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Homework Help: Time on watch dependent on air temperature?

  1. Mar 30, 2010 #1
    1. The problem statement, all variables and given/known data
    Can anyone give me a hand on how to start this? I would attempt a solution but I've tried over and over and can't seem to get anywhere.

    A man sets his watch at noon by the sound of a factory whistle 3 miles away. If the temperature of the air is 20-degrees C, how many seconds slow will his watch be by the time at the factory?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 30, 2010 #2

    DaveC426913

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    How fast does sound travel through air at 20C?
     
  4. Apr 1, 2010 #3

    Still a little lost here:

    In general, at 68 F (20 C) sound travels at 1130 ft/sec (344.4 m/sec.)
    3 miles = 15 840 feet
    3 miles = 4 828.032 meters

    but not sure how to put it together in equation format?
     
  5. Apr 1, 2010 #4

    DaveC426913

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    Really?

    You've got a velocity and a distance; you're being asked to solve for time.

    You say you're tried over and over. Show your attempts.
     
  6. Apr 1, 2010 #5
    T=V/D =
    T=15840 ft/ 1130ft/s = 14.018 seconds.
     
  7. Apr 1, 2010 #6

    DaveC426913

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    Does that sound about right? What does your intuition tell you? Think of thunder & lightning.
     
  8. Apr 1, 2010 #7
    I did find something that used c = d / Δ t - where c was speed of sound so
    Δ t = d / c = 15,840/344.4 = 46 seconds but that seems too long a delay for three miles no?

    As far as thunder and lightning, lightning strikes first b/c it travels faster in air than sound. The speed of a sound wave in air depends upon the properties of the air, namely the temperature and the pressure so it makes sense for
    Time=distance/speed =
    Time=15,840 ft/1130 ft/sec.
    14.018 sec

    but i'm still second-guessing it.
     
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