The police car moves faster than the speed of sound?

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Discussion Overview

The discussion centers around the scenario of a police car moving faster than the speed of sound and whether a person on the street would hear the siren. Participants explore concepts related to sound propagation, the Doppler effect, and the implications of relative motion in different frames of reference.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that if the police car moves faster than the speed of sound, the person will not hear the siren due to the sound being Doppler shifted to zero frequency.
  • Others argue that the speed of sound is constant relative to the medium (air) and that the speed of the sound source does not change the speed of sound itself.
  • A participant mentions that the speed of sound relative to the driver is not constant in all directions, especially when considering motion through the air.
  • Some participants discuss the implications of special relativity, noting that sound behaves differently than light and that the laws of physics apply differently to sound in moving frames.
  • There is a suggestion that if sound is generated inside a closed car, the perception of sound may differ based on the relative motion of the observer and the sound source.
  • Concerns are raised about the clarity of scenarios presented, particularly regarding what is moving relative to what.

Areas of Agreement / Disagreement

Participants express multiple competing views on the relationship between the speed of sound, the motion of the car, and the perception of sound by the observer. The discussion remains unresolved with no consensus reached.

Contextual Notes

Participants highlight limitations in their scenarios, including assumptions about the closed environment of the car and the effects of relative motion on sound perception. The discussion also touches on the complexities of sound propagation in different frames of reference.

  • #31
First, do you agree with the paragraph I referred.
If the driver is in the closed vehicle with no windows no air enters from outside because the vehicle is totally closed. The outside observer sees the vehicle moving relative to him. The driver will not know he is moving. This is what the paragraph clarify.
But A.T. said the speed of sound will be changed according to the driver. So the driver with this tool can know he is moving and this is conflict with what I explained.

Another thing I would like to raise,
Use another medium instead of the air. Use for example a long metal and hit the front end of it while it is moving and measure the speed by hearing form another side. Do same thing by hitting the back end of it. The measured speed should be the same regarding the first postulate of the SR. If not, the first postulate is wrong. Besides, is this measured speed same of the speed when the metal does not move (at the rest) or different? If it is different then the first SR postulate is wrong. As you know the sound wave transfers longer or shorter according to the outside observer. I would like some one understand my point.

If you want more clarification let me know please.

Regards,
 
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  • #32
Just how is SR the slightest bit relevant here? We ditched that two pages back as irrelevant. Also, what has the situation inside the car got to do with the OP? If you want to deal with a different situation the why not open a new thread?
 
  • #33
powermind said:
But A.T. said the speed of sound will be changed according to the driver.
For sound propagating outside of the car, obviously.
 
  • #34
powermind said:
First, do you agree with the paragraph I referred.
If the driver is in the closed vehicle with no windows no air enters from outside because the vehicle is totally closed. The outside observer sees the vehicle moving relative to him. The driver will not know he is moving. This is what the paragraph clarify.
But A.T. said the speed of sound will be changed according to the driver. So the driver with this tool can know he is moving and this is conflict with what I explained.
There is air outside the vehicle and there is air inside the vehicle. Sound waves move at the speed of sound relative to the air. So in any given inertial frame the sound waves inside and outside the vehicle will go at different speeds since they are propagating in air which is moving at different speeds.

The state of knowledge or ignorance of the driver is not relevant to the physics.
 
  • #35
powermind said:
Hi,
Suppose the police car moves faster than sound's speed and runs away from the person on the street. Will the person hear the siren?

Regards,



Go to an air show that has high performance military aircraft. For example the USN Blue Angels or the USAF Thunderbirds.
The high speed passes impressively demonstrate the aircraft is traveling close to (slightly below, at or slightly above) the speed of sound.
You can perceive the sound coming from behind the aircraft even when the aircraft is a few hundred feet distant. The aircraft is past you on the left but the sound is coming from the right.

A sonic boom is like thunder is short lived for a stationary observer on the ground. A large aircraft would make the two shock waves more noticeable than a small aircraft. However the roar of the engines sounds like it is tearing open the sky. And it is susceptible to a doppler shift that aircraft fans love to hear.


http://blueangels.navy.mil/show/
http://afthunderbirds.com/site/
 
  • #36
Good! I would like to be sure about your answer DaleSpam.
Same example of closed vehicle. The experiment will be inside the vehicle.
in case of moving:
1- The observer is in the back of the vehicle. The sound source is in the front. The observer will hear the sound after Δt1.
2- The observer is in the front. The sound is in the back. The observer will hear the sound after Δt2.
in case of no moving.
3- Same of #1 but he will here after Δt3
4- Same of #2 but he will here after Δt4

The all factors like the temperature are same in all cases

Is Δt1=Δt2 or not?
Is Δt3=Δt4 or not?
Is Δt1=Δt3 or not necessary?
Is Δt2=Δt4 or not necessary?

Answering these questions will clear the confusion.

Regards,
 
  • #37
powermind said:
The experiment will be inside the vehicle.
I assume that the air inside the vehicle is at rest wrt the vehicle (i.e. no air conditioning or other fans inside).

powermind said:
Is Δt1=Δt2 or not?
Is Δt3=Δt4 or not?
Is Δt1=Δt3 or not necessary?
Is Δt2=Δt4 or not necessary?
They are all the same (assuming that the vehicle is moving with some v<<c)
 
  • #38
powermind said:
Good! I would like to be sure about your answer DaleSpam.
Same example of closed vehicle. The experiment will be inside the vehicle.
in case of moving:
1- The observer is in the back of the vehicle. The sound source is in the front. The observer will hear the sound after Δt1.
2- The observer is in the front. The sound is in the back. The observer will hear the sound after Δt2.
in case of no moving.
3- Same of #1 but he will here after Δt3
4- Same of #2 but he will here after Δt4

The all factors like the temperature are same in all cases

Is Δt1=Δt2 or not?
Is Δt3=Δt4 or not?
Is Δt1=Δt3 or not necessary?
Is Δt2=Δt4 or not necessary?

Answering these questions will clear the confusion.

Regards,

You really think so? lol.
 

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