Question

A police siren emits a sound wave of frequency 440hz.The speed of sound is 330m/s.If police jeep moves with 20m/s following a truck moving at 10m/s. Then the frequency of echo from truck heard by policeman​

Answer 1

Answer:

The frequency of echo heard from truck is 454.19 Hz

Explanation:

Let us consider the police jeep as the source of emitting sound and truck be the listener

$v _{s} = 20m {s}^{ - 1}$

and

$v _{l} = 10m {s}^{ - 1}$

Given ,

frequency of sound wave produced by siren

$f _{s} = 440Hz$

And Speed of sound , v = 330m/s

The source and the listener is moving in same direction so

$f = f _{s}( \frac{v - v _{l}}{v - v _{s} }) \\ \implies f = 440 \times \frac{330 - 10}{330 - 20} \\ \implies f = \frac{440 \times 320}{310} \\ \implies f = \frac{44 \times 320}{31} \\ \implies f = 454.19</u></strong><strong><u> </u></strong><strong><u>H</u></strong><strong><u>z</u></strong><strong><u>$

Answer 2

The frequency of echo from truck heard by policeman is 454.19 Hz.

Explanation :

Given :

• The frequency of sound wave produced by siren :

$\sf \implies \: f _{s} = 440 \: Hz$

• The speed of sound :

$\sf \implies \: v = 330m/s.$

Now,

Now,Assumption :

The listener - Truck.

The source of emitting sound - Police Jeep.

So,

$\sf \implies \: v_{s} = 20 \: ms ^{ - 1} \\ \sf \implies \: v_{l} = 10 \: ms^{ - 1}$

As given :

The frequency of sound wave produced by siren :

$\sf \implies \:f _{s} = 440 \: Hz$

• The speed of sound :

$\sf \implies \: v = 330m/s.$

• The listener & source is moving in the same direction.

So,

$\sf \implies \: f = f _{s}( \dfrac{v - v _{l}}{v - v _{s} }) \\ \\ \sf \implies f = 440 \times \dfrac{330 - 10}{330 - 20} \\ \\ \sf \implies f = \dfrac{440 \times 320}{310} \\ \\ \sf \implies f = \dfrac{44 \times 320}{31} \\ \\ \implies f = 454.19$

•°• The frequency of echo from truck heard by policeman is 454.19 Hz.