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Homework Help: Tension of String/ Mass question

  1. Nov 7, 2007 #1
    1. The problem statement, all variables and given/known data
    The figure shows two masses hanging from a steel wire. The mass of the wire is 60.0 g. A wave pulse travels along the wire from point 1 to point 2 in 24.0 ms.

    What is mass m?

    2. Relevant equations
    v = sqrt(Ts/mu)

    3. The attempt at a solution

    t = 0.024s
    d = 4m
    v = d/t = 166.666m/s

    mu = m/L = 60g/8m = 7.5

    T = 208331Nsin40 = mg
    m = 208331Nsin40/g
    m = 13.66

    but the answer is 17.8kg. where did i go wrong?
  2. jcsd
  3. Nov 8, 2007 #2


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    the tension that you get from


    (I get 209 N - it is 60 grams) is the x-component of the tension in the inclined part of the wire.
  4. Nov 8, 2007 #3


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    Convert 60 g to kg and try again.
  5. Nov 8, 2007 #4


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    By using Lami's theorem You can find tension across point 1 and 2.
    T/cos40 = mg/sin40. Hence T = mg*cot40.
    m = (7.5^-3*166.67^2)/9.8*cot40
    = 17.8kg.
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