A toy-car, blowing its horn, is moving with a steady speed of 5 m/s… | JEE Main 2016 (10 Apr, Shift 1)

Q63JEE Main 2016 · 10 APR · SHIFT 1Difficult

Doppler Effect

A toy-car, blowing its horn, is moving with a steady speed of 5 m/s, away from a wall. An observer, towards whom the toy car is moving, is able to hear 5 beats per second. If the velocity of sound in air is 340 m/s, the frequency of the horn of the toy car is close to:

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Answer: (d)

Direct sound (car approaching observer): $f_1 = f\frac{v}{v - u}$
Wall receives $f\frac{v}{v + u}$ (car receding from wall) and reflects it unchanged to the observer: $f_2 = f\frac{v}{v + u}$
Beats $= f_1 - f_2 \approx \frac{2fu}{v}$
$5 = \frac{2 \times f \times 5}{340} \Rightarrow f = 170$ Hz
So the answer is (d) 170 Hz.

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Doppler Effect question from JEE Main 2016 (10 Apr, Shift 1)

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