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JEE Main 2016 · 10 Apr · Shift 1, Physics question 11

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 :

Choose one option
Answer and worked solution

Official answer: D

The observer hears the car directly (source approaching): f1=f340335f_1=f\frac{340}{335}. The echo comes from the wall, which hears a receding source: f2=f340345f_2=f\frac{340}{345}. The beat frequency is
f⋅340(1335−1345)=0.0294f=5  ⟹  f≈170 Hz.f\cdot340\left(\frac1{335}-\frac1{345}\right)=0.0294f=5\implies f\approx170\text{ Hz}.

Topics:Doppler effectBeats
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