JEE Advanced 2013 · Paper 2 · Physics Q8

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PhysicsMultiple correct+3 / -1Question 8 of 60

Two vehicles, each moving with speed $u$ on the same horizontal straight road, are approaching each other. Wind blows along the road with velocity $w$ . One of these vehicles blows a whistle of frequency $f_1$ . An observer in the other vehicle hears the frequency of the whistle to be $f_2$ . The speed of sound in still air is $V$ . The correct statement(s) is (are)

If the wind blows from the observer to the source,

f_2>f_1

If the wind blows from the source to the observer,

f_2>f_1

If the wind blows from observer to the source,

f_2<f_1

If the wind blows from the source to the observer,

f_2<f_1

Answer and worked solution

Official answer: A, B

Wind along the road. Sound speed relative to the ground is $V\pm w$ in the direction of propagation. With source and observer approaching each other at $u$ each:
$f_2=f_1\,\frac{(V\pm w)+u}{(V\pm w)-u}>f_1$
whichever way the wind blows (as long as $V\pm w>u$ ).

(A), (B)

Topics:Doppler effect

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JEE Advanced 2013 · Paper 2, Physics question 8