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Minimizing action of falling maple seed.

  1. Nov 6, 2011 #1
    In the following link is a slow motion video of a maple tree seed falling (you have to wait till the 7th second).



    From the Wiki article on Action

    http://en.wikipedia.org/wiki/Action_(physics)

    "In physics, action is an attribute of the dynamics of a physical system. It is a mathematical functional which takes the trajectory, also called path or history, of the system as its argument and has a real number as its result. Action has the dimension of energy × time, and its unit is joule-seconds in the International System of Units (SI). Generally, the action takes different values for different paths. Classical mechanics postulates that the path actually followed by a physical system is that for which the action is minimized, or, more strictly, is stationary."


    If we consider the falling seed and the air through which it falls as the dynamical system how can we use the above quote to help explain how the seed falls in the above video?

    Thanks for any help!
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Nov 6, 2011 #2

    Matterwave

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    I haven't actually seen a Lagrangian treatment for rigid body motion. The Newtonian treatment is much simpler I think when it comes to rigid bodies.
     
  4. Nov 7, 2011 #3
    The maple tree seed spins because it interacts with the air. The air flows around it in a highly complex, non-static, non-rigid way. If you dropped the seed in a vacuum with no initial spin, it would not spin. To use the action principle, you would have to model all the possible ways (possible mathematically, not necessarily physically) that the air can flow around it - this is the action - then minimize your expression to get the actual physical air flow - this is the least action. It's simple in concept, but would be difficult in practice to use a least action approach quantitatively to solve for fluid flow. If I remember right, the least action approach is useful in practice if there are very few degrees of freedom in the system (a bead on a hoop, a weight on a plane, etc.). For a fluid, each molecule is somewhat free, so your system would have a huge number of degrees of freedom.

    If you want to try to conceptualize the least action principle for a twirling maple seed, you could say that its twirling leads to the least amount of drag and least amount of air turbulence, because the air encounters its knife edge when it twirls instead of it's broad face. This similar to a simple paper airplane. If you simply drop a paper airplane, it will not fall straight down, but instead will glide at an angle, in the direction where the drag is minimized.
     
  5. Nov 8, 2011 #4
    I think what he is pointing to is that the seed falls, then surges upward, then floats or maintains its elevation... as if the seed's airfoil suddenly gets a better grip on the air, speeds up its rotation, then somehow "overspins" to lift the seed...
     
  6. Nov 9, 2011 #5
    That floating at the very end of the video must be a very subtle effect or be just the camera getting bumped. I have watched a lot of real maple seeds fall in my life time and I have never seen one stop falling suddenly.
     
  7. Nov 9, 2011 #6
    I think what happens at the end of the video is the seed lands on a table that can't be seen.

    Thanks for help in understanding this!
     
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