# Doppler effect in different scenarios

• takando12
In summary: You just have to be very clear about who is moving and in which direction.In summary, the conversation discusses the calculation of the number of beats per second heard by an observer when a car traveling at 5m/s and sounding a horn of frequency 256 Hz approaches a wall. The calculations are based on the speed of sound being 330m/s and the formula for calculating the frequency of sound heard when a source and/or observer are moving. The observer hears a reflected frequency of 260 Hz from the wall, and the driver hears a frequency of 260 Hz as well, as if a virtual car were approaching them at a speed of 5m/s. It is important to properly identify the source and observer and their respective velocities in
takando12

## Homework Statement

A driver of a car traveling towards a wall with a speed of 5m/s, sounds a horn of frequency 256 Hz. If speed of sound= 330m/s, find
a) No. of beats /sec if the observer is between the car and the wall.
b) No. of beats/sec if the car is between the wall the observer.
c) Frequency of reflected sound as heard by driver.

## Homework Equations

Velocity of observer =Vo
Velocity of source =Vs
Speed of sound= c
1) If source approaches observer
f' = (c/c-Vs)f
2) Source moves away from observer
f' = (c/c+Vs)/f
3) Source and observer move towards each other
f' = (c+Vo/c-Vs)f

## The Attempt at a Solution

I have attempted all sections and have got the right answers. But I am still not convinced with which formulas I'm using.I have a little trouble identifying the source and it's details.

a) The observer hears the car on the left and the reflection from the wall on the right. I understand why we use 1) for the incident frequency but for the reflection, the source becomes the wall and it is at rest. So why do we use 1) again and find the frequency heard? Shouldn't it be just the reflected frequency,as it is, without any change?
b) We use 2) for the incident sound and 1) for the reflected sound. Applying the same argument as before, why do we use 1) for reflection?
c) So now the source and the observer are the same. And I get the right answer if I use 3). Again we seem to be considering the source as the wall and that the wall is moving towards us, when it is not.

I am missing some key points of importance here, and that's why it's a messed up. Please help me set my understanding right.

For (a) the reflected frequency is the frequency 'heard' by the wall, which is the number given in formula (1) with car as source and wall as observer (260Hz). The wall then emits as an echo the 260Hz frequency it 'hears' and, as the observer and the wall are both stationary, that's the freq the observer hears.

For c, you have a stationary source emitting 260Hz and the driver as observer moving towards it at speed 5m/s. You apply formula (3) with ##V_O=5,V_S=0,f=260## and you'll get the right answer.

That's the same as what you get if you replace the source and its frequency by a virtual car that is the mirror image of the real car in the wall, with the virtual car moving towards the real one at a speed of 5m/s (rel to the ground) and emitting a 256Hz horn sound.

So it all works out.

## 1. What is the Doppler effect?

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the source of the wave. It is commonly experienced with sound waves, such as the change in pitch of a siren as it passes by.

## 2. How does the Doppler effect apply to different scenarios?

The Doppler effect can apply to various scenarios, including sound waves, light waves, and even ocean waves. It can also be observed in astronomy, where the shift in wavelength of light from galaxies moving away from us is used to determine their velocity.

## 3. How does the Doppler effect affect the sound of a moving object?

The Doppler effect can cause the sound of a moving object to appear higher in frequency when approaching, and lower in frequency when moving away. This is because the waves are compressed in front of the object and stretched behind it.

## 4. Can the Doppler effect be used to measure the speed of an object?

Yes, the Doppler effect can be used to measure the speed of an object by analyzing the change in frequency or wavelength of the waves it produces. This is commonly used in radar technology to measure the speed of moving vehicles.

## 5. How does the Doppler effect impact our daily lives?

The Doppler effect is present in many aspects of our daily lives, such as in the sound of passing cars, the pitch of a passing ambulance siren, and even in the colors we see in fireworks. It also has practical applications in various fields, including meteorology, astronomy, and even medical imaging.

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