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Before it hits

  1. Nov 11, 2007 #1
    Before an object hit terminal velocity does the shape matter.
    In other words if you drop a small rectangle will it slow down in any way or will it keep on falling till it gets to it's terminal velocity.

    then another part to that is why when you drop a pencil or anything else shaped like that does it fall on it's side
    Last edited: Nov 11, 2007
  2. jcsd
  3. Nov 12, 2007 #2


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    Yes, the shape will matter. As the object accelerates, aerodynamic drag will increase. If the object has a streamlined shape, the effects of drag will be slight. However, an object with poor streamlining will encounter little drag at first (because of its slow speed at the beginning), and then that drag will increase as the object accelerates. This in turn will slow the rate of acceleration, giving that object an acceleration curve that resembles that of the more streamlined object, but shallower.
  4. Nov 12, 2007 #3
    do you know of any types of programs that would do anything like that.
  5. Nov 12, 2007 #4
    I'm not completely sure but I think that V Python could do that (I think I got the name right).

    Or you could go online and look for an applet. Those are everywhere.
  6. Nov 12, 2007 #5
    Here's a challenging question: what shape creates the most drag?
  7. Nov 12, 2007 #6
    Described 3 dimensionally or 2 dimensionally?
  8. Nov 12, 2007 #7
    VPython will give you a nice visual represention of the problem, but to use it correctly you still need to be able to describe all the factors in a consice way and program it. (Yep, I program in Python...lol. Great language, but it is still programming....)

    - Depends who your Ex is?
    - What are the parameters for spill? Ie: Air leaving a chute, water a bag?
    - Best guess? The inverse of the one that creates the most thrust. A bell shape with the tip cut off where the ratio of sizes of the bottom and top are given in relation to the ambient pressure and the maximum pressure of the medium it is deployed in and the bottom opening relates to the mass to be slowed (even if the mass is the drag object itself alone)

  9. Nov 12, 2007 #8


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    This was sort of the first thing I thought of too, a scoop. But how would a scoop be any better at drag than a flat disc of the same cross-section?
  10. Nov 12, 2007 #9
    Dave: A flat disk is not optimal because of the air flow. (let's just use air as an example and then adjust for density and flow factors of other media as necessary OK?)

    Specifically the air does not hit the disk and stop, but creates a higher pressure zone in the middle where air becomes trapped against the disk and lower at the edges where it can flow off and around the disk. If you were to look at a falling disk with a pressure sensitive camera you would see that the disk would effectively become a lenticular object where air heading straight up at the center of the disk would get "shunted" off to the side before it ever hit the disk effectively wasting some of its drag. Granted some partial vaccum on the other side of the disk aids the drag process, but I suspect that it doesn't help any where near as much as releasing some of the pent up central pressure would to force the airflow to actually present as much "work" against the surface of the drouge shute as possible.

    Then again, I could be wrong.
  11. Nov 12, 2007 #10
    How about a shape (moving through initially 2-D or 3-D static air) that creates the most energetic reinforcing resonances - perhaps a foam-like small scale maximal surface?
  12. Nov 13, 2007 #11
    You wind up with a different problem. Remember that from the frame of reference of the drouge, in whatever medium, it's all about taking the incoming energy and translating it into kinetic energy. If you use a micro surface you get an initial boost due to surface area, but without releasing the pent up particles you effectively add mass to the drogue now moving at speed that new particles must slow down. In addition these pent up particles buiding up pressure over time (short or otherwise depending on the media) effectively "smooth" out the micro surfaces getting you back to a smoothish surface. Besides, turbulance doesn't help. In fact it hinders because you now have energy in the particles transferred into disturbance in the flow rather than transferring to the drogue as braking in a nice laminar manner.
  13. Nov 13, 2007 #12
    a piece of paper shape???
  14. Nov 13, 2007 #13


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    Precisely, which is why I don't know if a bell shape is any better at drag than a flat disc. It seems to me that the dead air in the cavity would cause it to act as if there were not a cavity at all.
  15. Nov 14, 2007 #14
    Dave, please reread my first post. The bell cavity I describe has a HOLE in the top to allow the built up pressure to release! Otherwise you would be correct and a cavity would actually be worse than a flat plate of the same material because it would weigh more and therefore require more media passing over it per unit time to slow it down. Make sense?
  16. Nov 14, 2007 #15
    However, you notice that the earlier statement suggests a bell shape with the tip cut off.

    EDIT: 2 minutes too late :(
  17. Nov 14, 2007 #16
    so you are saying before the object hits terminal velocity, it can go slower than 9.8m/s/s.

    it this just like dropping a piece of paper.

    then the other part you would have to have a stream line to go faster than 9.8m/s/s
  18. Nov 14, 2007 #17

    9.8 m/s/s represents an the force of pull between an object in an NEL (Near Earth Location) in general terms. It intentionally ignores other factors. Specifically drag. Such as air, water or whatever.

    The nature of the question is about drag, not about the Earth's gravitational pull. But if you insist, may I forward you to dropping a Kleenex, office Memo, or any other object with a large surface area to weight ratio. Manifestly they all accelerate slower than 9.8 m/s/s. The question isn't why doesn't that violate some law, but how does our understanding of physics extend beyond the simplistic inert body attraction equation?

    Remember that gravitational attraction assumes no other factors, but the whole question of a drouge implies there are other resistance factors.

    Case in point... Moon landing. No atmosphere = no drag ergo no parachute. But what if I built a parachute kilometers square using solar pressure to slow me down? Would it work? Sure! Am I enough of a moron engineer, (or just seriously over caffienated) to suggest it as a real world suggestion in a focus meeting. Heck no! (I have sense after all. My wife says I waste it on coffee and my Mom says I waste it on my wife, but somehow I don't think they are both talking about the same kind of sense/cents?)

    Anyway. Drag implies an operable medium that offers resistance to a motion. All we are exploring is the optimal geometries for the solution.

    In Earth atmosphere at near ground levels (say under 50 stories) it's all about how much mass can you trap and coerce into giving up potential energy for negative kinetic energy and then dump it for the next bit of air.

    Scratch that. The above paragraph is true for any situation where there are any two objects with a relative velocity and an elastic collision.
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