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Homework Help: Help with physics wave problem

  1. Jan 16, 2005 #1
    Hello everyone! I need help with some problems my physics teacher gave us. The problems I did not show any work for are the ones I have no clue how to do. I would just like a hint or something to get me started (I don't want you to do the whole problem for my because I need to know how to do it myself). I would like to know what I did wrong for the ones I showed work for but I don't want you to do the whole thing. The work is either typed or written. The correct answers are circled or typed. I'm sorry this is so long.

    5.) http://img117.exs.cx/img117/6494/number51oa.jpg

    7.) http://img117.exs.cx/img117/6541/number72mb.jpg

    11.) http://img117.exs.cx/img117/8550/number110wo.jpg

    13.) http://img120.exs.cx/img120/6853/number132rd.jpg

    14.) http://img120.exs.cx/img120/3610/number14pt12qr.jpg
    The answer is: A. pi

    15.) http://img120.exs.cx/img120/2436/number153bq.jpg

    16.) http://img120.exs.cx/img120/3938/number168gf.jpg
    The answer is: C. 120

    f= 1/(2Lv)

    19-22.) I don't know why I scanned 19. I don't need help with 19.

    20 work:
    f= 1/(2Lv)
    so why is the answer .25?

    The answer for number 21 is: B. pi


    The answer is: D. L/8

    24 - 27.) http://img149.exs.cx/img149/1576/number24279ia.jpg

    For number 24 why is the answer not f= 1/(2Lv)?

    30 - 32.) http://img149.exs.cx/img149/2849/number30329am.jpg

    How did they get the -10 I number 30? Would you integrate to get the answer? If so how did they get the -10?

    For number 31 would I use the formula v=sqrt(T/u)? If so how do I get the values of T and u??

    In number 32, I got (400,0) as a point on the graph. So it this why the answer is 4?

    Is the book wrong for this answer?

    Any help would be greatly appreciated.
    Last edited: Jan 16, 2005
  2. jcsd
  3. Jan 17, 2005 #2


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    Boy, does this look scary.

    5) I don't get why you write f=168 and then pick one of the choices that does not include this number. :confused:

    The condition for resonance is that the far (closed) end of the pipe have a node of the wave.

    So, for any integer n, you have resonance when [tex] L = \frac {n \lambda}{2} [/tex]

    Now use the relation for the speed of sound, and after plugging in numbers you should get something that looks like f = nF, where F is a number you get from the calculations. This implies that all frequencies that are multiples of this number F, will result in resonance. Now from the choices, find the set that has only multiples of this F.
  4. Jan 17, 2005 #3


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    7) I don't get how you did this. You've written f = 520(2/340). This is not correct. Also, you've written Vs = 340. That's not correct either. Check your text to see what each of the symbols means in the doppler formula.

    The wall is acting like a mirror, bouncing back the sound to the flautist. A clever way to solve this problem is to actually treat the wall as a mirror, and treat the image (on the other side of the mirror) as the observer. So, if the man is moving at 2 m/s towards the mirror, what will be the speed of his image, and in which direction will it travel ? Now you can simply forget about the mirror. The man is the source, and his image is the observer. This is a direct substitution problem now.
  5. Jan 17, 2005 #4


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    11) Still can't see how you're doing this. What's the formula for the speed of a wave in a stretched string?
  6. Jan 17, 2005 #5


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    13) This is just one step more than the previous problem. Having found the speed, you can find the frequency if you know the wavelength of the first harmonic. What must the wavelength be ?
  7. Jan 17, 2005 #6
    When I first did the problem I got 200.

    v= sqrt(T/u)
    v= sqrt(600/.015)

    but the answer is 632
  8. Jan 17, 2005 #7


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    Check the units on the linear density (mass per unit length) [itex]\mu [/itex]
  9. Jan 17, 2005 #8
    ok i know what i was doing wrong now. Thanks :smile:
    Last edited: Jan 17, 2005
  10. Jan 17, 2005 #9
    Wow I'm really stupid. Thanks :smile:
  11. Jan 17, 2005 #10
    I circled the correct answer. I see what I was doing wrong now. Thanks :smile: .
  12. Jan 17, 2005 #11
    For number 16:

    wavelength = (2/harmonic number)L
    Would the harmonic number be 3? If so why.
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