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Homework Help: Where'd I go wrong (sound problems)

  1. Jan 8, 2005 #1
    A mass of 5 kg hangs from a cord around a light pulley. The length between the vibrator and the pulley is 2 m. (A) When the vibrator is set to a frequency of 150 hz, a standing wave with 6 loops is formed. What must be the linear mass density of the cord?

    First I figured the wave length was 2/3 m because there were 6 loops over 2 m

    The Wavelength is equal to velocity over frequency

    2/3 = V/150
    V=100 M/s

    V = the squareroot of Ft (force of tension in the rope) divided by the mass of length

    The Squareroot of Ft/(m/L) = V
    Ft = 49
    m=desired variable

    I got .0033 kg for the answer when I plugged this all in. The correct answer is .0049. Thanks for the help
  2. jcsd
  3. Jan 8, 2005 #2
    It looks good to me... Are you sure the "correct" answer is actually correct?
  4. Jan 8, 2005 #3


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    Staff Emeritus
    Science Advisor
    Gold Member

    No, read the question again. m is NOT the desired variable, it is m/L that you are required to determine. You would have noticed this if you looked at the units of the solution, kg/m (not kg).

    What you've done so far is correct, but now you just have to find the linear density, [itex]\rho = m/L [/itex] from the equation :
    [tex]\sqrt{{\frac{49}{\rho}}} = 100 [/tex]
  5. Jan 9, 2005 #4
    Ahhh, thank you, that makes sense
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