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Puzzling Question about Rotation

  1. Jun 22, 2012 #1
    Is it possible to turn 180 degree for a person who is standing on a absolutely frictionless surface? Air friction is supposed to present. The person can jump vertically and spin in air.thats one possible scenario. Are there any others??
    Also if air friction is neglible is it possible to turn 180 degree?
     
  2. jcsd
  3. Jun 22, 2012 #2

    rcgldr

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    Re: Puzzling Question

    Ignoring the air and assuming a frictionless surface, angular momentum (in this case zero) is maintained, so a person can rotate their arms in a circle over their head, moving their arms clockwise to turn their body counter-clockwise or vice versa, to change the angular orientation.
     
  4. Jun 22, 2012 #3
    Re: Puzzling Question

    yeah, absolutely angular momentum would be conserved. But arms and feet are'nt two separate entities.They belong to the same entity-body.
     
  5. Jun 22, 2012 #4
    Re: Puzzling Question

    Feeling a bit difficult to visualise the situation.
     
  6. Jun 22, 2012 #5

    rcgldr

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    Re: Puzzling Question

    A cat can change it's orientation by bending at the middle, and whirling it's front and back body havles to rotate 180 degrees from inverted and land on it's feet. Link to youtube videos:



     
    Last edited by a moderator: Sep 25, 2014
  7. Jun 22, 2012 #6

    NascentOxygen

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    Re: Puzzling Question

    He may be able to turn his head at an angle and expel air forcefully to induce a jet-propelled rotation. Or he could empty his bladder tangentially ...
     
    Last edited: Jun 22, 2012
  8. Jun 22, 2012 #7

    Cleonis

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    Re: Puzzling Question

    But you are free to orient your limbs relative to your torso. It's only a single rigid body that cannot reorient itself in space.

    For visualisation:
    Take a cylinder shaped satellite, in orbit. Inside the cylinder, at the center of mass, there is a disk, with the axis paralllel to the axis of the cylinder. Let's say that disk's mass is 1/100th of the satellites' mass.

    Spin up the disk. Spinning up the disk causes the satellite to counter-rotate around its axis, but slower than the disk, as the satellite is heavier. After the satellite has rotated 180 degrees spin the disk down again.


    I assume astrounauts in weightless conditions do something like that. They will windmill one or both arms to reorient themselves.
     
  9. Jun 25, 2012 #8
    Re: Puzzling Question

    That was seriously awesome that i learnt from the posts. Thanks guys.
     
  10. Jun 25, 2012 #9

    A.T.

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    Re: Puzzling Question

    Yes or swing around the legs. See this video at 25:00 for a demonstration in the skylab:
    http://techtv.mit.edu/collections/l...cle-smarts-stability-translation-and-rotation

    They also can do what the cat does. They did some research on this in the 60's for the space program:
    http://www.theatlantic.com/video/archive/2011/09/can-an-astronaut-move-like-a-falling-cat/244829/
     
  11. Jun 25, 2012 #10

    HallsofIvy

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    Re: Puzzling Question

    In order to "spin in air" the person would have to have to have applied a rotary force before jumping. And he could not do that on a frictionless subject.
     
  12. Jun 25, 2012 #11

    D H

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    Re: Puzzling Question

    Nope. You're thinking of a person as a rigid body. People have arms, legs, and a waist. This is a vertical analogy of the "falling cat problem". Some journal articles about this problem: http://scholar.google.com/scholar?q=falling+cat+problem.
     
  13. Jun 25, 2012 #12
    Re: Puzzling Question

    This.

    :rofl:


    I think he can. That's how falling cats work, I suppose.


    Edit : Oh, DH added an article too.. :smile:
     
    Last edited: Jun 25, 2012
  14. Jun 25, 2012 #13

    jtbell

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    Re: Puzzling Question

    Another example, which I've done many times as a classroom demonstration: Sit on a frictionless rotating stool while holding a bicycle wheel horizontally. Everything is initally stationary. With one hand, start the wheel rotating clockwise. You and the stool start to rotate counterclockwise in order to conserve total angular momentum. When you've rotated 180 degrees, stop the spinning wheel. You stop too, in your new orientation.

    It doesn't even have to be a wheel. You can hold an object in your hands, out in front of you, and move it around in a horizontal clockwise circular motion. Same result.
     
  15. Jun 25, 2012 #14

    D H

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    Re: Puzzling Question

    And here's a graphic:

    tumblr_lurmysW0QC1r1w416o1_500.png

    This image is from TR Kane's and MP Scher's seminal paper on the subject, A dynamical explanation of the falling cat phenomenon. Thomas R. Kane is no slouch. Study robotics and you will inevitably learn about Kanes' dynamics equations. He is in a sense the father of modern robotics.
     
  16. Jun 25, 2012 #15

    A.T.

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    Re: Puzzling Question

    This is Kane's two-parts-cat model animated:

    https://www.youtube.com/watch?v=yGusK69XVlk

    A human standing on frictionless surface could try the same (basically a hula hoop swing), but would probably fall over. Swinging arms seems better here.
     
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