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Homework Help: Can anyone show me how to work this problem out?

  1. Sep 7, 2004 #1
    Can anyone show me how to work this problem out???

    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."
  2. jcsd
  3. Sep 7, 2004 #2

    Doc Al

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    Staff: Mentor

    Welcome to PF!

    Here's a hint: for all practical purposes you can treat the light as arriving instantly compared to the sound. So... how long does it take the sound to travel 1 km?
  4. Sep 7, 2004 #3
    I think doc al gave it away. :approve:
  5. Sep 7, 2004 #4
    [tex]\Delta t=t_{stop}-t_{start}[/tex]
    [tex]\Delta t=s[\frac{1}{v_{sound}}-\frac{1}{v_{light}}][/tex]
    For s = 1 km,
    [tex]\Delta t=2.9s=3s[/tex]
    n=3 s

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  6. Sep 7, 2004 #5
    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...
    Last edited: Sep 7, 2004
  7. Sep 8, 2004 #6

    Doc Al

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    Staff: Mentor

    But neither is 3 x 10^8 m/s the exact speed of light, nor 340 m/s the exact speed of sound.
    I think you've missed the point of this simple exercise. :wink:
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