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Homework Help: Wave problem

  1. Nov 21, 2007 #1
    [SOLVED] Wave problem

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

    a small stone fell into alake causing 40 waves after 4 seconeds, if the radius of the outter circle is 1.6 meters, find the length of the occurred wave and its velocity

    2. Relevant equations

    3. The attempt at a solution

    well i got two different solutions so far cause i couldn't determine what exactly was meant by (r=1.6m & length of the occurred wave):

    1- f = 10 Hz

    lambda/4 = 1.6
    lambda = 6.4 m

    v = lambda.f
    = 6.4 × 10
    = 64 m/s


    2- v = x/t

    x = 2pi.r
    = 10 m

    v = 10/4
    = 2.5 m/s (oO)
  2. jcsd
  3. Nov 21, 2007 #2

    Doc Al

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    Staff: Mentor

    Neither approach is good. Instead, picture what's going on. The waves travel outward from the center. The outer radius tells you how far the first wave traveled in the given time. Use that to find the speed of the wave.

    Also, you have 40 waves (and thus 40 wavelengths) stretched out over that radius. How big is each wavelength if 40 of them add to the outer radius?
  4. Nov 22, 2007 #3
    aha! so:

    v = 1.6/4
    = 0.4 m/s


    lambda = 0.4/10
    = 25 m

    i hope i got at least closer ^^
  5. Nov 22, 2007 #4

    Doc Al

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    Staff: Mentor


    Not good. If 40 wavelengths span 1.6 m, how can each wavelength be 25 m?
  6. Nov 22, 2007 #5
    I don't think i get it :confused:

    is it that i reversed the final result 25->0.04 (this happens all the time and its really annoying) or i simply skipped a step that i cant figure?
  7. Nov 23, 2007 #6

    Doc Al

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    Staff: Mentor

    That must have been what you did. You wrote .4/10 (which is correct), but you reversed the answer.

    What I would have done is this:
    40 Wavelengths = 1.6 m
    1 Wavelength = (1.6 m)/40 = 0.04 m
  8. Nov 23, 2007 #7
    I knew it :rofl:

    the problem was that i haven't known that the wavelengths make out the radius as this is my first encouter with circular waves

    anyways thanx alot Doc Al cya :cool:
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