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Homework Help: Lightning question continued

  1. Sep 7, 2004 #1
    The speed of light is 3.0*10^8 m/s and the speed of sound is 340 m/s. Find the vaule of the interger n in the following statement:"If you start counting seconds when you see something happen and stop when you hear it happen, for every n seconds counted the event was about 1 km away."

    but 3 is not the exact answer is it?

    I'll show my calculation.

    time take light to travel 1000 m is 1000/(3 *10^8) = a

    Sound travelled a distance during that time 340 * a = 1/750 m

    So sound should reach there when:

    (1000-(1/750)/340 = 749 999/255000 s

    so each n = 749 999/255000s for each km

    shouldn't the integer be 749999 which happen at 255 km , unless 749999 is not prime...
  2. jcsd
  3. Sep 7, 2004 #2


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    The value of the inverse of light speed is so much lower than the one of sound that your important digits are never touched by it. (this is a cheat, I know, but it works for checking the answer)

    So ... for every second, the sound travels a third of kilometer. The number of second is approximately 3 times the distance in kilometers.
  4. Sep 8, 2004 #3
    Sound travelled a distance during that time 340 * a = 17/(15X10^3) m; then, n = 14999983 / 5100000 second for each km.
    n=2.9 s = 3 s.

    No, 3 s is not the exact anwer, the exact anwer is as above. but since the question comes with to find the integer n; then i round it to the closest integer.
  5. Sep 8, 2004 #4
    look at it once more, 340 * a where a is the time for light to travel 1000 m.
    And the question was kinda screwed up. "For every n seconds the event was 1 km away". It should've just said, The event happened 1000 km away during the n seconds. Because before, it sounded like we have to find an integer n where for every n seconds the event is 1 km farther.
    Last edited: Sep 8, 2004
  6. Sep 8, 2004 #5
    Sound travelled a distance during that time 340 * a =
    [tex]340*\frac{10^3}{3*10^8}=\frac{34*10^4}{3*10^8}\\ =\frac{34}{3*10^4}\\=\frac{17}{15000}[/tex]

    Yes, maybe we can say that the question is not clear enough because it makes two person to have two different understandings about it.
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