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Homework Help: Mechanical Waves - Power

  1. Dec 1, 2007 #1
    1. The problem statement, all variables and given/known data
    A piano wire with mass 3.25 g and length 84.0 cm is stretched with a tension of 27.0 N. A wave with frequency 100 Hz and amplitude 1.30 mm travels along the wire.

    2. Relevant equations
    The only equation that I think applies to this one is P = .5*Sqrt(μ*F)*(omega^2)*A^2

    3. The attempt at a solution
    I've tried this freaking problem five times now, and I'm down to my last try on Mastering Physics (online homework program).
    I thought this was a pretty straightforward problem. I started out solving for μ by basically converting the mass (3.25g) to kg and the length (84cm) to m and then dividing the mass by the length. I solved for omega with the equation omega = frequency * 2PI. F I'm assuming is the tension.

    Then I thought it was simply a plug-in problem, but I can't seem to get it right. I had to do five different trials because at first I solved for μ wrong and then I wasn't sure what units the program wanted it in (just had Watts next to the solution, so I just assumed i needed to put everything in to kg and m and all that stuff). But yea, I don't know what I'm doing wrong.

    If anyone can help me out on this one I'd appreciate it, just can't seem to figure out what went wrong

    thanks in advance
  2. jcsd
  3. Dec 1, 2007 #2
    What's the question?
  4. Dec 1, 2007 #3
    woops...yea I'm an idiot
    The question is:
    Calculate the average power carried by the wave.
  5. Dec 1, 2007 #4
    what answers have you tried
  6. Dec 2, 2007 #5
    i have the exact same question on my Mastering Physics homework, and I just figured it out. You use that equation and just plug everything in, but the units must be converted for almost everything. So the mass must be in kg, length must be in meters, tension is in N already, frequency is in Hz already, but amplitude must be in meters (not mm). Once you convert everything and plug it into the equation, you get the right answer (mine was about 0.3 W, my numbers may be different than yours though!) For the part B, you just do (1/2) of the amplitude when plugging it in!

    Good luck!
  7. Mar 4, 2010 #6
    Clarification??? what did you change the formula into?
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