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Homework Help: Doppler effect

  1. Jun 6, 2014 #1
    1. The problem statement, all variables and given/known data

    can anyone explain why reletive velocity is (v-u), based on my understanding, as the obesrvers moving away, the distance of the f th wavefront and first waavefront increases. so i would say it's v+u . correct me if i'm wrong. thanks.

    2. Relevant equations

    3. The attempt at a solution

    Attached Files:

  2. jcsd
  3. Jun 6, 2014 #2
    The distance between wave fronts appears to increase, yes, but that is because the relative speed is also decreasing as the observer accelerates away. If u=v then the wave would never reach you.
  4. Jun 6, 2014 #3
    The wave and the observer are moving in the same direction. Their relative speed is v-u. If they were moving in the opposite direction than the relative speed would be v+u.
    Last edited: Jun 6, 2014
  5. Jun 6, 2014 #4


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    If two cars travel on the freeway at similar speeds in the same direction the relative velocity between them is small. It's not going to be the sum of their speeds.
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