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Homework Help: Help projectile particle motion (winnowing)

  1. Jul 24, 2004 #1
    Help!!! projectile particle motion (winnowing)

    Hi ,
    I have been given an assignment which is to investigate the effectiveness of winnowing-projecting particles into the air stream (horizontal wind) and obtaining seperation based upon particle size, shape and/or density.Look up drag coefficents for a number of shaes then use them in your studies.Investigate optimum launch conditions for maximum particle seperation. :uhh:

    write a report of your findings to include numerical algorithm and theory, description of modell and theory ect....

    OK, i sort of know what i am doing but i still feel a little confused by the question...could some one tanslate it into simple english please...what is it really asking?

    help...i just want to get it finished...i just need pointing in the right direction not they answer....thank you :rofl:
  2. jcsd
  3. Jul 24, 2004 #2
    I'll bite: winnowing is the process of separating seeds from chaff. I believe grain was harvested, and then allowed to dry. Once dry the grain was stirred or tumbled to cause the chaff to separate from the grain. The resulting chaff/grain mixture needed to be separated so small amounts of the mixture was placed in large shallow bowls and tossed into the air on moderately windy days. Chaff is paper like while the grain was larger and more massive. This difference in shape and mass allowed the women(usually) to separate the chaff from the grain because the wind would "catch" the chaff more so than the grain. The grain would fall back down into the bowl while the chaff was blown away by the wind.

    It sounds like your being asked why this procedure actually works. You're asked to investigate why the grain and chaff both don't simply fall down at the same rate back into the bowl. You're going to have to make some assumptions about grain and chaff shape (grain is spherical or tear drop shaped with chaff is flat/thin like paper) then investigate how these shapes affect the drag coefficients. Using your assumptions about shape you then need to find a condition(s) which would blow the most chaff away while allowing the grain to fall at some catchable angle i.e. 100-mph wind would be too much while .1-mph wind would probably cause the tosser to spend hours on a single bowl full.

    For the report you need to address you assumptions (size, shape, mass, and max catchable angle) finding a relation between the chaff and grain. You need to address what air resistance is and how it works.

    That's my best guess.

    Good luck.
  4. Jul 24, 2004 #3
    Hi , thanks alot..that has made it alot more clear for me.I have managed to complete the first part of the assignment on excel which was to model the motion of a projectil e with air resistance..but this part (winnowing) was a littbe bit scary for me.Do u think i can assume drag coefficients for them or find them on net?
    once again thank you
  5. Jul 24, 2004 #4
    You should be able to find info somewhere. Try the NASA web site. Great place full of information about prettymuch everything.

    Develope an experiment to determine the drag coefficient. Drop various materials from a known height and time how long it takes for them to fall. You should be able to develope a drag coefficient though it may take awile. Don't use dat from 1 or 2 drops to build your model. Use a minimum of 7 drops then build your model. Test your model using 7 more drops (decide on some error factor say all drops must fall within 10% of the predicted drop). If all drops don't fall within the predicted range re calculate you coefficient using all 14 points of data. Repeat as necessary. I'd do this somewhere relatively calm to limit the external factors. If you do the above enough you'll come up with a workable function.

    Good luck.
  6. Jul 26, 2004 #5
    Thank you once again...i feel really confident now about finishing the assignment..thanks
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