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Sound and supersonic motion - a question from Feynman's lectures

  1. Jun 26, 2013 #1
    Sound and supersonic motion -- a question from Feynman's lectures

    In chapter 51, volume 1 of the FLP, Feynman writes
    Unfortunately he didn't give a justification for this claim. I was hoping someone in Physics Forums could fill this gap in.
     
  2. jcsd
  3. Jun 26, 2013 #2

    russ_watters

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    I don't like it at all. Sound is just disturbances in the medium and a moving object most certainly disturbs the medium at subsonic speed.
     
  4. Jun 26, 2013 #3
    Presumably he had in mind sound waves. (This is clear from the context the quote appears in). Subsonic objects definitely disturb the air around them but they don't create sound waves like supersonic objects.
     
  5. Jun 26, 2013 #4

    russ_watters

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    Sound waves ARE disturbances in the air. The sonic boom shock wave is just the piling-up of all the disturbances on top of each other.
     
  6. Jun 26, 2013 #5

    boneh3ad

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    He is alluding to the presence of shock waves, which are essentially a form of sound waves that result simply from an object moving through a gas at supersonic speeds.
     
  7. Jun 26, 2013 #6

    russ_watters

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    Yes, but he seems to be implying that objects moving at subsonic speed don't also "generate waves on each side".
     
  8. Jun 26, 2013 #7
    Ok so the only difference between subsonic and supersonic motion is that the small air disturbances created by the motion reinforce each other to form a shock wave in supersonic flight, but don't in subsonic flight?
     
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