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Homework Help: Simple Harmonic Motion - Conflicting equations!

  1. Mar 10, 2010 #1
    1. The problem statement, all variables and given/known data
    What is the equation for the displacement of a sin wave (eg something in SHM)?

    My book says x(t) = Acos(ωt + ɸ0)

    My prof's notes say y(x,t) = Asin(ωt - kx + ɸ0)

    2. Relevant equations

    3. The attempt at a solution
    I'm not confused about why one is sin and the other is cos. I'm confused why one has the extra term -kx. When do you use which?

    I have a question that asks me to write the displacement for a sine wave travelling to the left with frequency 200 Hz, speed 400 m/s, amplitude 0.010 m, and initial phase 90 degrees. So I'm not sure which of the above to use.
  2. jcsd
  3. Mar 10, 2010 #2


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    Homework Helper

    First one is the general equation of SHM.
    The second one the equation of the sine wave propagating in a medium towards the positive x direction.
    You have to use the second equation to find the displacement with a small modification.
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