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Homework Help: Water bell; got answer but shape's weird?

  1. Aug 3, 2012 #1
    1. The problem statement, all variables and given/known data

    This question is about finding the smallest bowl to contain all the water being spurt out of a water spray head.

    They used the lagrange multipliers to solve it which I understand, but the the envelope doesn't look like a parabola to me??

    Isn't the envelope supposed to be a curve that is parallel to a certain point on every member of a family curve?

    3. The attempt at a solution

    I dont even see how it's possible to draw a curve that's tangential to all water-jet curves..

    Attached Files:

  2. jcsd
  3. Aug 3, 2012 #2
    They did not use Lagrange multipliers. An envelope of a family of curves F(x, y, p) = 0, where p is parameter, is obtained by [itex]\frac {\partial F} {\partial p} = 0[/itex], which is what they do.
  4. Aug 3, 2012 #3


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    Your picture is very strange. You seem to have water coming from a head that is convex downward and raised above the surface of the water. The situation described in the post has a hemispherical (convex upward) head at the level of the surface of the water.
  5. Aug 4, 2012 #4
    LOL i thought it was the bottom hemisphere. That was the first picture that came to my mind when i thought of a tap and a basin (come on, who builts a tap that spurts upwars? :P)

    I finally see it, thank you!

    I seem to lack the imagination.
  6. Aug 4, 2012 #5

    Ray Vickson

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    Well, who builds a water feature with a spray head at water level, pointing down?

  7. Aug 4, 2012 #6


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    Think of it not as a "tap" but as a fountain in an outdoor pool.

    (But they are making some very peculiar bathroom fixtures now. I recently had my bathroom remodeled. I looked at some bathroom sinks that are acryic hemispheres sitting on top the counter. I stared at them for a while, shook my head, and walked on.)
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