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Homework Help: Pendulum Problem?

  1. Dec 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A cord of mass .65 kg is stretched between two supports 28 m apart. If the tension in the cord is 150 N, how long will it take a pulse to travel from one support to the other?


    2. Relevant equations
    umm... the only equation to find helpful was...

    T = 2pi[tex]\sqrt{m/(mg/L)}[/tex] = 2pi[tex]\sqrt{m/(F/L)}[/tex]?


    3. The attempt at a solution

    Knowns:

    F (tension) = 150 N
    m = .65 kg
    L = 28 m

    2pi[tex]\sqrt{.65 kg/(150 N/28 m)}[/tex] = 2.19 s?

    Though the book says it is 35 s...

    Help? Should there be another equation that I should be using or did I miss something?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Dec 4, 2008 #2

    alphysicist

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    Homework Helper

    Hi Kandycat,

    This is not a pendulum, so these equations do not apply here. In this problem, a cord is stretched and a pulse is made to travel along the cord, so you want to find an expression for the wave speed of a stretched string. What would that be?
     
  4. Dec 4, 2008 #3
    Oh... so is it v = [tex]\sqrt{F(t)/(m/L)}[/tex]?

    v = [tex]\sqrt{150 N/(.65 kg/ 28 m)}[/tex] = 80.35 m/s

    T = L/v = 28 m/80.35 m/s = .35 s

    Thanks!

    Sorry about that.
     
  5. Dec 4, 2008 #4

    alphysicist

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    Homework Helper

    That looks right to me!
     
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