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Doppler Effect of a vehicle

  1. Jul 27, 2010 #1
    1. The problem statement, all variables and given/known data
    Firstly, whilst reading all this more complicated (and far more interesting) physics in other threads I feel like a dunce posting something which is probably of 'lower' level here as it were, but here goes:

    If we have a vehicle like a police car (with siren going) moving at a constant velocity, and then another vehicle (carrying an observer) travelling in the same direction and moving at the same speed as the police car, would the observer witness any Doppler effect?
    2. Relevant equations
    Not really relevant. Logic only.

    3. The attempt at a solution
    If neither the observer or the police car were moving there wouldn't be Doppler effect obviously, but when the police car is moving he's effectively moving through his own waves and therefore creating a higher pitch of sound and consequently a red shift.

    Does this only occur when the observer is at a different velocity compared to the emitter, or does it also occur when they are both moving at the same velocity?
    Sorry for clogging up your board with easy questions, couldn't find a site which was going to be as efficient with an answer as this one :P! Thanks in advance.
  2. jcsd
  3. Jul 27, 2010 #2
    Doppler Effect only exists when there in relative motion between source and observer. Police car is not moving through its own waves - speed of sound is at least 7X faster than the car would move.

    Besides, motion of the car does not affect how often the audio speaker in the siren vibrates. If the two cars are not moving toward or away from each other the wave crests pass by the car at the same rate as if they were at rest.

    Finally, if you look at the Doppler equation and plug in the same velocity for source and observer you see that the perceived f is the same as the broadcast f.
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