Question

A locomotive approaching a crossing at a speed of 20m/s aound a whistle of frequency 640hz when 1 km from crossing. there ia no wind and the speed oof aound of air is 330m/s. what frequency is heard by an observer 3 under root km on the straight road from crossing at right angle. ans is 660 hz.

Answer 1

Answer:Yeah! It's 660 Hz

Explanation:

First find the Velocity in the direction of Observer from Source

Which is vcos∅ = 20m/s × 1/2 {cos∅=Base/Hypotenuse}

... v = 10m/s

Now apply formula of Doppler's effect

μ'=μ×v/(v-v°)

μ'=640×330/330-10

μ'=640×330/320

μ'=660 Hz

Where μ'→Final Frequency and μ→Initial frequency

Study hard! And SMART!

Answer 2

Answer:

The frequency heard by an observer is 660 Hz.

Explanation:

Given that,

Speed = 20 m/s

Frequency = 640 Hz

Speed of sound = 330 m/s

We need to calculate the velocity of the locomotive in the direction of motion of sound

Using formula of velocity

$v=v_{x}\cos\theta$

Put the value into the formula

$v=20\times\dfrac{1}{\sqrt{1+(\sqrt{3})^2}}$

$v=10 m/s$

We need to calculate the apparent frequency

Using formula of frequency

$f=\dfrac{v}{v-v_{x}\cos\theta}f_{0}$

Put the value into the formula

$f=\dfrac{330}{330-10}\times640$

$f=660\ Hz$

Hence, The frequency heard by an observer is 660 Hz.