First slide

Doppler's effect

Question

A person feels 2.5% difference of frequency of a motor-car horn. If the motor-car is moving to the person and the velocity of sound is 320 m/sec, then the velocity of car will be

Difficult

A

8 m/s (approx.)
B

800 m/s
C

7 m/s
D

6 m/s (approx.)

Solution

By Doppler’s formula $n' = \frac{nv}{(v - v_{S})}$
Since, source is moving towards the listener so $n' > n$ .
If n = 100 then n' = 102.5
$\Rightarrow 102 .5 = \frac{100 \times 320}{(320 - v_{S})} \Rightarrow v_{s} = 8 m / \sec$ (approx.)

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