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Science Forum

  1. Standard member Ramned
    The Rams
    04 Mar '08 23:34
    A bullet is fired from a powerful gun 5 meters above the head of an observer who is 150 m away. The observer hears 2 loud "cracks."

    A) Why does the observer hear 2 cracks?
    B) Which of the two cracks does the observer hear first? Why?
  2. 05 Mar '08 08:29
    Originally posted by Ramned
    A bullet is fired from a powerful gun 5 meters above the head of an observer who is 150 m away. The observer hears 2 loud "cracks."

    A) Why does the observer hear 2 cracks?
    B) Which of the two cracks does the observer hear first? Why?
    The bullet traveled faster than sound.
    As the bullet leaves the gun it emits a 'crack' lets call it A.
    As the bullet is traveling, it slows down to less than the speed of sound, and thus emits a 'sonic boom', call it B.
    The observer will hear B first.
  3. 05 Mar '08 10:29
    Originally posted by twhitehead
    The observer will hear B first.
    On the second point - it is probably possible, if the initial velocity of the bullet was high enough, that the sonic boom caused by the deceleration through the speed of sound could happen some time after it had passed the observer. In which case you might hear it later.
  4. 05 Mar '08 10:50
    Originally posted by mtthw
    On the second point - it is probably possible, if the initial velocity of the bullet was high enough, that the sonic boom caused by the deceleration through the speed of sound could happen some time after it had passed the observer. In which case you might hear it later.
    Good point, I didn't think of that. Maybe we should fire the gun directly at the observer so that we can be certain.
  5. 05 Mar '08 15:45
    Originally posted by twhitehead
    The bullet traveled faster than sound.
    As the bullet leaves the gun it emits a 'crack' lets call it A.
    As the bullet is traveling, it slows down to less than the speed of sound, and thus emits a 'sonic boom', call it B.
    The observer will hear B first.
    I think that the observer can hear A first, B first, or both of them at the same time - depending on the observer's position relatively to the bullet's trajectory.