Understanding Doppler
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Physics › Understanding Doppler
An ambulance turns on its siren as it is driving away from you. What happens to the sound pitch?
The pitch will sound lower.
The pitch will sound higher.
The pitch will stay the same.
You will be unable to hear the pitch.
There is insufficient information to tell.
Explanation
The Doppler effect states that the frequency of sound increases as it approaches you and decreases as it goes away from you.
Since it is going away from you, the pitch of the sound will appear to get lower.
An ambulance turns on its siren as it is driving towards you. What happens to the sound pitch?
The pitch will sound higher.
The pitch will sound lower.
The pitch will stay the same.
There is insufficient information to solve.
You will be unable to hear the pitch.
Explanation
The Doppler effect states that the frequency of sound increases as it approaches you and decreases as it goes away from you.
Since it is coming towards you, the pitch will appear to be higher.
You and a friend are running down the street at 
There is no change in the frequency
The frequency of the sound goes up
The frequency of the sound goes down
The frequency of the sound pulses between higher and lower levels
We must know the distance between the source and the observer to solve
Explanation
Change in frequency due to relative motion is described by the Doppler effect, given by the equation:
The numerator terms are summed when the observer is moving toward the source. The denominator terms are summed if the source is moving away from the observer. You and your friend are both moving with the same velocity in the same direction, meaning that the source (friend) is moving toward the observer (you) AND the observer (you) is moving away from the source (friend). The result is that both the numerator and denominator terms will be subtracted.
The fraction simplified to one. The perceived frequency is equal to the transmitted frequency.
An ambulance drives past a stationary man. Its siren has a frequency of 
We would need to know the velocity of the ambulance
We would need to know how far the man is from the ambulance
Explanation
When the ambulance passes the man, there is blue shift according to the Doppler effect. That means that the perceived frequency will be lower than the given one. Only one answer choice has a lower observed frequency than the actual frequency: 
You are standing on a street corner when an ambulance rushes past you, blaring its siren. At what point do you actually hear the frequency that the ambulance siren is emitting?
When it is right in front of you
When it is moving towards you
When it is moving away from you
You can never hear the actual frequency, since the ambulance is moving
Throughout the entire motion of the ambulance
Explanation
The Doppler effect states that the frequency of sound shifts proportionally to its velocity, either moving towards you or away from you. As it moves towards you, the perceived frequency increases. As it moves away, the perceived frequency decreases. The only time you will hear the correct frequency is when the observer and sound source are directly aligned, so that the frequency is neither increased nor decreased.
An ambulance drives past a stationary man. Its siren has a frequency of 
We would need to know the speed of the ambulance
We would need to know the distance the man is from the ambulance
Explanation
When the ambulance approaches the man, there is red shift according to the Doppler effect. That means that as it approaches, the observed frequency increases. There is only one answer choice with a higher observed frequency than the actual frequency: 