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Water wave physics problem

  1. Aug 19, 2009 #1
    1. The problem statement, all variables and given/known data
    In a ripple tank experiment, students generate water waves at a speed of 4.0 cm/s and a wavelength of 0.5 cm. If the waves are refracted into shallower water where their speed decreases to 3.0 cm/s, what is their new wavelength?


    2. Relevant equations



    3. The attempt at a solution
    i have no idea about anything here!!!!!!!please help?!
     
  2. jcsd
  3. Aug 19, 2009 #2

    jgens

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    Re: wavelength

    Well, one of the first places to start is with the equation relating the velocity of a wave, its frequency and its wavelength. Next, when the waves refract into shallower water and begin travelling slower, does this change the waves' wavelength, frequency, or both? Please provide your reasoning for these answers.
     
  4. Aug 19, 2009 #3
    Re: wavelength

    ooo so i would find the force using f=v/ λ, then plug that force and the new speed in to find the new wavelength. do i have to change everything from cm to m?
     
  5. Aug 19, 2009 #4

    jgens

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    Re: wavelength

    Alright, backtracking it looks like you know the equation:

    [tex]v = \lambda \nu[/tex]

    Where [itex]v[/itex] is the velocity, [itex]\lambda[/itex] is the wavelength, and [itex]\nu[/itex] is the frequency (be careful not to get the force and frequency mixed up, they are two very different things).

    Now, it also looks like you realized that when the waves refract, this does nothing to the frequency. Using, this frequency you can calculate exactly what you need! I think that you can answer your question about unit conversions if you give it enough thought (look at the mathematical expression you're dealing with and make sure units cancel).
     
  6. Aug 19, 2009 #5
    Re: wavelength

    ok, so if the units for frequency is Hz and thats what im finding, it gives me T because the meters cancel. so would i go 1/T to get the frequency then plug that number in to get the actual new wavelength?
     
  7. Aug 19, 2009 #6

    jgens

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    Re: wavelength

    Why don't you write out what you're thinking mathematically and I'll check that ;-). It's a bit difficult to follow what you wrote.
     
  8. Aug 19, 2009 #7
    Re: wavelength

    F= v/ λ
    F=0.04m/s/0.005m
    F=8s----->frequency isnt measured in s, its measured in Hz. so..
    F=1/T
    F=1/8s
    f=0.125Hz
    λ=v/f
    λ= 0.03m/s/0.125hz
    =0.24m

    or do i just keep the f in seconds and do this instead.
    F= v/ λ
    F=0.04m/s/0.005m
    F=8
    λ=v/f
    λ= 0.03m/s/8
    λ=3.8 x 10^-3m
     
  9. Aug 19, 2009 #8

    jgens

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    Re: wavelength

    Okay, it looks like you're confused in a couple of places . . .

    First, you need to be careful with your math, one of your manipulations with the units essentially amounts to:

    [tex]\frac{m}{s}*\frac{1}{m} = s[/tex]

    Hopefully you can understand why that's incorrect. Second, the equation

    [tex]\nu = \frac{v}{\lambda}[/tex]

    Gives you the frequency of the wave, not the period! It's very important to get these concepts correct. Try again and we'll see where it goes from there.
     
  10. Aug 19, 2009 #9
    Re: wavelength

    from what your telling me.. im not sure what is wrong with the first way? cuz i did change the frequency into the period
     
  11. Aug 19, 2009 #10

    jgens

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    Re: wavelength

    Here's what I mean more explicitly . . .

    [tex]\nu = \frac{v}{\lambda} = \frac{4 \mathrm{cm/s}}{0.5 \mathrm{cm}} = 8 \frac{1}{\mathrm{s}} = 8 \mathrm{Hz} \neq 8 \mathrm{s}[/tex]
     
  12. Aug 19, 2009 #11
    Re: wavelength

    i knwo that 8 hz doesnt equal 8 seconds, which is why i went f=1/t, f=1/8 which gives me Hz does it not?
     
  13. Aug 19, 2009 #12

    jgens

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    Re: wavelength

    Okay, you're still confused. The frequency is measured in Hertz and the period is measured in seconds. The equation that you're using gives you the frequency, not the period. It can, however, be easily modified to obtain the period. To check that you have the right frequency, find the product of the wavelength and the frequency (if this doesn't give you the speed, you've done something wrong). Does this clear things up? As an aside, is this algebraic manipulation okay with you?

    [tex]\frac{a}{b} * \frac{1}{a} = b[/tex]
     
  14. Aug 19, 2009 #13
    Re: wavelength

    ok, so when i tried solving the problem, what part did i do wrong.

    and yes, i understand that. the a's are cancelling eachother out
     
  15. Aug 19, 2009 #14

    jgens

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    Re: wavelength

    Well, the part that you did wrong is that after you calculated the frequency, you went on and calculated the period and called that the frequency. Seriously, find the product of the wavelength and frequency and if it doesn't give you the velocity then your frequency is incorrect. I'll do this for you using the values you calculated:

    [tex]0.125 \mathrm{Hz} * 0.5 \mathrm{cm} = 0.0625 \frac{\mathrm{cm}}{\mathrm{s}}[/tex]

    You should note that this isn't the velocity specified in the problem. Hopefully you wouldn't have understood that "algebraic" manipulation because it was completely fallacious. It should be equal to 1/b, not b.
     
  16. Aug 19, 2009 #15
    Re: wavelength

    ok so 8hz * 0.5cm does work out to the right speed. (4). in that case what is wrong with the second way i tried.
     
  17. Aug 19, 2009 #16

    jgens

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    Re: wavelength

    What's wrong with the second way is that it still seems that you're confused about the distinction between the period and frequency since you said:

    Try to present your solution again, this time using the correct names for concepts.
     
  18. Aug 19, 2009 #17
    Re: wavelength

    k just look at this part of the equation.
    1)F= v/ λ
    2)F=0.04m/s/0.005m
    3)F=8
    ----> m/s *1/m. so since the m's cancel out, were left with 1/s which is Hz right? because if u plugged a number in fr seconds it would be like the equation, f=1/T.

    4)Λ=v/f
    5)Λ= 0.03m/s/8
    6)=3.8 x 10-3
    in step 5 im plugging in the new speed and the found frequency
     
  19. Aug 19, 2009 #18

    jgens

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    Re: wavelength

    Yes, your work looks correct. You're correct about the meters cancelling and leaving you with 1/s which is the same unit as Hz. Also, you don't need to convert everything into meters - leaving everything in terms of centimeters works just fine. Good job!
     
  20. Aug 19, 2009 #19
    Re: wavelength

    phew!! thank you so much for your time. :)
     
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