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Homework Help: Doppler effect and signs of the equation

  1. Mar 29, 2016 #1
    1. The problem statement, all variables and given/known data
    A motor scooter moving at 10 m/s approaches and passes a police car moving at 50 m/s in the opposite direction. The frequency of the siren on the police car is 700 hz when the car stands still. the difference in frequency heard by the driver of the scooter between when the two vehicles are approaching each other and when the two vehicles are separating from each other is closest to..

    2. Relevant equations
    doppler effect equation

    3. The attempt at a solution
    I know I must find the frequency at one point and subtract it from the other to find the difference, but in the doppler equation I never know when the velocity of the source/listener must be subtracted/added to the speed of sound
    Last edited: Mar 29, 2016
  2. jcsd
  3. Mar 29, 2016 #2


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    I hope you mean "Speed of Sound" not of light.
  4. Mar 29, 2016 #3
    i do indeed lol
  5. Mar 30, 2016 #4


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    You know that is always a matter of multiplying or dividing by 1+v/c or 1-v/c, where c is the speed of sound and v is one of the other velocities.
    You also know that when the source is moving towards the receiver the frequency will increase, so for that case it must be either multiply by 1+v/c or divide by 1-v/c.
    A simple thought experiment will tell you which. Suppose the source is moving at the speed of sound. What will happen to the wavefronts? What frequency will that seem like to the receiver when they meet?
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