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Homework Help: Wave Changing Air Pressure (should be easy, help please!)

  1. Nov 27, 2005 #1
    . [Tipler5 15.P.049.] A sound wave in air produces a pressure variation given by the following formula, where p is in pascals, x is in meters, and t is in seconds.

    p(x,t) = 0.70 cos /2 (x - 330t)

    (a) Find the pressure amplitude of the sound wave.
    correct check mark Pa
    (b) Find the wavelength.
    wrong check mark m
    (c) Find the frequency.
    wrong check mark Hz
    (d) Find the speed.

    The only part i got is (a) which is obvious 0.70....

    i cant get the other ones and i only have 1 more go at them. I tried frequency = w=2(pi)f so w/2(pi) = f... but that didnt work... also 330 by itself didnt work? i tought the frequency in a wave was determined by the number before t? =\ Im really confused and i read over the book 3 times already and i cant figure it out? (theres only half a page on pressure and waves)

    Any help appreciated... thanks a lot!
  2. jcsd
  3. Nov 27, 2005 #2


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    Homework Helper

    you should edit your post so others know that it is "cos pi/2 ...".

    As in oscillations, the frequency is the reciprocal of the repeat time.
    cosine repeats each 2 pi ... if t = 0 at x=0, cos = 1
    if t = 1/330 [sec], at x=0 you have cos (0+ pi/2) = 0 .
    if t = 2/330 [sec], at x=0 you have cos (0+ pi) = -1 .
    If t = 4/330 [sec], at x=0 you have cos (0+ 2 pi) = 1 .
    . . . the repeat time is 4/330 .
    We do this "properly" by remembering the solution "cos (omega t)"
    which means that omega is (pi/2)330 .

    In a similar way, the wave number "k" is in solution "cos (kx)"
    Don't forget to distribute the pi/2 (that is, multiply thru by it).

    Speed is a different matter. There we only need the argument of the function,
    writing it so it looks like (x - vt)/lamda ... or as ... k (x - vt) .

  4. Nov 28, 2005 #3
    thanks a bunch. =)
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