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The rotation of a Diver

  1. Apr 25, 2016 #1
    1. The problem statement, all variables and given/known data
    I am doing a report or physics homework where I have to talk about the rotation of an Olympic high diver and am slightly confused as to how it all works. I have a few questions which would help clarify.

    1) So I know that in the air, angular momentum is conserved as in the air the diver experiences no torques. This means the momentum is provided by the jump. Is the torque provided because the center of mass of the diver is in front of their contact point on the springboard so when the springboard provides the force(and person provides force on springboard), the force is applied at a distance and at an angle to the center of mass, meaning torque is provided, allowing them to spin in the air

    2) Does simple harmonic motion and resonance apply for the springboard where the diver swings their arms during the run up in time with natural frequency in order to increase amplitude/displacement which in turn increases restoring force so it generates a larger torque for the diver

    3) I also need a brief starting point for how twists along the vertical axis(head to toe) is created to get me started. I found out it works similar to the way a cat can rotate in the air but I don't fully understand. Just a couple sentences as a starting point will do.

    2. Relevant equations
    L=Iw
    L=Fr
    change in L = torque * change in time

    3. The attempt at a solution

    I have said what I have thought in the questions
     
  2. jcsd
  3. Apr 25, 2016 #2

    Simon Bridge

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    Why not try simple jumping and running and arm-swinging for yourself?
    Can you jump up and twist at the same time?
    Can you jump so you flip over onto something (suggest: bed)?
    Does it help you to swing your arms and so on?

    When you are familiar with the effects on yourself - look at pictures and video of high divers in motion: plenty on youtube.
    You can go slowly through the action to get an idea what physics are involved and you'll understand better having tried yourself.
     
  4. Apr 26, 2016 #3

    haruspex

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    Yes, but it is important to distinguish between twisting in the sense of a nonzero angular momentum and in the sense of a net change in orientation. The cat's trick is to achieve the second without the first.
    Forty years ago I failed to persuade a visiting Russian professor that an astronaut can reorient in space by extending arms, twisting the trunk, bringing the arms in, separating the legs, and undoing the twist. Perhaps we had both had too much vodka.
     
    Last edited: Apr 26, 2016
  5. Apr 26, 2016 #4

    CWatters

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    Find a chair that will rotate. Sit in it an wave your arms or a broom in a circle above your head.
     
  6. Apr 26, 2016 #5

    Nidum

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  7. Apr 26, 2016 #6

    Simon Bridge

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    Aside:
    In your defence, it is not easy ...
    https://en.wikipedia.org/wiki/Falling_cat_problem

    And I have to include this one for the references:
    https://lostinscience.wordpress.com/2011/01/25/how-cats-land-on-their-feet/
    It repeats the wikipedia thing, but less technically and with easier references; includes a more general overview of what can only be called "the physics of cats". There is a link to a New Scientist article explaining how humans can rotate to similar effect. The study example is a highdiver ... so on topic :)

    ... and there's video of cats trying their self-righting trick when weightless. Near the end of the video, one of them seems to have hit on the idea of orienting towards the nearest surface just before colliding ... which makes me want to try a longer experiment. So that was fun: thanks.
     
    Last edited: Apr 26, 2016
  8. Apr 26, 2016 #7

    haruspex

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    In regard to cats, I only wrote that they achieved the trick of changing orientation while maintaining zero net angular momentum. Your link bears that out. The specific motions I mentioned were for how an astronaut might do it.
     
  9. Apr 26, 2016 #8

    Simon Bridge

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    I wasn't clear: I'll try that again:
    1st getting the context right...
    ... in your defence, it's not easy (to convince someone that it is possible to do this).
     
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