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Homework Help: Am I being punked? (Physics problem)

  1. Oct 13, 2004 #1
    This was problem 7 in the homework from the 2nd chapter(Kinematics in one dimension) of my physics book:

    According to a rule of thumb, every five seconds between a lightening flash and the following thunder gives the distance of the storm in miles. Assuming the flash of light arrives in essentially no time at all, estimate the speed of sound in m/s from this rule.

    The answer was 300 m/s. I figure I would need speed of light and sound but the speed of sound isn't anywhere in this book. Wouldn't I need to know how many miles would equal 5 seconds? I need help, thanks
  2. jcsd
  3. Oct 13, 2004 #2
    arent they saying that it takes 5 seconds per mile for the sound to get there? how they said every five seconds between them gives the distance in miles.... so that would be like saying every 5 seconds is one mile.... i think
  4. Oct 13, 2004 #3
    If a mile is 5280 feet and sound takes 5 seconds to travel that far then it travels about 1000 feet per second. The speed of light is about 186,000 miles per second so for all intents it is instantaneous.

    The actual speed of sound is a function of temperature and pressure (altitude) but it doesn't change all that much. To understand a concept a first order answer is more than adequate.

    Pay attention in your physics class as it is the study of the physical world and will be of value to you all your life. That is more than I can say of many classes.

    Good luck
  5. Oct 13, 2004 #4
    Speed of sound

    If you're not given an air temperature, you can just use 331 m/sec as the speed of sound.
  6. Oct 13, 2004 #5

    Chi Meson

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    The speed of sound was supposed to be estimated. THis means you do not need the speed of sound to begin with.

    Every track star knows that the "metric mile" is 1500 meters. (This is actually .92 miles, but it's close). If 1500 meters in 5 seconds, then how fast?
  7. Oct 13, 2004 #6
    300 m/s

    So for the problem, every 5 seconds is 1 mile. 1 mile = 1.61 km = 1610 meters. That's 322 meters per second which is about 300 m/s. Whoa DUDES! Thanks a lot, I'm on the road to my noble prize!!!!
  8. Oct 13, 2004 #7
    You mean Nobel Prize? :tongue2:
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