1}}$ . The reflected signal is received by the source and superposed with the original. What will be the beat frequency of the resulting signal in Hz? (Given that the speed of sound in air is $330\ \mathrm{ms^{-1}}$ and the car reflects the sound at the frequency it has received).

Your answer

Answer and worked solution

Official answer: 6

Reflected frequency. The car receives $f_0\dfrac{v+u}v$ as a moving observer, then re-emits it as a moving source:
$f'=f_0\frac{v+u}{v-u}.$

Beats.
$f'-f_0=f_0\frac{2u}{v-u}=492\times\frac4{328}=6\text{ Hz}.$

Answer: 6

Topics:Doppler effect Beats

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JEE Advanced 2017 · Paper 1, Physics question 11