First slide

Doppler's effect

Question

A car C blowing a horn of frequency 650 Hz is moving at a speed of 25 m/sec on a straight path CP. A person is standing at a point O at a distance of 40 m perpendicular to CP as shown in figure. When car is 30 m away from P, the apparent frequency of sound as heard by the person is $(V_{s o u d} = 345 m / \sec)$

Moderate

A

625 Hz
B

650 Hz
C

675 Hz
D

680 Hz

Solution

Here the observer is at rest and source is in motion and observer will receive sound at a component of source velocity.
$O C = \sqrt{{(30)}^{2} + {(40)}^{2}} = 50 m u co s θ = 25 \cos θ = 25 \times \frac{C P}{C O} = \frac{25 \times 30}{50} = 15 m / \sec A p p a r e n t F r e q u e n c y n^{1} = n (\frac{V}{V - u c o s θ}) = \frac{650 \times 345}{(345 - 15)} = 680 H z$

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