First slide

Doppler's effect

Question

A police car moving at 22 m/s, chases a motorcyclist. The police man sounds his horn at 176 Hz, while both of them move towards a stationary siren of frequency 165 Hz. Calculate the speed of the motorcycle, if it is given that he does not observes any beats

Easy

A

33 m/s
B

22 m/s
C

Zero
D

11 m/s

Solution

n₁ = Frequency of the police car horn observer heard by motorcyclist
n₂ = Frequency of the siren heard by motorcyclist.
v₂ = Speed of motor cyclist
$n_{1} = \frac{330 - v}{330 - 22} \times 176; n_{2} = \frac{330 + v}{330} \times 165$
$∵ n_{1} - n_{2} = 0 \Rightarrow v = 22 m / s$ .

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