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Homework Help: Doppler effect homework problem help

  1. Apr 9, 2007 #1
    Doppler effect homework problem help (still lost)

    1. The problem statement, all variables and given/known data

    A block with a speaker bolted to it is connected to a spring having spring constant k = 17.0 N/m, as in Figure P17.40. The total mass of the block and speaker is 44.00 kg, and the amplitude of this unit's motion is 0.475 m. Assume that the speed of sound is 343 m/s.

    (a) If the speaker emits sound waves of frequency 420 Hz, determine the highest and lowest frequencies heard by the person to the right of the speaker to the nearest 0.01 Hz.

    2. Relevant equations
    d = mg/k
    vs = gt
    [v/(v (+ or -) vs)]*f

    3. The attempt at a solution

    When my friend and I were working on this, here's what we did:
    Got variables:
    k = 17N/m
    mass = 44kg
    amplitude = .475m
    v = 343m/s
    freq = 420Hz
    I = 1000Hz

    Our way of solving was using k = mg/d and solving for d.
    d = mg/17
    d = 25.36 = 1/2 * 9.8 * t^2
    t = 2.28

    From here, we used the formula [v/(v (+ or -) vs)]*f.

    My answers are 449.27 Hz and 394.36 Hz
    Now, here's the problem, our answers end up being within 10% but we have no idea what we did wrong and any ideas as to how to get the correct answer from here. Thanks for any suggestions.
    Last edited: Apr 10, 2007
  2. jcsd
  3. Apr 9, 2007 #2
    Is there something I'm missing that is causing my answer to be about 10% off?
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