The police car moves faster than the speed of sound?

  • Thread starter powermind
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  • #26
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So they can know that they are moving relative to the air!!
Obviously yes.
 
  • #27
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Sure. You don't need sound for that. You can feel the wind in your face, or use an anemometer. What is your point?
But I mentioned that the all windows are closed and nothing enters inside! I add you cannot see anything outside.

http://jcer.com/index.php/jcj/article/viewFile/80/78 (P 532)
Surely, we ask, he must know that he is moving! This explains the difference easily. But, said Einstein, perhaps he does not know that he is moving. Perhaps he thinks he is at rest. Perhaps he really is at rest. Perhaps it is you who are moving. How do we know? This became the essence of the first of two major postulates proposed by Einstein and which underpin his theory.
The postulate is stated as, “the laws of physics are the same (invariant) in all inertial (reference) frames.” It can equally be called the “reciprocity of reference systems.” It implies that any observer has the right to declare himself at rest and all others in motion with respect to him.
I hope you understood my point!
 
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  • #28
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Sound is not light and air is not vacuum. I hope you understood my point.

You can do the analysis in any reference frame that you like, and by doing so you can change the speed of the air and the speed of the sound waves. What you cannot do is change the speed of the sound waves relative to the air, nor can a change of reference frame change the speed of the air relative to the car or whatever other apparatus or observers that you might choose to include.
 
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  • #29
A.T.
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But I mentioned that the all windows are closed and nothing enters inside!
You need to be more clear in your scenario. What is moving relative to what? What is inside, what outside the car?
 
  • #30
sophiecentaur
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This thread is getting to be full of people trying to justify things they said near the top and missing the main point of the original question. It is hard to avoid, I know.
 
  • #31
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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
sophiecentaur
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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
A.T.
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But A.T. said the speed of sound will be changed according to the driver.
For sound propagating outside of the car, obviously.
 
  • #34
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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
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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 travelling 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
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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
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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).

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
sophiecentaur
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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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