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Calculate speed of tornado

  1. Jan 11, 2016 #1
    1. The problem statement, all variables and given/known data
    Some tornado researchers see a faraway tornado moving straight toward them. They see a bolt of lightning from the cloud above the tornado and 20 seconds later hear thunder. Exactly 40 seconds after seeing the first lightning , they sees second bolt and hear the sound thunder 18 seconds later. If the speed of sound in air is 340 m/s. The average speed of the tornado is nearly:

    17m/s
    34 m/s
    150 m/s
    310 m/s
    680m/s

    2. Relevant equations
    V=x/t

    3. The attempt at a solution

    We can say that the lightning and the tornado are moving together. So when lightning occur that's exactly where the storm is.
    So in first part, sound takes 20 seconds to reach. For a distance of 340*20= 6800 m
    Then there is a 20 second of no thunder or lightning.
    (I don't know what to do from here)
     
  2. jcsd
  3. Jan 11, 2016 #2

    HallsofIvy

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    Light from the storm reaches you almost instantaneously. The sound, traveling at 340 m/s arrives 20 s later so, as you say, the lightning must have occurred 340*20= 6800 meters away. Now do the same thing for the second lightning. How far from you was that second lightning? So how far did the storm move during those 40 seconds? How fast was it moving?
     
  4. Jan 11, 2016 #3

    JBA

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    The 40 seconds separation time and the two sound travel times are independent of each other. One indicates the time between the two strikes and the others are the distances from the strikes to you.
     
  5. Jan 11, 2016 #4
    In 18 seconds it moved 6120 m.
    So did the storm move 680 m?
    680/40= 17m/s

    Thanks :)
     
  6. Jan 11, 2016 #5

    JBA

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    That is not correct method. The you need to calculate the difference in the distance between the first strike and the second strike based upon the different times the sound traveled.
     
  7. Jan 11, 2016 #6
    Like how? Cause what I understood from your scentence is that it is the same as what I did
     
  8. Jan 11, 2016 #7

    Ray Vickson

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    You calculated the first distance (from your location to the tornado) as D1 = 340*20= 6800 m. Why can't you calculate the second distance D2 in a similar way?
     
  9. Jan 11, 2016 #8
    18*340=6120 m

    Then I subtracted 6800-6120= 680m
    to find the distance the tornado moved.
    What am I missing?
     
  10. Jan 11, 2016 #9

    Ray Vickson

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    You are missing the speed calculation for the tornado, which is what the question asked for.
     
  11. Jan 11, 2016 #10

    haruspex

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    That's correct. I think you may have confused people by your statement "it moved 6120 m." I don't think that's what you meant.
     
  12. Jan 11, 2016 #11

    HallsofIvy

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    In 18 seconds, the sound of the lightning flash moved 6120 m. Was that what you meant?

    Yes, at the first flash the storm was 6800 m from you. At the second flash the storm was 6120 m from you so in the 40 seconds between flashes it had moved 6800- 6120= 680 meters closer to you.

     
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