[Dog4444]

http://www.edexcel.com/migrationdocu...e_20090115.pdf

Question 10, on the test.
Help please.

[UserInvalid]

D.

As the fire engine moves away from the observer, the waves stretches, hence the wavefront increases.

But I am confused, C is also a correct answer, if the pitch decreases then the frequency must decrease also, but the actual answer is D. Someone can guide me also?

[Dog4444]

(Original post by UserInvalid)
D.

As the fire engine moves away from the observer, the waves stretches, hence the wavefront increases.

But I am confused, C is also a correct answer, if the pitch decreases then the frequency must decrease also, but the actual answer is D. Someone can guide me also?

The waves stretches only is the speed is increasing? That's why I was thinking of B. Why it's not a B?
Pitch = frequency, it decreases only if wavelength increases, what can happen only if the car speeds up?

[Stonebridge]

It's a typically poor multiple choice question that should never have got past the "shredding" stage.
A is wrong. The wavelength of the waves reaching the observer are longer, not shorter.
B is wrong but you could interpret it as correct if you don't read the question in the way the writer intended.
The fire engine is moving away at constant speed and the question actually meant to say that the pitch of the siren is less than when the engine wasn't moving. "The pitch of the siren decreases" is actually ambiguous. It could be interpreted as the pitch is actually decreasing as the engine moves away. This would require the engine to increase its speed.
C is wrong because the question means the frequency (pitch) of the actual siren itself. Not the observed pitch. The actual frequency of the siren on the engine doesn't change, just the frequency you hear (because of the motion), changes.
D is correct. As the engine moves away the waves are "stretched". This means the wavelength is longer and as velocity = frequency x wavelength, if the wavelength increases, the frequency decreases for the same velocity. The speed of sound doesn't change.

So D is correct

The question is a poor example and you are welcome to cut and paste my post into an email and send it to the examining board.

[Dog4444]

(Original post by Stonebridge)
It's a typically poor multiple choice question that should never have got past the "shredding" stage.
A is wrong. The wavelength of the waves reaching the observer are longer, not shorter.
B is wrong but you could interpret it as correct if you don't read the question in the way the writer intended.
The fire engine is moving away at constant speed and the question actually meant to say that the pitch of the siren is less than when the engine wasn't moving. "The pitch of the siren decreases" is actually ambiguous. It could be interpreted as the pitch is actually decreasing as the engine moves away. This would require the engine to increase its speed.
C is wrong because the question means the frequency (pitch) of the actual siren itself. Not the observed pitch. The actual frequency of the siren on the engine doesn't change, just the frequency you hear (because of the motion), changes.
D is correct. As the engine moves away the waves are "stretched". This means the wavelength is longer and as velocity = frequency x wavelength, if the wavelength increases, the frequency decreases for the same velocity. The speed of sound doesn't change.

So D is correct

The question is a poor example and you are welcome to cut and paste my post into an email and send it to the examining board.

Does "how much the wave stretched" depend on speeding up/down? If it moves with constant velocity, the stretch of the wave stays the same, so does the frequency?

[Stonebridge]

(Original post by Dog4444)
Does "how much the wave stretched" depend on speeding up/down? If it moves with constant velocity, the stretch of the wave stays the same, so does the frequency?

If the engine moves at constant speed the pitch of the siren will stay the same, but will be lower than it was when the engine was stationary.

You may be thinking of the sound you hear when a formula one car passes on the track. You hear the pitch actually get lower as it moves. This is because the pitch is higher when the car moves towards you and lower when it moves away. You hear the sudden change as it passes by.

[Dog4444]

(Original post by Stonebridge)
If the engine moves at constant speed the pitch of the siren will stay the same, but will be lower than it was when the engine was stationary.

You may be thinking of the sound you hear when a formula one car passes on the track. You hear the pitch actually get lower as it moves. This is because the pitch is higher when the car moves towards you and lower when it moves away. You hear the sudden change as it passes by.

Well, yes. It's a Doppler effect.
Ok, I'll assume it's examiners fault. Thank you.

[ElvishJumpSuit]

(Original post by Dog4444)
Does "how much the wave stretched" depend on speeding up/down? If it moves with constant velocity, the stretch of the wave stays the same, so does the frequency?

Imagine the new wave front being emitted further away than the previous one.

This change of position is the amount of 'stretching' that occurs but it is also the distance moved by the fire truck in one PERIOD of the vibration.

A faster fire truck will move further in each period so the 'stretching' will be greater, resulting in further reduction in 'perceived' pitch.