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Hearing sound 3 seconds late

  1. Aug 31, 2008 #1
    I was at tattershall lakes the other day and a skier went over a jump, about 3 seconds later after he landed you'd hear the slap of his skis hitting the water.

    I want to know why? I know that light travels faster than sound, but we wasn't very far away, not far enough for it to make that big-a difference - at least, I don't think so.

    Sorry if this is a stupid question :3
     
  2. jcsd
  3. Aug 31, 2008 #2

    Doc Al

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    How far away was he? Sound travels about 340 m/s--about a mile every 5 seconds.
     
  4. Sep 1, 2008 #3
    We was about 80m away from where he was landing. I guess I can try and work out how long it'd take for the sound to reach me using my GCSE physics :P

    D = S/T? So T = D/S? So it'd take about ~0.24 seconds for the sound to reach me?
    and it'd take ~2.7 x 10^-7 for the light to reach me :3 That's not 3 seconds difference?

    I'm probably wrong with that, I was just using a formula from class.
     
  5. Sep 1, 2008 #4

    Doc Al

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    Your calculations look OK to me. Perhaps you were hearing an echo.
     
  6. Sep 1, 2008 #5
    Why would I not hear the initial splash?
     
  7. Sep 1, 2008 #6

    Doc Al

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    Beats me.
     
  8. Sep 1, 2008 #7

    uart

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    The other possibility is that you are hearing something other than the ski landing, and incorrectly attributing it to the ski slapping the water.

    For example a ski-jumper will normally land with the rope slack, but within a few seconds the boat must take up the slack and resume towing the skier before he sinks. You might for example be hearing the sound of the rope slapping as it regains tension.
     
  9. Sep 1, 2008 #8
    The speed of sound in air depends on the temperature. The 331 m/s is for a temperature of 0 degrees C. The formula to adjust for temperature is V = 331.4m/s+0.61m(s.˚c)t.
    So that will likely make your time even shorter.

    However when a sound wave passes from one medium into another of different density, its speed is changed. It can bend the direction of the wave and is called refraction. Around a lake you often get a difference in temperature between the water and land. The water takes longer to heat up in the day but also cools slower at night. Also while heating land the air next to the surface heats first and is warmer than air at higher levels, which bends the sound wave up. At night the wave is bent downward and you can often hear people out in the middle of the lake or on the opposite shore as if they beside you.

    The terrain where you are at makes a difference too, but it is possible that the front of the sound wave was bent and went overtop of you
     
  10. Sep 1, 2008 #9
    I was standing on the jetty, and behind me was a small hill.

    uart's theory sounds reasonable, I wouldn't be surprised if it was that although, when I was knee boarding I got quiet alot of slack at one point but I didn't here a crack when the rope was pulled tight again.
     
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