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Homework Help: Wave in a string question

  1. Aug 12, 2008 #1
    1. The problem statement, all variables and given/known data

    "A uniform rope with length L and mass m is held at one end and whirled in a horizontal circle with angular velocity w. You can ignore the force of gravity on the rope. Find the time required for a transerve wave to travel from one end of the rope to the other."

    2. Relevant equations

    w = 2*pi*f
    wave speed = frequency*wavelength = sqrt(tension/mass per unit length)

    3. The attempt at a solution

    I'm a bit confused here. If the force of gravity on the rope is being ignored, then what is the tension in the rope?
  2. jcsd
  3. Aug 12, 2008 #2


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    Hi kidsmoker,

    Think about what has to be true for the mass to travel in a circular path.
  4. Aug 12, 2008 #3
    There's a force mw^2A acting towards the centre? Do I take this as the tension in the string? I'm still not sure what to do sorry...
    Last edited: Aug 12, 2008
  5. Aug 12, 2008 #4


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    What is "A" in your equation?
  6. Aug 12, 2008 #5
    Sorry, the radius of the circular motion. I called it A cos it's like the amplitude.
  7. Aug 12, 2008 #6


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    In your problem that is actually "L" and yes that is the centripetal force on the string which is your Tension that you are looking for.
  8. Aug 12, 2008 #7
    Are you sure? Why would it be equal to the length of the string?
  9. Aug 12, 2008 #8


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    Perhaps this will help?

    Wait a minute is there a mass at the end of the rope or is the rope itself mass M total?

    Maybe I am misreading the problem?
    Last edited: Aug 12, 2008
  10. Aug 27, 2008 #9
    I ws completely misreading the question too. I thought that the string was hanging vertically and you were whirling the top end in a circle, so that these circular waves travel down the string. I realise now that it means that the whole string is horizontal and it's being whirled in a circle like this, so as to keep it taut. In that case the problem is easy :P

    Thanks anyway!
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