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Homework Help: Doppler effect problem

  1. Dec 5, 2013 #1
    1. The problem statement, all variables and given/known data

    A parachutist leaps from a hovering helicopter, after 4.0s of freefall shouts back at 425Hz. What frequeny is heard at the helicopter?

    2. Relevant equations


    3. The attempt at a solution

    I'm stuck on finding the speed for the jumper. I'm sure it's simple but i just can't think of it for somereason.
    Last edited: Dec 6, 2013
  2. jcsd
  3. Dec 5, 2013 #2


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    Read the problem carefully. It is free-fall, the parachute is not yet opened . What is the speed of a falling body after 4 s?

  4. Dec 5, 2013 #3
    Last edited: Dec 5, 2013
  5. Dec 5, 2013 #4

    vy=Vyi-gt right?

    initial speed is 0 so its just -9.8*4=-39.2
  6. Dec 6, 2013 #5


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    Yes, it is the velocity if you consider upward positive.
    The parachuter shouts back. How is his sound heard on the helicopter? (Hm. The helicopter is very noisy --think it is a super helicopter making only low noise so the parachuter's sound can be heard :wink:)

  7. Dec 6, 2013 #6
    But it is strange that i get a higher frequency of what it should be. The answer is 381.4 and i keep getting 479. I'm getting my signs wrong somewhere...

    I used f'=f/(1+Vs/V) for a source moving away from reciever.
  8. Dec 6, 2013 #7


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    You need to calculate with the speed of moving away. The speed is positive, magnitude of velocity. Substitute Vs=39.2 m/s.

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