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View Poll Results: Will the circles made by the rock thrown in the stream stay circles?

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  • Yes they will

    7 53.85%
  • No they won't

    6 46.15%
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Thread: Rock thrown in water!

  1. #1 Rock thrown in water! 
    Forum Sophomore L.E.A.P.'s Avatar
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    If you throw a rock in calm water, it will create concentric circles hat many of you will call waves.

    Now let's say we have a little stream of water, will the circles created by the rock thrown in the water stay circles or will the change to an elliptic form


    Footnote:

    William, billco and river_rat, please, I know you can do this very easily (please PM me for the answer)


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  3. #2  
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    Ummm uhhhhhhhhhhh dohhhh ughhh.... they'll get stretched out. The wave front will get elliptical.


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  4. #3  
    Forum Ph.D. william's Avatar
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    Pretty good question LEAP.

    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  5. #4  
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    i put yes, but really i think they wont stay circles or eliptical because they would be destroyed.
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  6. #5  
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    Ripples move out in circles BUT the water does NOT!. I voted circles, purely from first instinct.
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  7. #6  
    Forum Ph.D. william's Avatar
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    I know LEAP requested that a certain three of us not answer this but I'll make a vote based only on observation. I fish quite a bit in the river so I'll go with circles.

    cheers
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  8. #7  
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    In my sobre state of mind I'd say circles as well. Moving circles
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  9. #8  
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    Quote Originally Posted by billiards
    In my sobre state of mind I'd say circles as well. Moving circles
    Not quite sure whether you mean 'sober' or 'sombre'.... 8)
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  10. #9  
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    Solution 1 - Laminar Flow:
    Stream velocity varies with distance from the bank (lateral) and with water depth (vertical). Therefore the circles will become distorted. This will not be a distortion to ellipses, but rather to a concave/convex lens shape, with the convexity pointing downstream.
    The magnitude of this distortion will depend upon the relative velocities of the outgoing ripples and the velocity profile across the stream.

    Solution 2 - Turbulent Flow:
    The ripples will be rapidly distorted in a random manner, then destroyed.
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  11. #10  
    Forum Professor river_rat's Avatar
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    I would say circles that move with the flow of the water
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  12. #11  
    Forum Sophomore L.E.A.P.'s Avatar
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    Well, it is a good question, because most of you are wrong! :-D

    Let's do a normal transformation:

    t= x+2, y

    Now that transformation does not specify wich point of a graph to transform... When not specified, points move all at the same time and move the same distance. Wich gives us a circle.

    Flow does not select wich part of the water to move, so in the case of a translation (flow of water), all the points move at the same time, in the same direction and at the same distance.

    I got a few more for you boys!
    P------------y--------------i----------------s
    ------h-------------s----------------c

    Alternate energy... or should we rather say alternate life?

    Are you interested in scientific high school projects? If you are, please contact me!

    "I must say that determination, devotion, and taking on challenges bigger then yourself is the way to become a master at anything" L.E.A.P.
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  13. #12  
    Forum Professor river_rat's Avatar
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    so whats my prize?
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  14. #13  
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    Quote Originally Posted by river_rat
    so whats my prize?
    2nd prize you mean... 8)
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  15. #14  
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    I think you overlooked Ophiolites answer. The circles will remain circles only under the assumption of uniform speed. Given the fact, that no stream in the world shows perfectly uniform speed (not even close), you should have stated this assumption in your question. Otherwise, I'd have to agree with Ophiolite, and add one more thing: Water speed usually varies in streamwise direction as well, due to variations in waterbed slope and stream width. At a location where the water accelerates, you will see the circles stretched in streamwise direction. Where the water decelerates you'll see the circles compressed in the streamwise direction. Hence, they become elliptical, even without the boundary effects or turbulence that were mentioned. I understand you weren'y thinking of accelerating streams, but as that's a more natural condition, you simply have to state your model assumptions to get the answer you expect.
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  16. #15  
    Forum Bachelors Degree The P-manator's Avatar
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    I white water paddle a lot and i can tell you that in those kinds of streams the ripples don't stay circles. So please tell us wether the current is strong or not.
    Pierre

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  17. #16  
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    In theory yes in practise never.
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  18. #17  
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    I would say the adjective "calm" in the original question excludes white water, but it doesn't exclude calmly accelerating streams or steady boundary layers. (Unless you consider white water calm.) The trouble is, that "calmness" is a very subjective quality. I think it was meant in the sense of "uniform speed". Right?
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  19. #18  
    Forum Bachelors Degree The P-manator's Avatar
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    Again it depends what type of whitewater. Some streams I can consider wuite "calm".
    Pierre

    Fight for our environment and our habitat at www.wearesmartpeople.com.
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