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A Debate

  1. Oct 13, 2007 #1
    Me and a friend had an argument. Say you had a wire hanger and bent it into the first shape shown in the attachment (a straight portion and then the rest curved into a half "U"). You attach the straight portion to a machine that rotates the object at a very high speed and creates a bowl shape with the half U portion, as shown in the second half of the picture. The argument is this: could the rotational speed of the hanger be fast enough to hold water, or would it fly upward and out of the created bowl?
     

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  3. Oct 14, 2007 #2

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    It cannot hold the water. What an idea!

    Suppose you have an actual bowl spinning but with a hole in it. The water will obviouly come out of this hole due to the centifual force (and gravity). Your roating hanger is nothing but a bowl with a big big hole.
    (Or a hole with a broken bowl!)
     
  4. Oct 14, 2007 #3
    A good question! I assume you're thinking the viscosity of the water is large enough to hold it in place till the wire comes around again. You might think of using a rotating shape that has some restorative force (kind of like a propeller blade) since any fluid that "droops" will not be picked up on the next rotation unless you have a fluid much closer to molasses on a cold day.

    You might look at common kitchen mixers to see how your idea works. In fact, if you're willing to make a little mess, you can come pretty close to your answer experimentally. Keep up this kind of thinking - this is the kind of strange question Richard Feynman was famous (infamous?) for.
     
  5. Oct 14, 2007 #4

    arivero

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    other perspective: could this wire-cup hold a tennis ball, or a pingpong ball?
     
  6. Oct 14, 2007 #5

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    It could hold a solid object, due to the balancing of three forces -- gravity, friction and the centrifugal force.

    The shape of the bent portion matters very much. The vector sum of the centrifugal force and the weight of the body has to be normal to any rotating surface if the object is to keep still wrt the rotating surface without friction. A paraboloid, the surface of revolution of a parabola about its axis, is just such a surface. So, if the wire were bent in the shape of a parabola, a ball placed properly would rest on it. But if it moves just a bit wrt the rotating frame, the coriolis force would shift it out of equilibrium.

    A small drop of water, whose dimensions are smaller than the thickness of the wire, will cohere and be carried along the bent portion of the wireif placed properly. But if the volume is more, the drop will tend to push out around the wire due to centrifugal force.
     
  7. Oct 14, 2007 #6

    russ_watters

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    It won't hold a solid object. What you have there is a blender.

    What you are suggesting is that if you spin it fast enough, an object won't be able to fall through. The faster it spins, the less an object will be able to drop before hitting it. But then also, the faster it spins, the larger the force when it does hit.
     
  8. Oct 14, 2007 #7

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    Hi Russ,

    Before posting a reply, I just wanted to ask you whether your posting is a reply to the general discussion, or to my posting specifically? I have a feeling that what I’ve said could been horribly misunderstood.

    After all, it was I who mentioned that it could hold a solid object. The stability of that is exactly the same as keeping an object on a slanted wire on earth. (But theoretically it's possible.) The rest of my writing just describes ordinary mechanics in a rotating frame. Also, in my mind I have no picture of blenders or propellers or the likes.
     
  9. Oct 14, 2007 #8

    russ_watters

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    Both. It sounds like youa are talking about an object that is rotating with the wire. That would not be a stable situation because the object would have to be balanced on the wire.
     
  10. Oct 14, 2007 #9

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    Isn't that what I said? I have mentioned all the points you are talking about.
     
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