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Sound and wave vibration help

  1. May 20, 2003 #1
    PHYSICS help plz!!!! Due 2morrow!!!

    I'm learning about vibration and wave and sound. And I have to explain and solve this problem below, can someone plz help me out...Thanks
    You watch a distant farmer driving a stake into the ground w/ a sledgehammer. he hits the stake at a regular rate of one stroke per second. You hear the sound of the blows exactly synchronixed with e blows u see. And then you hear one more blow after u see him stop hammering. How far away is the farmer? (speed of sound is 330m/s)
  2. jcsd
  3. May 20, 2003 #2
    Wow, if questions were always this easy...

    ...I might have been a physicist.

    Hello miyuki9,
    Just think about it for one second and maybe it will come to you...
  4. May 20, 2003 #3
    is it 330m? Is that right?
  5. May 20, 2003 #4
    I think it is better to understand the process and make a simple mistake that gives a wrong answer than it is to have the correct answer and misunderstand how you got it.

    Can you explain your thinking to arrive at that figure?
  6. May 20, 2003 #5
    b/c u see and hear the sound @ the same time, according to the problem, so I take 330m/s * 1s = 330m.
    if I'm wrong or misunderstood, Plz explain and show me how to do it plz!!
  7. May 20, 2003 #6
    You have done fine!
    You were given the exact time between hammer-blows (1 second), but the most important hint is that you hear the sound 1-sec after the last blow. That means it must take 1-sec for the sound to travel the distance from the hammer to your ears. Knowing how fast sound travels lets you set up the problem just as you have done; (330m/s)(1s)
    You end up with ‘s’ in both the numerator and denominator of the equation where they cancel out (to 1), leaving you with 330m as the distance.

    Pretty cool, huh?
    Last edited by a moderator: May 20, 2003
  8. May 20, 2003 #7
    You are correct. You are hearing each hit exactly one second after it happens. Thus because you hear it at 300m/s, he must be one second-unit of sound away. Thus 330m

    I think the physics people here, me included, don't like to just give an answer, as it doesn't promote the learning process. I agree.

    So, If this farmer was at the same distance from you. But there was a small wall in front of you, and an infinite one behind you, so the sound goes from farmer to large wall and back to you. And the distance of the large wall from you another 660m.

    What is the time change between the sound for the farmer and the time you hear it?

    How many hits will you hear after the farmer stops hitting?

    HARDPART: At what speed would the farmer have to run from where he is hitting to you, in order to hear the last hit at your location? heh, answer it and you get a cyber-kiss.
  9. May 20, 2003 #8


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    Brrr!!! Cold day.
  10. May 20, 2003 #9
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