# Frequency involving moving observer and source

• n77ler
In summary, the conversation discusses a scenario where a person is driving at 27.5 m/s on a highway and hears a police car approaching from behind. The perceived frequency of the siren is 1250 Hz, but after the police car passes, it is perceived as 1202 Hz. The question asks for the speed of the police car, with the given information that the speed of sound in air is 343 m/s. The equations and attempts at solving the problem are also mentioned.
n77ler

## Homework Statement

You are driving along a highway at 27.5 m/s when you hear the siren of a police car approaching you from behind and you perceive the frequency as 1250 Hz. You are relieved that he is in pursuit of a different speeder when he continues past you, but now you perceive the frequency as 1202 Hz. What is the speed of the police car? The speed of sound in air is 343 m/s.

So, U_o= 27.5 m/s
U_s = ??
v= 343 m/s

## Homework Equations

It states that the perceived frequency from behind is 1250 ad the 1202 after it is gone by

## The Attempt at a Solution

I used the equation f ' =(1+- u_o/v)/(1+-u_s/v)f
Because it gives you information when the car is behind you and then after it has passed you I used two equations and let them equal each other but I am unsure if the perceived frequencies are f or f ' ??

Ok so I've wrote out my equation but I'm finding it really hard to get through because I don't know which givens go with the variables in my equations... In my equation the left side is the first part of questions and right side is the second side

((1 - u_o/v)/(1 - u_s/v))f - f ' = ((1 + u_o/v)/(1+ u_s))f - f '

anybodyy ? :(

## 1. How does the frequency change when the observer and source are both moving?

When both the observer and source are moving, the frequency of the sound wave is affected by the relative motion between them. If the observer and source are moving towards each other, the frequency will be higher. On the other hand, if they are moving away from each other, the frequency will be lower.

## 2. What happens to the frequency when only the observer is moving?

If only the observer is moving, the frequency of the sound wave will be affected by the speed and direction of their motion. If the observer is moving towards the source, the frequency will be higher. If the observer is moving away from the source, the frequency will be lower.

## 3. How does the frequency change when only the source is moving?

When only the source is moving, the frequency of the sound wave will be affected by the speed and direction of its motion. If the source is moving towards the observer, the frequency will be higher. If the source is moving away from the observer, the frequency will be lower.

## 4. Is the change in frequency the same for all types of waves?

No, the change in frequency due to the motion of the observer and source is different for different types of waves. For sound waves, the change in frequency is known as the Doppler effect. For electromagnetic waves, such as light, the change in frequency is known as the relativistic Doppler effect.

## 5. Can the frequency change in the absence of motion?

Yes, the frequency can change even if there is no motion involved. This can happen due to other factors such as the medium through which the sound wave is traveling, or the presence of objects that can reflect or absorb the sound waves.

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