The police car moves faster than the speed of sound?

[powermind]

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,

 

[maajdl]

Yes.

 

[powermind]

But the speed of the siren's sound related to the outside person=The speed of the sound relative to the dirver-the speed of the car=343-343=0!
I am talking about the sound which goes to the person.
If this is wrong, when will the person hear the siren if you have the following:
1- The speed of the car which equals 343 m/s.
2- The speed of the sound relative to the driver which equals 343 m/s.
3- The distance between the siren and the persion when the siren starts working.

 

[Borek]

You don't need anything more than time = distance / speed.

 

[powermind]

Do you mean that the speed of the siren's sound is not affected by the siren speed?

 

[Borek]

Yes. Frequency is affected, but the speed of the source doesn't change the speed on the sound itself.

 

[A.T.]

The speed of the sound relative to the driver which equals 343 m/s.

That is wrong. The speed of sound relative to the air is 343 m/s. If you move though the air, you see different speed of sound, in different directions.

 

[Borek]

I have moved the sonic boom discussion, please continue it here: https://www.physicsforums.com/showthread.php?t=732594

 

[boneh3ad]

I have moved the sonic boom discussion, please continue it here: https://www.physicsforums.com/showthread.php?t=732594

I'd argue that a good chunk of that was not sonic boom related and rather about the propagation of waves in air, which is relevant to the OP's question still.

 

[berkeman]

Thanks for all the cleanup work, Borek!

 

[powermind]

I am sure he cannot hear the siren because the sonic boom is more loud than siren :)

That is wrong. The speed of sound relative to the air is 343 m/s. If you move though the air, you see different speed of sound, in different directions.

It means that the speed of sound is directly proportional to the speed of its source!

 

[A.T.]

It means that the speed of sound is directly proportional to the speed of its source!

In the rest frame of the medium (air) the speed of sound is the same in all directions. In a frame that moves relative to the medium the speed of sound depends on the propagation direction. The speed of the sound relative to the driver is not 343 m/s in all directions. Especially not forwards and backwards.

 

[sophiecentaur]

I am sure he cannot hear the siren because the sonic boom is more loud than siren :)


It means that the speed of sound is directly proportional to the speed of its source!

I don't think you really mean that. The speed and frequency is perceived relative to the speed of the observer (e.g. by two microphones and an on board timing device). The source velocity will affect the frequency of the sound radiated in different directions (Doppler) but, once launched, the sound travels at the asme speed in all directions appropriate to the atmosphere carrying it.

Sound laid down (the siren) by the receding car will be doppler shifted to zero frequency if the car's speed is Mach 1+. So I don't think that's an issue as the siren sound would just be infra-sonic.
Disturbances in the air, left behind by the car are less easy to analyse. Apart from the fast motion of the when it's disturbed by the car moving through it, the air will soon not be moving at Mach 1+ so you are bound to hear something. Perhaps a general low amplitude 'rumbling' due to turbulence(?) but nothing coherent like in the cone of the boom in the forward and side direction.

 

[powermind]

Many thanks

 

[powermind]

In the rest frame of the medium (air) the speed of sound is the same in all directions. In a frame that moves relative to the medium the speed of sound depends on the propagation direction. The speed of the sound relative to the driver is not 343 m/s in all directions. Especially not forwards and backwards.

Does this disagree with special relativity: the laws of physics are invariant in all inertial systems?

 

[sophiecentaur]

No. SR is irrelevant. Sound is not EM.

 

[russ_watters]

Let me expand on that a bit:
You are mixing together the two postulates. The postulate that the laws of the universe are the same everywhere applies everywhere, including here.

The postulate that says the speed of light is constant in all frames applies only to light (setting aside the broader implications of the theory that don't show-up at low speed). In other words, the fact that the speed of light is the same in all frames does not imply that the speed of sound should be the same in all frames.

 

[powermind]

A.T, your answer will be differnet if the sound is generated inside the car which is nothing enters from outside. i.e the windows are closed?

 

[Dale]

What about it? Sound moves at the speed of sound wrt the air.

 

[A.T.]

A.T, your answer will be differnet if the sound is generated inside the car which is nothing enters from outside. i.e the windows are closed?

The speed of sound is the same in all directions relative to the airmass in which it propagates.

 

[A.T.]

Does this disagree with special relativity: the laws of physics are invariant in all inertial systems?

Which law of physics would be different?

 

[powermind]

Which law of physics would be different?

You said:

In the rest frame of the medium (air) the speed of sound is the same in all directions. In a frame that moves relative to the medium the speed of sound depends on the propagation direction. The speed of the sound relative to the driver is not 343 m/s in all directions. Especially not forwards and backwards.

 

[A.T.]

You said:

Yes, and?

 

[powermind]

If the observer and the sound source are stationary relative to the air the speed of the sound is 343m/s.
But if both moves relative to the air, you said that, as I understood, the speed of the sound relative to them will be not equal 343m/s. So they can know that they are moving relative to the air!
Suppose the observer stands at the back of the car and the sound source is located at the front. In case of moving, he will hear the sound in different time.

 

[A.T.]

So they can know that they are moving relative to the air!

Sure. You don't need sound for that. You can feel the wind in your face, or use an anemometer. What is your point?

 

[Dale]

So they can know that they are moving relative to the air!

Obviously yes.

 

[powermind]

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!

 

[Dale]

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.

 

[A.T.]

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?

 

[sophiecentaur]

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.

 

[powermind]

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,

 

[sophiecentaur]

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?

 

[A.T.]

But A.T. said the speed of sound will be changed according to the driver.

For sound propagating outside of the car, obviously.

 

[Dale]

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.

 

[pikpobedy]

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/