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Traverse E wave travel Time on wires

  1. Nov 18, 2008 #1
    1. The problem statement, all variables and given/known data
    A 30.0m steel wire and a 20.0m copper wire, both with 1.00mm diameter,
    are connected end to end and stretched to a Tension of 150N. How
    long does it take a transverse e wave to travel the entire length of
    the two wires?

    2. Relevant equations
    Voulume of Rod = pie r^2(L)
    v = sqrrt T/mass per unit length
    v = dt/dx

    3. The attempt at a solution
    I found the volume of the steel rod to be pie(.0001m)^2(30.0m) = 9.42x10^-7 m^3
    I found the volume of the copper rod to be pie(.0001m)^2(20.0m) = 6.42x10^-7 m^3

    I looked up the density of steel to be 7850 kg/m^3
    I looked up the density of copper to be 8230 kg/m^3

    The weight of steel rod should be 9.42x10^-7 m^3/7850 kg/m^3 = 1.2 x 10^-10 kg
    Then divide by 30.0m = 4.0x10^-10 kg/m

    The weight of copper rod should be 6.28 x 10^-7 m^3/8230 kg/m^3 = 7.63 x 10^-11 kg
    Then divide by 20.0m = 3.82 x 10^-12 kg/m

    Using V = sqrrt T/mass unit of length

    Steel wire is sqrrt 150N/4.0 x 10^-12 kg/m = velocity of steel = 6.12 x 10^6 m/s
    Copper wire is sqrrt 150N/3.82 x 10^-12 kg/m = velocity of copper = 3.93 x 10^13 m/s

    I added them together amd divided by 2 to get an average of 1.97 x 10^13 m/s
    across the whole wire. Then divided that number by 50.0m to get 3.93 x 10^11s

    I know that isn't right as the answer is .329s
    I'm not sure what I'm doing wrong. Any help would be appreciated.
    Thanks,
    Kevin
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Nov 20, 2008 #2

    Redbelly98

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    Staff Emeritus
    Science Advisor
    Homework Helper

    That "shortcut" of averaging the two velocities is a common misconception, but wrong. Even if the wires were the same length!

    How long does it take the wave to travel down the steel part?
    How long does it take the wave to travel down the copper part?
     
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