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Conservation of angular momentum on a rope

  1. May 30, 2014 #1
    If a person hangs still from a rope some distance above water and then the rope is cut off, is it possible for the person to start somersaulting in the air before hitting the water? When I try to imagine myself in this situation, I would think that "of course it is, one can just start moving in the air to start rotating". But according to the principle of conservation of angular momentum, it is not possible, since it says that when the sum of average external torques is 0 then pf = p0 <=> I*wf = I*w0 and in this case w0=0 so that means wf must also be 0.

    When a physical law goes against intuition like this, I find it hard to understand. Although I don't have the ability to try the experiment myself at the moment, I would definately imagine that it would be possible to start rotating in the middle of the air.

    Any ideas how this can be explained in a way that doesn't go against intuition?
     
  2. jcsd
  3. May 30, 2014 #2
    In the real world, there is air resistance that is acting on the body. Once the man starts to fall, he may experience different amount of force due to air resistance on different parts of his body which may create an unbalanced torque. This unbalanced torque enables the man to somersault.
     
  4. May 30, 2014 #3

    jbriggs444

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    The human body is not rigid. It is possible to change its orientation without an external torque and without performing a rigid "rotation". So even though it is not possible to start rotating in mid-air without exploiting air resistance, it is possible (given sufficient time) to assume a head-down posture before hitting the water.

    One of the simple ways to accomplish this is by windmilling the arms in the opposite direction.

    You can Google for "cat landing on feet" and find hundreds of pages on the phenomenon. The cat's approach is more subtle than windmilling the arms.
     
  5. May 30, 2014 #4

    A.T.

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    In this video at 25:00min an astronaut does it in free fall.
    http://techtv.mit.edu/collections/l...cle-smarts-stability-translation-and-rotation

    Here the falling cat:

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

    That might be an useful mechanism for sky divers, not for small fall heights.
     
  6. May 30, 2014 #5
    In that case, I guess, as Jbriggs pointed out, changing moment of inertia can generate an angular momentum
     
  7. May 30, 2014 #6
    Just watch a springboard diving competition on how to change body orientation during a short fall into the swimming pool.
     
  8. May 30, 2014 #7

    A.T.

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    No, it cannot.
     
  9. May 30, 2014 #8

    jbriggs444

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    A.T. is correct that you cannot change angular momentum that way. You cannot change your angular momentum without an external torque.

    Nor does my post talk about changing your moment of inertia. Zero angular momentum would involve zero angular velocity no matter how small you make the moment of inertia.

    What is important is that you can change the way you face without having any net angular momentum. Windmilling your arms is a simple way. Your body rotates one way and your arms rotate the other way. Angular momentum is conserved, but your face is now pointed in a new direction.
     
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